electrical engineering, electrical fire, and safety, electrical safety audits, electrical training, for process plants oil and gas

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A STUDY NOTE ON
ELECTRICAL RISK MANAGEMENT
By
Dr.A.M.Salsingikar, Ph.D.
Chartered Engineer
&
Govt. Accredited Electrical Safety and Energy Auditor.
Electrical Safety is a Serious Issue
No one can replace a worker or a loved one that has died or suffered by the
consequences of an electrical accident. Electrical Safety in the workplace is
the most important job of an electrical worker.
No matter how much training one has received or how much employers try
to safeguard their workers, Electrical Safety is ultimately the responsibility
of the person himself working on electrical lines and equipment.
It has long been recognized that electrical accidents do not just happen but are caused due
to the non observance of certain fundamental safety measures as contained in the I.E.
Act, 1910 and I.E. Rules 1956, made there under. A study of electrical accidents shows
that, In general by and large, accidents at consumer's premises are due to the following
reasons:
Amateur workmanship & repairs
Defective wiring
The lack of proper maintenance
Use of non-standard & shoddy appliances
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Ignorance about fundamentals of electricity
2. WHAT ARE ELECTRICAL HAZARDS?
Hazard means anything which can cause harm and Risk means the chance, great or
small, that someone will actually be harmed by the hazard. The main hazards are:
contact with live parts causing shock and burns.
faults which could cause fires;
fire or explosion where electricity could be the source of ignition in a
potentially flammable or explosive atmosphere.
The absence of accidents does not necessarily mean there are no hazards. A risk
management process must be adopted and repeated at regular intervals.
Hazard identification is not a one-time exercise.
Hazards grow with time and with change in process. As time goes by, statutes as well as
technology change. To comply with updated statutory regulations, periodic compliance
assessments are needed. Hence, hazard identification has to be done periodically.
(1) ELECTRICALHAZARDS
The following hazards are with in the use of Electricity:
A. PRIMARYHAZARDS
a. Electrical shock
Joules Burns
b. Burns
Flash over burns
c. Fire &Explosion
Spark
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Static Electricity
B. Secondary Hazards
(Involuntary reflex action)
(a) Persons falling from height
(b) Dropping of tools and object
(2) DEFINATION OFELECTRICSHOCK
(a) Electric shock is sudden and accidental stimulation of the body’s
nervous system by electric current. Current will flow due to voltage
difference.
(b) EFFECTOFELECTRICCURRENT ONMAN
Table 1 gives the effects of electric current on man.
Table 2 gives human resistance to electric current.
EFFECTS OF ELECTRIC CURRENT ON MAN
Current in Milliamperes
Direct 60Hz 100Hz
Men Women Men Women Men Women
Slight sensation on hand 1 0.6 0.4 0.3 7 5
Perception threshold 5.2 3.5 1.1 0.7 12 8
Shock not painful muscular
control not lost
9 6 1.8 1.2 17 11
Shock painful muscular 62 41 9 6 55 7
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control not lost
Shock –painful, let go 76 51 15 10.5 75 50
Threshold
Shock-painful and severe
muscular contractions, 90 60 23 15 94 63
breathing difficult
Shock-possible ventricular
fibrillation effect from 500 500 100 100
8-Second shocks
Short shocks testing seconds 165 165
High voltage surges 50* 50* 18.6* 18.6*
* Energy in watt-seconds or joules.
TABLE-II
HUMANRESISTACETO ELECTRIC CURRENT
Body Area Resistance (ohm)
Dry skin 100,000 to 600,000
Wet skin 1,000
Internal body hand 400 to 600
Ear to ear (about) 100
(c) HOWSHOCK OCCURS
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Shock occurs when the body becomes part of the electric circuit. The current
must enter the body at one point and live at another. Shock may occur in one of three
ways.
(1) With both wires of the electric circuits
(2) With one wire of an energised circuit and the ground.
(3) With a metalic part that has become hot by it self being in contact with an
energised wire.
The severity of the shock depends on following factors,
(1) The rat of flow of current throw the body measured in amperes
(2) The path of the current throw the body.
(3) The length of time the body is in the circuit.
Other factors which may effect the degree of shock are the frequency of the
current, phase of the hart cycle when shock occurs and the physical and
psychological condition of the person.
(d) Jouleburns : The passage of an electric current along any conductor is
accompanied by the dissipation of it according to Joule’s Law the heat dissipated
is directly proportional to I2
RT where I is current in ampere; R the resistance in
ohms and T the time in second. As the skin is the site of the highest resistance in
the body it is here that burning is most likely to occur when contact is made with
a live conductor. Such burns may be deeper than may first appear on clinical
examination. Consequently healing is often slow and maybe accompanied by
much scarring.
Flashburns : If an earthed conductor is brought close to a both another
conductor at a high voltage, the insulation of the air between team may break
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down giving rise to a spark. This ionizes the air considerably lowering its
distance which in turn allows the current to increase an electric arc is set up. If
the earthed conductor is a human being too close to a high voltage line, he will be
burnt by the arc without actually coming into contact with the conductor.
Because of the reduced electrical resistance of the air and the large area of the
skin burning (which reduces the skin resistance) large currents may flow. Thus
the victim is the subject of a double events a flame burn from the arc and an
electric shock from the current which passes.
This burns are often made worse as a result as the result of clothing catching fire.
Because flash burn accidents are usually associated with high voltages, the
currents which flow are often to great to cause ventriculator fibrillation. Also,
because the victim does not usually touch the conductor he is practically never
“held on” but falls away from the conductor, thus extinguishing the arc. The
current therefore usually passes through the victim for only a brief time.
Secondary effect : Many after effects of electric shock have been reported but
these reports often describe isolated cases of a disease following a shock and there
is no real evidence of a casual relationship. However, several sequels are well
substantiated.
Angina electrical may follow a shock in a relatively young person and is clinically
in distinguishable from angina pectoris . It nearly clears up within a few weeks or
months leaving no after effects. Electrical cataract is a permanent condition which
may insure after certain types of electric shock, usually severe and passing
through head.
Chromoproteinura sometime leading to severe disturbance of renal function may
follow a severe electric shock which has caused strong muscular contraction with
release of myoglobin . It is analogous in all respects into circulation from
damaged muscles .
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Various neurological sequel have been described and it is possible that some of
these may be due to injury to the spinal resulting from the violent movement of
the vertebre caused by the strong muscular contractors occuring during shock.
Nevertheless it should always be remembered that shocks of several hundred
millamperes are passed from the temples to the other in electroconvulsive therapy
and that side effects on the central nervous system from this are rare.
(e) RESON FOR SHOCK
(1) Victim touches bare conductor live
(2) Victim touches poorly insulated conductor
(3) open and short circuit due to equipment failure
(4) Static electricity
(5) lightning
3) COMPARISON BETWEEN ACANDDC
What is more dangerous-AC or DC ?
A question is often asked whether AC is dangerous or DC is dangerous.
Alternating Current (AC) and Direct Current (DC) have slightly different effects on the
human body, but both are dangerous above a certain voltage. The risk of injury changes
according to the frequency of the AC, and it is common for DC to have an AC
component (called ripple). Someone with special equipment can measure this, but the
effect on a particular person is very difficult to predict as it depends upon a large number
of factors. As a consequence you should always avoid contact with high voltage electrical
conductors, regardless of the type of electrical current they are carrying.
It is the voltage ( AC or DC) that is more dangerous than the current in any case.
What voltages are dangerous?
A wide range of voltages can be dangerous for different reasons. A very low voltage
(such as that produced by a single torch battery) can produce a spark powerful enough to
ignite an explosive atmosphere. Batteries (such as those in motor vehicles) can also
overheat or explode if they are shorted.
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If a person comes into contact with a voltage above about 50 volts, they can receive a
range of injuries including those directly resulting from the electrical shock (stopped
breathing, heart, etc), and indirect effects resulting from loss of control (such as falling
from a height or coming into contact with moving machinery). The chance of being
injured by an electric shock increases where it is damp or where there is a lot of
metalwork.
Electrical or thermal burns can also occur from the flow of electrical current or hot
surfaces.
When a person comes in contact with live or exposed electrical parts he gets electrocuted.
The severity of electrocution then depends upon the path of current through the body and
the duration of the contact with live parts.
We have all experienced a electric shock without any damage because we got away from
the dangerous parts immediately when we got the shock, thereby stopping the current
flow through our body to the earth.
But that is not the case with voltage. The voltage develops an electromagnetic field
around the live parts and when a person approaches the live parts, this field produces an
electric arc which jumps upon the person nearby and injures that person.
Alternating Current and Direct Current have slightly different effects on the human body,
but both are dangerous above a certain voltage. The risk of injury changes according to
the frequency of the AC, and it is common for DC to have an AC component (called
ripple). Someone with special equipment can measure this, but the effect on a particular
person is very difficult to predict as it depends upon a large number of factors. As a
consequence you should always avoid contact with high voltage electrical conductors,
regardless of the type of electrical current they are carrying.
4) HIGHVOLTAGEHAZARDS
High voltage equipment should have secure enclosures and these enclosures must
be locked. Keys for these locks should be with responsible people and with those
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authorised to enter e.g.electricians. Small enclosure doors, panels drawers should
be locked or with interlock which will stop the current when enclosure is opened.
Additional shielding around bus and bars and live contact should be provided.
Warning should be placed on both access to area and on machine to warn men.
Work on high voltage lines should be discouraged. When necessary it should be
done only by qualified men who knows aritificial respiration.
5) INSULATION FAILURE
Insulation failure has been the cause of electrical accidents. The following are the
reasons for it :
1) Due to non-uniform material there are localized specific heat and resistances.
Hence deterioration due to heat, from current destruction of chemical bonds in
insulation by electrons in conductor. So electrical resistance is reduced. Current
produces heat which degrades polymer.
2) Moisture increases conductivity (e.g.) Nylon absorbs 8 % moisture. Hence in
damp locations, don’t use electrical tools and extension cords. The reduced
resistance earths current through user.
3) In enclosed spaces (rotating electrical machinery) Ozone is a problem. Electric
discharge due to sparking and arcing produces Ozone, which attacks insulation.
4) Ultra violet and nuclear radiation degrade insulation.
5) Photochemical processes by solar radiation cause breakdown of polymers like
synthetic rubber, vinye chloride, vinylidence chloride and HCL produced which
degrades insulation.
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6) Mechanical damages : Mechanical damage results from abrasion, cutting, flexing
and crimping. Vibration also acts in the same way.
7) High voltage causes sparking or corona which breaks insulation, corona produce
NO which with humidity forms No.3
6) BIOLOGICAL REASONS
Some insulations are eaten by insects, Rats also do this. In most atmosphere
fungus forms.
7) PRESSURE
Vaccum removes gases from organic insulators. So resistance is reduced.
8) INTER LOCKS
When circuit switch is installed in a drawer, door, lid or pane of enclosure the
current is cut off when closure is opened. They must be operable when you want
to check live circuits.
9) SHOCK PROTECTION METHODS
(a) Isolation
Electrical equipment (eg. HV ) should be isolated from approach by untrained
and unauthorised men. Large transformers with exposed terminals can be
located in vaults or fenced enclosures to which only permitted persons can
go. Panel boards, generators, large meters, batteries bus bars should be
enclosed or grounded.
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(b) Marking
Access to dangerous electrical equipment should have warning signs. Inside
may be painted with colors that will show when they are open.
The following general shock protection methods can be also employed :
(1) Insulation of parts routinely or accidentally touched by men (Knobs, dials,
handles etc)
(2) Rheostat and potentiometer control shafts can be coupled to non-
conductive rods and knobs.
(3) Rubber mats should be provided near switch boards.
(4) Rubber gloves and non conductive shoes should be worn.
10) GROUNDFAULT CIRCUITINTERRUPTER(GFCI)
GFCI interrupts current to load when current to ground exceeds some
predetermined value which is less than that required to operate the over current
protective device of circuit. Circuit breakers and fuses will open under
comparatively large current which is fatal to men.
GFCI is sensitive to small currents less than the injurious current. Within short
time GFCI opens circuit before men is affected. If current is greater than GFCI
level passes to earth by any other route due to imbalance in current circuit breaker
opens. GFCI cannot help when amount flows through man to earth. But most
accidents are of line to ground type.
11) LIGHTNING
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This is a massive natural discharge of static electricity characterised by high
potentials and high current flow. Lightning arrstors (rod and ground) provide
easiest path. Lightning rods are higher than their neighbourhood. Person in boat or
open field are easy targets of lightning. Trees increase the hazard. All metal
buildings are also dangerous.
The following precautions should be used to avoid lightning stroke.
(1) Protected buildings should be used as shelter.
(2) In unprotected building, metal should not be touched.
(3) In outdoors lie down or go to depressions. Metal fences should not be
touched.
12) ELECTRICAL HEATING
Fires occur during electrical heating. Over currents due to short circuits or surges
heat will cause fuse, switch off or circuit breakers to explode. Turn outs of
operating equipment create fire and equipment fails due to resetting high
temperature.
13) OVERLOAD PROTECTION
Circuits are protected by fuses and circuit breakers and sometimes by cutouts. The
following are different types of fuse (1) Screw in plugs (2) Ferrules (3) Cartridges
(4) Knife edge Cartridge.
Time lag fuses which carry overload for a short time (e.g.) motor draws more
current during starting. Cold fuses colder than conductor has less resistance and
hence greater overload blows it. Circuit breakers may be thermal, magnetic or
combination of both.
14) UNIT PROTECTION
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Individual equipment like motors will have fuses or protective devices. In
parallel operation one equipment may be overloaded and hence separate fuses are
necessary. Relays are used for unit protection. They are also called cut outs. In
thermal relay bimetallic disks hold 2 sets of contacts (closed or expanding metals
close contact). There are magnetic cutouts.
Resets are of manual or automatic types. In manual reset button or switch must
be pushed. So attention is drawn to open circuit. In automatic current flow is out
off, system cool and goes to original only when actuated. Warning lights are
provided in automatic type to show that system is open.
15) ACCIDENTAL STARTING
In maintenance jobs after opening the circuit it shoals be locked open and the
person working should have the key with him. Accidentally people hit switch or
buttons and start current e.g. drills. So button should be recessed. Those doing
maintenance jobs should know the switch which cuts off current by operating the
machine with the switch. If more than one is doing maintenance both should have
separate locks. Warning tags should be put at controlling point.
BASICELECTRICALSAFETYRULES
(1) Only qualified men should do electrical work.
(2) Don’t work on live circuits. If necessary, use buddy system and/or
use protective equipment (gloves, mats etc)
(3) Don’t touch bare conductor.
(4) Verify from terminal box or switch box that a circuit is open.
(5) Inform concerned people.
(6) Do not meddle with interlocks.
(7) Know voltage and frequency of the circuit.
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(8) Check suitability of fuses and circuit breakers. Consult an
electrical engineer.
(9) Inspect cords of portable tools.
10) Do not connect earthing to pipe lines.
11) Enclose uninsulated conductors.
12) When cranes pass near overload lines, they should be de-energised.
13) Don’t use wet clothing or shoes or wet feet.
14) Use wooden board.
15) Relive capacitor of changes before working on them.
16) Don’t use metal scales or tapes near electrical circuit.
17) Use flame proof equipment.
18) Design grounding properly
19) Use lighting arrestores.
ELECTRICALEQUIPMENTINSPECTION.
1. INTRODUCTION-
The purpose of this presentation is to highlight some of the methods used and
their implementation to perform quality control function relating to electrical
equipment. The many activities in which quality assurance /control fucnctions
involved are:
1. Design
2. Manufacture
3. Construction
4. Maintenance
This note deals with aspects relating to inspection during manufacture.
The main objective is to ensure that material is that go to make the ultimate
product are so designed and processed through various stages of manufacture, that
they confirm to stipulated drawings and specification to enable the design intent
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to be met in full and the product performs in service as it is intended to do. The
degree or extent of inspection activity mainly depends upon:
(1) Safety (ii) Investment, and (iii) Reliability.
2. PLANING
The degree of success of quality control program entirely depends on how
effectively and exhaustively the various steps and stages of inspection are
planned. The above requirement demands a clear understanding of the ultimate
goal by all personnel involved in the inspection. A through knowledge of design
specification is also a pre- requisite. Once there is a clear description and
understanding of the above aspects, the inspection personnel initiate the firs step
of production a quality control plan. This plan is the basic document which
greately reduces the subjective component of inspection and clearly spells out the
various steps in manufacture, the check various test procedures, assembly checks
and ultimately the final step of performance tests
3. ELECTRICALEQUIPMENT
Electrical equipment are broadly classification under various heads such as:
1. Rotating machinery like generators, motors etc.
2. Stationary equipment like power transformers, control and instrument
transformers which can be further sub-divided into various voltage,
medium voltage etc.
3. Control gears like circuit breakers of various types which can also be
further sub-divided into various voltage class, Motor control centers, etc.
4. Cables and line materials.
5. Protective devices like relays, releases etc
These can be further conveniently broken down to following components to
enable formulating a quality control plan.
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1. Strucutural -1.1 Main frame, tanks shaft, couplings
2. Conducting -2.1 Magnetic Electrical
3. Insulating -3.1 Main Support
4. Auxiliary -4.1 Coolers
4.2 Bushings
4.3 Fans
5. Protection -5.1 Embedded temperature detectors
5.2 Buchloz Relays
5.3 Current Transformers
6. Hardware
7. Pre-assembly
8. Final Performance tests.
The list above is typical and not exhaustive. The most commonly used electrical
equipment i.e. Generators, Motors and Transformers demand a high degree of
safe and reliable operation in power generation units and process industries.
4. Typical quality control planes developed in line with the guidelines
mentioned earlier is discussed below:
4.1 Structural Parts: The main structural parts of rotating machinery are the
frame and shield, which house the bearing fabricated. Normally larger
frame sizes are easy to fabricated and involve welding. The types of
discontinuity that are expected in welding are lack of fusion, lack of
penetration, destructive examination. Methods like Radiography, Mag-
particle, Dye penetrant etc. In the case of castings the discontinuities can
be shrinkage, gas and below holes, cracks etc. These can also be detected
by NDE methods referred to earlier. End shields which house the bearing
can be either cast or fabricated. The inspection requirements will be same
as those of the frames. In the case of shafts and couplings, the NDE
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method used is Ultrasonic combined with Dye Penetrate test to locate
both sub-surface discontinuities.
A pre-requisite for the above checks is the raw materials meeting the
specification requirements and a thorough visual examination. The above
also form part of inspection function .
In the case of tanks of transformers and large generator frames pressure
test ensure integrity, leak tightness and also ability to with stand specified
vacuum in the cases of transformer tank is carried out. Parameters like
deflection, leak rate, etc. are measured to ensure that specified limits are
not violated. While deciding on the inspection methods one has carefully
look into requirements of design safety reliability, application etc.
4.2 Conducting
4.2.1 Electrical :copper and aluminium form the main electrical conducting
material through copper finds more extensive application. In large
generators which have water cooled stator coils NDT liked eddy current is
carried out to ensure integrity in addition to leak test at elevated pressure .
Normally the checks carried out on conducting materials are :
I . Physical and chemical
II. Electric conductivity measurements
III Visual
IV Dimensional
4.2.2 Magnetic : Magnetic material used in electrical machine is silicon still.
There are varying grades and choice is left to the designer of the machines
and availability too. The checks carried out are:
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I . Physical and Chemical
II Iron loss measurements
III Bending test
IV Waviness and thickness
V Insulation test
VI dimensional
VII Visual
43. Insulating material :
There are various types of insulating material used in machines. They are
classified on the basis of temperature with stand capability for example
Class A – 1050c - paper, cotton silk impregnated with oil suitable varnish
Class E – 1200c - mica , fiber class etc.- with suitable bonding varnish
Class B- 1300c - -do-
Class F -1550c- -do-
Class H- 1800c- Mica, porcelain glass, quartz, with or without an organic
binder.
Among the above, most widely used are class A in transformers Class H, B
and F in rotating machinery. The checks on a sample of insulating materials
are:
i) Break down strength
ii) Tensile strength
iii) Visual
In case of transformer oil the following test are carried out.
i) Check for electrical resistivity
ii) Checks for electrical strength
iii) Check for water content
iv) Check for gas content
v) Check for ten-data
vi) Visual examination for suspended impurities.
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4.4 Auxiliaries: Auxiliaries are those components required as additional
firments to the main equipment’s for successful operation. Example are-
coolers in the case of air or water cooled machine Radiators external
mounting, tap changers etc. Though classified as auxiliary they have an
important role to play and as such demand as such care in inspection as the
main equipment. Failure of an auxiliary can either seriously effect the
operation or berate the machine which may not be acceptable to process
systems. Inspection carried out are:
Water cooler: I) Material and Fabrication tests
ii) Hydrostatic test
iii)Visual control
Fans: i) Materials and fabrication tests
ii) NDT depending on whether the I an is
iii)Dimensional
iv) Visual
v)Performance if application
Bushings: I) Dimensional
ii) Visual
Top Changer: i) Visual and dimensional check
ii) check for interlocks
iii) Measurement of contact resistance
iv) Check for electrical clearances
vi)Heat run as a qualifying tests
Protection: There are either embedded elements like Resistance Temperature
Detectors or additional figments which aid protection like Buchloz relay, current
transformers for over current, differential, earth-fault protecting magnetic oil level
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gauges, flow meters, pressure gauges etc. It is needless to impress on the
necessity to inspect these. Very often these do not lend themselves fir
replacement as in the case of RTDs embedded in stators of generators and
motors. It is a must to ensure that these are of very high quality and perform
reliably in service.
RTDs :i) Resistance characteristic R0 and R100 values
ii) Response time
iii) Temperature cycling
iv) Self heating.
Buchloz : I) High voltage and insulation resistance test
ii) Gas Volume test
iii) Loss of oil and surge test
iv) Operational check of test coil and trip contacts
V) Visual inspection
4.6 Hardware: Classified under this are the various fasteners, valves, piping
etc. Unfortunately, these receive very little attention but have been the
cause of many failures of equipments. The tendency to use commercial
quality fasters in critical application must be discouraged. The best way to
ensure quality is to obtain these form reputable manufactures and monitor
quality by a will formulated statistical quality control plan.
4.7 Pre-assembly : Materials released after inspection are processed and form
in to major sub-assemblies like starter Rotator etc. in the case of rotating
machinery, tank, core and coil assembly etc. in the case of transformers.
It is of utmost importance to inspect these sub-assemblies before they are
completely assembled. For examples, in the case of H.V. Motors, a few of
the typical checks are :
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4.7.1 Startor : i) Visual inspection of coils and end coil connection
for
good workmanship, cleanliness, dama insulation,
and
ii)
coil anchorage, wedge, integrity, etc.
H.V. Test
iii) Resistance measurement
iv)
v)
Insulation resistance
Check of all embedded parts
4.7.2 Rotor : i) Visual inspection for proper and secure mounting of
ii)
all fitments like fans, balancing weights etc.
Review of dynamic balancing records
iii) Visual check on brackings of short circuit rings in
cage rotors.
iv) Run-out check of shaft and journal concentricity.
4.8 FinalPerformance : Prior to final performance is the most important
activity of assembly. It has been often found helpful to witness this
operation. Inspite of various stages checks, difficulties do arise at
assembly which need careful study before corrections are made. It has
been observed to be the normal practice with many manufacturers to
violate good engineering practices and some-how make an assembly for
test. A good example is non-uniform ‘Air Gap’ reduced clearance to live
parts, improper mounting of bearings, etc adversely the performance
figures and as such go unnoticed. Which later in service lead to either
failure or unreliable operation and perhaps endanger safety. Witnessing
assembly helps to ensure that a good acceptable assembly is made and that
any eviation requiring correction is carried out in line with accepted
norms.
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It is been found to be extremely helpful to prepare a complete test
schedule to advance with all details of test instruments, accuracies
indicated in a schematic. The test guaranteed performance values and
other relevant design data.
ELECTRICALEQUIPMENTINSTALLATION
The basic norms followed in the any electrical installation work have to
may objective as follows,
(a) Safety of personal operating the installation
(b) Ease of maintenance work to be done during the life of the installation.
Indian electricity rules, 1956 the varies product wise standard
specifications issued by BIS for “code of practice “ for installation and
maintenance “ also lay emphasis on the two objectives maintain above. In
this context the most important criteria, applicable to all types of
equipment are –
(1) Effective earthing provisions.
(2) Safe clearances of live part of earth
(3) Accessible of working space and ventilation for maintenance personal
Earthing :-
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This consist of equipment earthing and also system earthing. For
equipment earthing it is obligatory that the gasing is provided with to
separate and distinct connection to earth. That is why all manufacturers
provide to earthing points at suitable places on the casings that on tanks in
the case of transformers on the base plate in the case of motors on the
panel sides in the case of switch boards etc. The idea of providing two
connections is that even if by accident one connection is snapped either
near the equipment or in the run of the earthing strip, positive connection
to earth always maintain.
Entire earthing system is designed with the regard to the maximum likely
fault current base on the equipment rating. In this connection it is pertinent
to observe that when ever any plant extension are made, which might
results in increase earth fault current, The size of earthing is checked for
correctness of size this aspects is many times forgotten and when new
equipment is added people only run two earth strips from the same and
connect them to the existing earth ring main. Following guidelines for size
of earthing strips to be used of copper should be useful :-
Earth fault current (KA) Cross section (sq.in.) size of copper strip
-----------------------------------------------------------------------------------------
Not Exceeding 30 0.3 11/2” x 3/16
Not Exceding 40 0.5 2” x ¼”
----------------------------------------------------------------------------------------
These earth strips are connected to” earth pits “ where it is normal practice
to install cast iron plates buried 6 ft. below ground level and surrounded
by wet and moist coke. For large power station and measured sub stations
it is customary to install for cast iron plates (4ft.x 4ft.x ½”thick) at four
corners so that the distance between any two of them is not less than 40ft.
For smaller substations smaller size Cast Iron plates (2ft x 2ft x
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3/8ft”thick) or even ½” thick cast iron pipes (6” dia x 10 ft .long ) are
used “. The earthing can be considered as “Effective “ if the resistance to
Earth of the whole system does not exceed the following figures:-
(1)Power stations - 0.5 ohms
(2) Measure substations - 1.4ohms
(3)Other substations - 2.0 ohms
(4) Distribution transformer station - 5.0 ohms
(5) overhead lines supports - 25.0 ohms
For effective Earthing it is necessary to ensure that the Eartrhing
conductors are not set in concrete and also preferably not run in metal
conduits. With the above instruction Earthing the voltage drop between
“Nominal” Earthed parts with which anyone is liable to be in
simultaneous contact should not exceed 55 volts which is quite a safe
figure for operating personnel.
For the system earthing it is common practice to earth the neutral point
either directly or through a resistance to reduce the strain on the system
insulation.
SafeClearancesfromliveparts:-
All standard equipment manufacturers ensure in their designs that within
the equipment live points are always kept with adequate clearance to the
flashover. In the case of overhead line construction clearances above
ground of the lowest point of the sagged conductor are stipulated in
chapter VIII of “Indian Electricity Rules 1956”. These are also applicable
to outdoor installations of high voltage and extra high voltage equipment
like busbars, Transformers, circuits breakers etc.
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For low voltage switchgear and control gear the degree of protection
provided by enclosures is spelt out in BIS 2147 under IP Classification
like IP54, IP55 etc. In this classification the first numeral denotes the
measure of protection offered to personnel against contactinglive or
moving parts housed within the enclosure and ingress of solid foregin
bodies. The second numeral denotes the measure damage by ingress of
foregin liquids. Depending on the requirements of any particular
installation the correct type of enclosure is specified while ordering out the
equipment. The following guidelines might be useful:-
FirstCharacteristicsNumeral SecondCharacteristicNumeral
No. Degreeof Protection No.DegreeofProtection
5 Complete Protection 4 Protection against splashing
of water from any direction.
6 Complete protection as above 5 Protection against water jets
Plus protection against ingress of dust from any direction.
Isolation:-
Any equipment installed should be capable of being “totally isolated form
the rest of the system” This is very necessary not only when the
equipment develops some fault and has to be physically removed but also
during routine maintenance when the personnel concerned can isolate the
equipment and earth it affectively before commencing on any work.
To achieve the above every transformers is provided with either circuit
breakers or Gang operated Switches fuse units on both HT & LT sides
High and Medium voltage switch gear is usually of the draw out type
double isolation feature motors usually have their starters located close by
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26. 26
or when centrally controlled from MCC’s additional local push button
control is usually provided. This list can go on.
WorkingSpaceAndVentilation :-
Every electrical equipment. When in service, carries current and is
therefore a source of heat. Manufacturers always include in their
specification the parameter of temperature rise over ambient. For
convenience and safety of the personnel it is necessary to give thought to
this problem right at the time of initial installation especially for
equipment installed indoor. Installation of exhaust fans in transformer
cubicles and switch rooms for this purpose is a common practice.
It is also necessary to ensure right at the time of intial installation that
adequate working space and access t all parts needing periodical attention
are available. That is why the clear space to be provided “around” an
indoor transformer is recommended as follows:-
Wall on one side - 0.55 Meters
Wall on two sides - 0.75 Meters
Wall on three Sides - 1.00 Meters
Wall on four sides - 1.25 Meters
Also for switchboards the “Clear Space” stipulations are that
(a) A clear space of not less than 3 feet in width shall be provided in front of
the switchboard.
(b) Space, if any, at the back of the switchboard shall be either less than9
inches ( so nobody can attempt to get there) or more than 30 inches in
width-in which case there shall be a passage way from either end of the
switchboard clear to a height of 6 feet.
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It should be noted that the above space clearances also ensure that emergency
escapes to safety are automatically available to the personnel.
FireHazards :
Every installation must take care to minimise fire bazards in the first instance and
also keep adequate facilities for fire fighting in an emergency. Some of the
common meqasures to eliminate fire risk are to provide adequate number of
“Soak pits” for oil filled in transformers and to fill cable trenches with sand
pebbles or similar noninflammable material. Also if cables run on racks in the
trenches the cables should run clear of the walls. It is of ours assumed that while
designing the cable routes, derating on account of grouping factor is considered to
ensure that cables don’t get overhead.
Electrical equipments have always been subject in some degree to the twin risks
of fire and explosion. Fortunately such cases are few and for between but
nevertheless we must be prepared for such an eventuality. For large Power
Stations, Major Substations and Factories it is customary to install smoke
detectors, Gas detectors, Heat detectors and Flame detectors with associated alarm
circuits so that advance warning of incipient conditions leading to a possible fire
is obtained. It is also customary to install fire fighting equipment like fire
extinguishers of CO2 type Dry chemical type or and to have trained personnel to
extinguish fires quickly.
INSTALLATION OF ELECTRICAL EQUIPMENTS AND THEIR
INSPECTIONBEFORE COMMISSIONING
The performance of the electrical system largely depends upon the proper
installation of individual equipments. The initial precautions and quality of the
erection work will be responsible for the trouble free service. The little efforts at
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the beginning will pay later. The electrical equipments can be classified in two
types; stationary type and rotating type. We will now consider the installation of
stationary equipments like transformers, switch gears and cables.
Before commencing erection work the electrical layout of the system is
first drawn. The layout of building is also considered. The former is incorporated
on the building layout so that utilization of space is optimum without sacrificing
the other benefits of the process control. The electrical layout should be such that
it will have proper sectionalising facility and proper descrimination in the
protective system. Another aspect of layout i.e. location of equipment will be
such that, it will ease out maintenance and will finally result into high reliability.
The location of the equipment should be such that proper clearances are
maintained from the existing structures and in between them. The dissipation of
heat generated in the equipment like transformer is also taken into account.
Appropriate space is left around the equipments taking into the
requirement of maintenance, e.g. enough space has to be kept in front of
distributions panel boards for easy replacement of fuses in fuse units checking and
testing of relays etc. for draw out type equipments like breakers, front space is
kept free. Trouble free access to these equipments is also considered with a view
of fire hazards and exit passage during emergencies.
There are certain statutory provisions. The electrical layout of the high
tension equipment has to be sent to the competent authorities like Electrical
Inspector. He studies these plans from the point of clearances, safety angles and
other statutory requirements. On getting the approval from the electrical
inspector, the erection work can be undertaken. Similarly for the works to be
carried out at public places, the permission from the local authorities like
Municipal Corporation, Municipalities and Gram Panchayats is also obtained.
The concurrence of the telephone and telegraph authorities is required for th
outdoor structures, where it may cause interference to the communication circuits.
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Before taking the actual erection work in hand, the foundation requirement
of these equipments, cable trenches, cable ducts or trays is studied and foundation
is contructed strong enough to meet these requirements. We will now discuss the
procedure followed up for the installation of individual equipments.
TRANSFORMERS
I Inspectionafterreceipts :
When the transformer is received from the manufacturer, it is immediately
inspected to find out whether there is any apparent damage and short supply of
materials as per the packing list. This is done to ensure that the damages and
shortages if any, are informed to the manufacturer as well as insurance company
in time. Following points re to be checked up :
1. Any physical damages on transformer tank, cooling fins and
transformer auxiliaries.
2. Any damage to insulators or cable boxes.
3. Any oil leakage from transformer top cover, oil drain valve tap
switches and welded joints.
4. Check all bolts because due to vibration in transit, some bolts may
get loose.
5. Any damage to oil level indicator glass.
II Loading/Unloadingtransformers :
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While loading or unloading transformers care has to taken that it is lifted
without causing harmful vibrations due to crane or any other means. It
should always be lifted by means of lugs or shackles provided for this
purpose. All lugs or shackles are used simultaneously to avoid
unbalancing while lifting the transformers. The strength of lifting
equipment like crane or lifting machine should be adequate to lift the
transformer weight.
When hydraulic jacks are used for lifting, looking mechanism should be
checked earlier. This will ensure that the hydraulic jack will not fail while
raising the transformer tank. Similarly the screw jacks of sufficient
strength should be used. These are used on arms specially provided on
transformer tank. These should not be placed below weak point like
transformer tank, radiators, drain value etc.
III ErectionofTransformers :
BIS 1986 which deals with installation and maintenance aspect, is
followed for the erection practices. Normally small distribution
transformers are despatches duly filled with oil. Accessories like
explosion vent, conservator tank are normally mounted on these
transformers. If not the auxiliary equipments together with breathers are
sent separately and these are to be assembled at site.
For power transformers, auxiliary equipments are sent separately in
packings. Following auxiliaries are assembled at site after the transformer
tank is put on rails or on suitable supports on rollers.
1. Conservator Tank.
2. Buchholz relay chamber and its pipe work.
3. Dehydrating breathers
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4. Explosion vent
5. Cable boxes
6. Temperature gauges
7. Thermodynamic joint
8. Thermodynamic joint.
IV Transformerlocationandfoundation :
It is firstly to be considered whether the transformer is to be mounted on
outdoor plinth or inside the structure. If it is outdoor installation,
ventilation aspect need not be studied, otherwise proper ventilation has to
be provided. A levelled concrete or masonary plinth of correct size is built
up. The depth of the foundation depends upon the soil, its consolidation
and weight of depends upon the soil, its consolidation and weight of the
transformer. Suitable rail or tracks are provided on which transformer is
put up at final position. Locking arrangement or masonary stoppers are
provided so that the rollers are locked up. The width of the foundation
should not be more as this will enable any human being to stand on this
foundation particularly when bushings are mounted on transformer.
The height of plinth above ground level is decided from water level
position during rainy season. Plinth height should be more than that level.
For installing transformer on pole mounted structure, the location should
be selected in such a way that it will not obstruct the vehicular traffice and
have sufficient clearances from neighbouring structures, trees, electric and
telephone poles. These should be straight and strong enough to carry
transformer load. These poles may be guided to avoid leaning of the
structure. Transformer is mounted on double channel structure. Side
clamps are provided to hold the transformer in position. The height of the
channel should be more than 8 feet from the ground level.
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V Cabletrenchesandoildrain arrangement :
Cable trenches are built up in the substation taking into consideration the
number of cables passing through the cable trench, position of equipments
like transformer, switch gear, panel boards. These trenches are filled up
with sand, soft earth or completely covered by means of non-inflammable
prefabricated reinforced concrete slabs or chequer plates. For transformer
with oil contents more than 2,000 liters, oil drain arrangement is provided
around the plinth and is connected with oil sump. This is then filled up
with pebbles. During the fire, those pebbles will reduce spreading of fire
as hot oil will drain quickly to oil sump and limit supply of oxygen.
VI For distribution of transformer, tank is always filled with oil. After
connecting auxiliaries like oil conservator, the transformer is topped with
oil confirming to BIS – 335. Transformer oil sample is taken from the
down drain valve. If dielectric strength of oil is low, the under vaccum at
the temperature of 80o
C.
For the power transformer, accessories are first fitted to the tank. The hot
oil is then pumped through the bottom drain valve by means of filter
machine. Care has to be taken that no dust or moisture have entered
through oil and oil is warmed atleast upto the ambient temperature. Use of
metal hoses is preferable. The sulphur from rubber hoses got mixed up
with hot oil reducing its dielectric strength. This sulphur may also affect
metal surface including transformer winding.
VII DryingProcess
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When the transformer is manufactured, there is penetration of moisture in
the paper insulation of the transformer coils. The removal of this moisture
is very important. The precautions have to be taken during the drying
process so that moisture from inside coil is removed completely. The hot
transformer oil (80o
C) is circulated for a long time by means of centrifugal
or vacuum filter machine. The samples of oil are initially tested and
insulation resistance of transformer winding between phase to earth is
noted. Initially insulation resistance is always high. As hot oil is
circulated more and more, moisture from the coils oozes out reducing
insulation resistance tremendously. After few circulations of hot oil,
insulation resistance improves fast and finally remains steady. Oil
samples are also tested periodically.
Normally transformer tank and cooling fins are filtered separately after
closing transformer main valve in case of transformer with separate and
radiator tanks. This accelerates drying process.
If the insulation resistance value does not got built up, variable low
voltage supply is applied on the primary side of the transformer. The
secondary side is short-circuited through a conductor of adequate size
suitable for taking full load current of the transformer. The primary
voltage is adjusted in such a way that transformer secondary current is
close to full load current. Glass wool packings are also used to prevent
radiation through cooling fins and tank. Care has to be taken to that the
transformer oil temperature is not excessive.
A close watch has to be kept on flow and temperature of oil during the
drying process. If it is not kept properly, transformer may get excessively
heated damaging insulation and if oil flow is more, it will damage air vent
diaphragm. After the drying process is complete, oil valves on
transformer tanks feeding the headers are opened. Otherwise some fine
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will not have free circulation of oil finally resulting to higher temperature
of oil in service. Transformer is then topped up to the pre-determined oil
level. If the oil is more, it may overflow in the service, if it is low it will
give also minimum oil level alarm.
VIII Cablesconnections :
Cables are laid in the cable trenches as per the code of practice i.e. as per
BIS-255. Before connecting these to the transformer, their insulation
resistance has to be checked. On ascertaining their healthiness, they are
connected with the transformer. Cable termination can be made up both
bituminous compound method or by using heat shrinkable or epoxy joints.
Heat shrinkable method is becoming more and more popular due to its
ease and time conservation and requirement of less skill. Bituminous
boxes are filled up with hot compound (with 80o
C). It is poured slowly to
avoid the formation of air pockets and then it is topped up. If the
transformer is installed outdoor, the metal sheet hoods are provided in the
cable boxes. For the transformer with bushings, jumpers with crimpped
lugs are provided. They are adequate tapped by PVC or cambric tape. At
joints plastic compound (or toughest B) is used. Jumpers are also taped up
suitably with PVC sleeve.
IX Cablesconnection :
When the transformer is operated in parallel with the existing network,
phasing has to be checked up so that it will run successfully in parallel
with existing network.
X Breatherconnection :
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Breather is connected to the conservator with a pipe work. This pipe
should be suitably clamped in between to prevent vibrations. Breather is
then filled up with the oil upto a suitably marked level. Seal provided
during the transportation has to be removed for free entry of air. Silica gel
colour should be blue, if not it is reactivated in hot air chambers.
XI Precommissioning tests
Transformers are tested at the manufacturer work and test certificates are
provided with ratio, polarity phase relationship insulation resistance,
copper and iron losses magnetising current etc.
Transformer polarities are checked to enable it to run in parallel with
existing net work. Transformer protection schemes including buchholz
relay, transformers oil and winding temperature indicators thermal relay
connections differential and restricted earth fault connections are tested
before energising the transformer.
SWITCHGEARS
On receipt of switchgear packages, these should be inspected to ascertain
damages following points are checked while inspecting the switchgear
packages:-
1. Damages to the cubicle.
2. Damages to the insulators.
3. Damages to operating mechanism.
4. Damages to relays, terminal boxes etc
Both the shortages noticed in comparison with the packing list and damages
are informed to the manufacturer and transist insurance company.
I. Preparationoffoundation:
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Indoor type switchgears are normally erected on finished and well-cured
floor. The floor should be reasonably leveled. (Inclination should not be
more than 1 in 1000). As per foundation drawings provided by the
supplier, grouting holes are marked drilled in the floor. The depth of such
grounding holes should be around 4 to 5 inches. The switchgear position
is decided by required for making cable boxes and drawing out circuit
breaker tank.
II. Installation ofhousing:
Circuit breaker tank is removed form the cubical and then cubical is put
into position. If the leveling of the floor is not proper the first cubicle
should be installed at highest level point and other cubicles are then
aligned. If erection is started from one end, the leveling will be improper
and it will be difficult to raise circuit breaker tank. Metal shims are used
below these cubicles. Housing should be plumed absolutely upright. A
cubicle base is set squarely. Runner rails are leveled and grouted parallel
to each other. After putting the first subicle in position it is wedged and
then second cubicle is positioned next to it. Bus bars are connected
temporarily to ensure the correct height. Care has to be taken that the first
cubicle, which is leveled and positioned, should not get disturbed. After
positioning the cubicle foundation, bolts are provided and quick setting
cement m8ixture or lead can be used for grouting. Cement mixture should
be in proportion 1:3 with stand.
While connecting bus care has to be taken that it should not cause any
strain on insulators or connecting studs. Cover the bus bar joints with
plastic stage. Compound is put on joints evenly to avoid air voids. These
joints are then tapped with PVC and cambric tape. If polythene covers are
available, these are provided. After setting of cement, foundation bolts are
tightened with nut.
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III. each equipment is subjected to insulation that with 1000 volt or 2500 volt
megger. If the value of the insulation resistance is unequal in phases and
lower than 200 megohms between phases and phase to earth, the
equipment should be dried up in a warm atmosphere until insulation value
is improved and remains steady it is recommended that equipment be
placed in a dry and ventilated room for at least 48 hours if the insulation
resistance value are on lower side. The temperature of room should be
around 500. During this process bolted and hinged covers are removed.
All insulators are cleaned up and dried.
Phase barriers are removed from the tank after removing tank cover.
The tank is then cleaned and washed with oil. Care has to be taken that
tint free cloth should be used for cleaning and not cotton waste. Wedges
are removed. Fit the turbulators and phase bar iers. Fill the tank to correct
level with clean and moisture free oil confirming to BIS –335. Tank
gasets are inspected before bolting top cover. Operating mechanism is
checked cleaned and lubricated . Singer oil should be used for the
mechanism. If oil is thick, dust may accumulate on it forming a compact
mass.
Breaker should be checked for raising and lowering operation. Tripping
and closing fun ction should also be checked before breaker tank is
inserted in the cubicle. Interlocking arrangement and shutter
arrangement is also checked . If shutter arrangement is not proper, the
tank insulators may get damaged while raising the breaker.
IV. Mounting VoltageTransformers:
The carriage type voltage transformer is mounted on guide rails on the top
of the current transformer and busher compartments of the cubicile. It is
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inserted or removed by means by means of two handles. Voltage
transformers tank is cleaned and filled up with tested oil confirming to BIS
– 335. High repturing capacity fuses are provided on primary side while
on secondary side rewirable fuses are provide. Shutters are actuated by the
voltage transformer carriage when moving it into or out of service
position. Facilities are provided for padlocking the carriage under service
or isolated position and for shutters in closed.
V. Mounting instrumentpanels :
Instrument and panels generally some as separated consignment. These are
mounted on the cubicles they are grouted by means of suitable bolting
arrangement.
As per the requirement control table are made and connected .Indicating
motars like ammeter , voltmeter, energy meter and productive relays are
mounted on these panels the control wiring is carried out as per wiring
diagram. Suitable ferrules with number are used for proper identification.
VI Cableconnection :
Different methods of cable connection can be used. Normal method is to
used bituminous compound in cable boxes. Use of epoxy, heat shrinkable
or plastic type moulded jointes is now increasing. While pouring
bituminous compound temperature of compound should be around 80
deg.c. care has to be taken while pouring the compound . It should poured
slowly to avoid air trapping and it is subsequently topped up. While
connecting cable phasing of new cable with existing cable is checked up
and similar phasing is maintained.
VII Commissioning Protection system :
Protective system is decided on the basis of the duty circuit breaker has to
perform. Say for controlling cables , O.H. line, transformer , capacitors
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etc. Control wiring is check as per wiring diagram . Relays function is
checked by use of secondary injection method. Before commissioning the
operation, of the system is checked .
VIII Highvoltagetest:
The switch gear is subjected to high voltage test at power frequency with
alternating current for one minute. Care has to be taken that voltage
transformers are either racked out or H.T. fuses provided in the voltage
transformers are removed. The A.C. test voltage applied are (twice the
rated voltage plus 2kv ) as follows.
Ratedvoltage TestVoltage
33kv 68kv
22kv 46kv
11kv 24kv
6.6kv 15.2kv
Sometimes the high voltage A.C. test is not carried particularly after connecting
cable in such instances, it is permissible to test the equipment with direct current
voltage. Direct current test are applied for 15 minute duration. The voltage to
earth should not exceed the following:
6.6kv switchgear 10.5kv
11kv 18kv
22kv 37kv
CABLES
Inspection : On receipt of cable drums , these are inspected immediately to
ascertain the damages during the transit. If the drums are in physically damage
condition, there are open an inspected to check whether cable on drum is damage.
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End cables / seals are check in case of PILC cables. If they are not in tact ,
moisture penetration is to be checked before they are resealed . Some times cable
get damage due to nails. This piece is cut an removed. The information and extent
of damage is informed to the manufacturer and insurance company.
All cables are generally install according to code of practice for cables (BIS –
1255)
I. SelectionRoute :
The selection of the route is very important following point are the to be
considered:
1. Shortest length of cable
2. Route should cover some vergin area
3. The road in reinstatement cost
4. The route should be such that there should not be frequent road openings by other
5. Future load growth in the area.
6. After taking trial pits, position of other utilities are checked and route is decided
accordingly.
In the case of industries, the length of the route is limited and therefore the cable
route should be chosen in such a way that it will be easily accessible eg. the in
such route should be with shortest length and it should be away from chemicals,
hot surface across etc.
II. Making ofcabletrench :
The cables are laid in trenches at not less than the following depth:-
Upto 1.1 K.V - 0.45 metre plus rad. Of cable
Upto 11 K.V. - 0.75 metre plus rad. Of cable
Upto 33 K.V. - 1.0 metre Plus rad. Of cable
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The trench should be wide enough so that person can stand in it and guide the
cable when cable laying process is in progress. The trench should be in line and
clear so that stones or anyhard material should not protrude out causing damage to
cable . Soft bodding of sand or earth is provided before laying cable.
III Pullingofcable:-
Cable rollers are provided throughout the cable trench. Cable is then pulled by
connecting cable sock or cable grip at one and pulled in its direction . All
mazdoors should apply pull or push simultaneously. If there is not proper
synchronism in the above activity, cable may get twisted or kink also can be
formed. Mechanical winch can also be used for pulling. The advantage of winch
is that the pull remains constant. Normally cable coil is kept near the cable end
terminations.
After laying of cable , layer of soft earth or sand is provided on it. The thickness
of this layer should be around 3”. Top cover either of R.C.C. tiles or rod bricks is
provided on it. The trench is then filled with balance road material. Care has to
be taken that big stones should be used for refilling purposes.
IV CableJointsandmakers:
Joint makers are provided at the location. Sometimes joints are safeguarded
with masonary work filled with sand when road traffic is heavey .
Joint position is chosen in such a way that it is away from cable bend, road
junction bus stop fire hydrant etc. The pit should be wide enough to
accommodate jointer gang. The depth of trench small pit is made and water is
diverted towards it. It is then pumped out. Tents are provided at joint locations.
This will avoid entry of dust in joint. In soldered joint, flux is used which is
having fluorine compound . It removes the oxide film from exposed strands of
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conductors. Solder temperature is maintained about 1000
C. First conductor is
cleaned and then conductor is untwisted and heated upon 1100
C and flux is then
applied. The bested ferrule is then put on the conductor. The ferrule is normally
weak back. The holder is then poured at 3200
c.
It is observation that there should be compatibility between flux and solder
otherwise the joint remains weak and gives at full load.
Now days crimping of ferrules is done by Hydraulic or Hand operated crimping
tools. The crimping is done from the center of ferrule towards the ferrule edge.
Welding method is also used for jointing conductor.
Heat shrinkable or epoxy joints are now days widely used both for termination
and straight joints.
V CableTesting:-
Cable is after laying and jointing. For PILC cable (DC voltage is applied for 15
minutes. AC voltage for one minute for earthed systems.)
Voltage Belted Cables
------------------------------------------------------------------------------------------------------------
AC Voltage DC Voltage Between Between
Between
conductor
between
conductor
conductor
and sheath
conductor
and sheath
(AC Voltage) DC Voltage
------------------------------------------------------------------------------------------------------------
1100 2000 3000 2000 3000
6600 12000 18000 7000 10500
11000 20000 30000 11500 17000
22 KV 40000 60000 23000 35000
------------------------------------------------------------------------------------------------------------
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For Unearthed System
3300 6000 9000 6000 9000
6600 12000 18000 12000 18000
11000 20000 3000 20000 30000
-----------------------------------------------------------------------------------------------------------
ELECTRICAL HAZARDS
IN
POWER GENERATION AND DISTRIBUTION
1.0 INTRODUCTION:
As a source of power, electricity should be handled very carefully as
compared to other sources like steam or fuel as electricity is not visible.
Failure to take suitable precautions in its use, creates conditions which are
certain to result in body harm or equipment damage or both. However,
accident statistics show that if electricity is handled with precautions, the
probability of accident is much lower compared to other sources. In spite of
control of hazards, there are injuries and fatalities due to electrical shock.
Control of almost all the electical hazards is neither difficult nor expensive but
ignoring or neglecting them normally leads to serious accidents.
2.0 ELECTRICAL INJURIES:-
Current flow is the factor that causes injury in electrical shocks i.e. the
severity of electrical shock is determined by the amount of current flow
through the victim. In general an alternating current of 100 milli amperes at
the supply frequency of (50 Hz) may be fatal if it passes through the vital
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organs. Also it is estimated that a value of 16 mill amperes is the average
current at which an individual can still release himself from an object held by
the hand. Because current flow depends on voltage and resistance, these
factors are important. Other factors affecting the amount of damages done are
the parts of the body involved, the durations of current flow through the
victim and the frequency of power supply (in case of alternating current).
Resistance to current flow is mainly to be found in the skin surface. Dry skin
has a fairly high resistance but a sharp decrease in resistance takes place when
the skin is mosit.
Once the skin resistance is broken down, the current flows easily through
blood and body tissues. High voltage alternating current of 50 Hz causes
violent muscular contraction, often so severe that the victim is thrown clear of
the circuit. Although low voltage also results in muscular contraction, the
voltage often prevents the victim from freeing himself from the circuit makes
exposure to it dangerous.
6.1 Death or injury by electric shock may result from the following.
a) Contraction of the chest muscles which may interfere with breathing to
such an extent that death will result from asphyxiation when the
exposure is prolonged.
b) Temporary paralysis of the nerve center which may result in failure of
respi, a condition which often continuous until long duration after the
victim is freed from the circuit.
c) Interferance with normal rhythm of the heart causing ventricular
fibrillation. In this condition the fibers of the heart muscles instead of
contracting in a co-ordinated manner, contract separately and at
different times. Blood circulation ceases and death ensures; since
apparently the heart spontaneously recover from this condition. It has
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been estimated that 100 milli amperes is sufficient to cause ventricular
fibrillation.
d) Suspension of heart action by muscular contraction. In this case the
heart may resume its normal rhythm when the victim is heat, as heavy
current flows through the body.
e) Hemorrhages and destruction of tissues, nerves and muscles due to
heat, as heavy current flows through the body.
2.2 In general the longer the current flows through the body the more serious
may be the result. Considerable current is likely to flow from high voltage
sources and in general, only very short exposure can be tolerated if the
victim is to be revived.
Injuries from electric shock are less severe when the current does not pass
through or near nurve center and vital organs. But in majority of electrical
accidents, the current and the lungs resulting in serious effects.
Another type of injury is burns form electric flashes. Such burns are
usually deep and slow to heal and may involve large areas of the body.
Even victims at relatively good distance from the arc, may receive eye
burns.
With high voltage, flashes of explosive, violence may result. This instant
arcing is caused by short circuits between bus bars, opening isolators or
links carrying a heavy load current inadvertently, etc.
2.3 In many cases the victims may be saved by prompt application of artificial
respiration, since a common effect electrical accidents is failure of that
part of the nervous system which controls breathing. Artificial respiration
should be immediately applied to a victim of electic shock and should be
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continued until the revives or until qualified doctor arrives at the accident
site.
3.0 SAFETYPRECAUTIONSANDPRACTICES:
3.1 The organizations should take care of the following safety aspects in
planning
a) To integrate safety practices completely with production and operation
requirements.
b) To provide safe working conditions, proper and adequate tools equipments
and protective devices.
c) To see that persons are at all times kept familiar with the general and
special practices for the safe conduct during their work.
d) To see that these safety measures and recommendations which apply are
in all cases carried out by the persons meticulously.
3.2 FUNDAMENTALSOFSAFETY:
a) Prevention of accidents requires the whole-hearted co-operation of all
members of the organisation. A capable, mentally alert employees will
avoid accidents at all times.
b) An unsafe man is a liability. He is a danger to himself his follow workers
the equipment the public and the organisation as a whole.
c) Accidents do not “Just Happen”. Accidents include the following:
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i) Operating an equipments without authority or warning.
ii) Operating or working without proper instructions.
iii) Making safety devices in operative.
iv) Using defective equipments or using equipments improperly.
v) Working nearby dangerous or live electrical equipments that could
conveniently be de–energised.
Unsafe conditions which may cause accidents include the following:
i) Unguarded equipments.
ii) Defective material or equipment.
iii) Improper illumination.
iv) Non standard design and construction.
3.3 GENERAL SAFETY PRECAUTION:
All voltage levels, even low volts shall be considered dangerious even though
voltage may not be considered dangerous even though voltage may not be
high enough to produce serious shock.
All electrical circuits are to be treated live and no work (maintenance, repair,
overhaul, cleaning testing etc.) is to be carried out on any part of electrical
apparatus or circuit unless such parts are.
a) De-energised and and confirmed .
b) Isolated and all practical steps taken to back off from live conductors and
energy sources.
c) Effectively connected to earth at all points of disconnection of supply such
apparatus or between such points and points of work.
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Maintenance, repair and construction work on electrical apparatus shall not be
done until after making sure that working conditions are safe and obtaining proper
authorization for doing the work.
3.4 HOTLINE WORK:
Hot line work is done with the help of special tools and with special precautions.
The type of work includes re-placement of insulators, making or breaking hot lines
jumpers, hot line washing of insulators, etc. Hot line tools should be kept dry and
clean. No hot line work should be carried out when weather conditions would
result insurface wetting (humidity above 60%). Hot line washing should also be
avoided during inclement weather condition.
While working on equipments adjacent to live equipments, the following minimum
safe clearances must be maintained:
VoltageRange Minimum SafeDistances
750 - 3,500 Volts 0.3 Meters
3,501 - 10,000 “ 0.6 “
10,001 - 50,000 “ 1.0 “
50,001 - 100,000 “ 1.75 “
100,001 - 250,000 “ 3.5 “
4.0 ELECTRICAL EQUIPMENTS:
4.1 SELECTION:
Most items of electrical equipments are designed and built for specific types of
services. They will operate with maximum efficiency and safety only when used
for the purposes and under conditions for which they are intended. In selection of
equipment, it is advisable to follow the recommendations of the engineering
consultant services, manufacturers, various codes and standards etc.
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4.2 INSTALLATION
Group operated disconnectors, circuit breakers, current transformers, potential
transformers, power communication station transformers, power line carrier
communication equipments, rectifier, motor generators sets, capacitors,
condensers, generators, bus bars, cable dividing boxes and other electrical
equipments should be so installed that the possibility of accidental contact with
energised parts is totally avoided. When an interlock is used as a safety device, it
should be fail-safe i.e. steps must be taken to ensure that failure of this device will
not jeopardise the safety of personnel doing the operations.
Inter locks selected should meet the following standards:-
a) Fail –safe features i.e. failure or mal-function of the interlock mechanism
will inhibit the normal operation.
b) Interlocks should be tamper proof.
Where space and operating requirements permit, electrical equipments should be
placed in the less congested areas of the plant or, where practicable in the place to
which only authorised and competent persons who have full knowledge about the
operating instructions of the equipments and the plants have access.
If the current carrying parts must be exposed, they should be made inaccessible by
elevating them at least 3 meters above the work area of enclosures, barriers of
guards should be provided to prevent contact.
In addition, warning sings should be displayed near exposed current-carrying
party and in especially hazardous areas, such as high voltage installations. These
signs should be large enough to attract attention and should be visible from all
approaches to the dangerous zone.
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4.3 Periodical inspection and testing of electrical equipments and competent
supervision of supervision of maintenance people are extremely important. Often
during the periodical checks, maintenance people can spot hazards before they
cause injuries.
The features and precautions to be taken for major equipment in power plant/
electrical installation are covered in subsequent section.
5.0 MAJORELECTRICALEQUIPMENT :
5.1 DISCONNECTS :
These are meant for isolating circuits. There are two types of disconccectors,
Single Pole Disconnectors and Group operated Disconnectors.
a) SINGLE POLEDISCONNECTS :
They are used as devices for isolating currentless circuits. They can only be used
to isolate a dead circuit or an equipment from a voltage source after the main
current is interrupted by a circuit breaker. They should never be used for
charging or discharging overhead lines, cables, transformers, capacitors etc.
While opening single pole disconnectors , it is advisable to open the
disconnecting blade a little about an inch or so, to see if there is any arcing. If
there is any appreciable arcing, after the blade has moved by an inch or so from
the jaw, then the blade should be immediately closed back. The disconnect blades
should not be subjected to any arcing.
b) GROUPOPERATED DISCONNECTS (GOD):
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GOD are primarily meant for isolating of some equipments. They are not
normally designed to make or break load currents. The operating is done by
circuit breaker. If attempts are made to interrupt load current besides severe
arcing, this can create a phase to phase or phase to ground faulty, thus resulting in
hazardous conditions to the equipment and the operator. Similar hazard can also
occur while closing a GOD on load.
Also certain types of special GODs having features like arcing horns, positive
closing acting etc. are designed for making or breaking charging currents of small
transformers, and small lengths of overhead lines and cables. GODs are never
used for test charging a faulty of inspected faulty equipments. Such test charging
should be done by the circuit breakers with adequate protection. GODs provided
for generators and synchronous condensers and other rotating machines should
never be opened when connected to any voltage source even when the machine is
carrying no load. Such an operating may develop severe arcing and power
oscillations. In such a case the voltage should be made zero.
5.2 CIRCUIT BREAKER :
They are used in high voltage circuits with large current capacities. Their use has
become more common. They are available in variety and sizes. They
may be instantaneous in their operating for equipped with time relay
protection schemes. They may be operated manually or remotely. They
are used for taking rated load current and breaking them. They are also
used for interrupting fault current without any distress. Also used for test
charging faulty suspected faulty equipments. Considering its above
functions, no underated circuit breaker rupturing capacity is less then the
fault level of an installations the circuit breaker may be alright for normal
load current but will be inadequate for clearing the faults. However, the
operation of the circuit breaker for test charging should conform to the
designed duty cycle of the breaker and total number of such test charging
will be governed by the preventive maintenance requirements of the
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circuit breaker. Also no circuit breaker should be operated beyond
stipulated operating duty.
5.3 POWERTRANSFORMER:
They are used for either stepping up or stepping down the voltage level. In
generating stations they are used for stepping up the voltage and at receiving
stations for stepping down the voltage. Normally transformer is a very sturdy
equipment. If it is operated as per its rating and within voltage, frequency and
temperature limits, there is no hazard on it. If a transformer trips out especially
by the action of Buchholtz Relay, it should never be test charged unless it is
proved healthy by carrying out the necessary tests. Charging a faulty transformer,
may aggravate the fault and may lead too intense damage beyond repair and also
cause fire. While isolating transformers are connected, such potential transformers
shall be isolated and low tension fuses to be removed, to prevent any possibility
of transformer being made live through synchronizing or volt-meter plug. Before
commencing any work on a transformer, the transformer winding should be
discharged to ground. Neutral ground of a transformer should never be accepted
in place of the grounding of phase terminals before commencement of work.
5.4 INSTRUMENTTRANSFORMERS
The enclosure of all instrument transformers should be effectively grounded.
Current transformer secondaries should never be open circuited when current is
flowing in the primary. Potential transformer secondaries should never be
shorted.
5.5 SHUNTCAPACITORS :
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For the power factor improvement, shunt capacitors are used in a almost every
efficiently operated distribution system. When switching bank into a network, the
capacitor acts like a short circuit at the instant of switching in. Current surges are
specially high, if a capacitor is connected in parallel with one or more capacitors,
which are already in the circuit. To avoid this camping reactors are provided.
Discharging of the residual charge, for taking out the capacitor bank for work.,
discharge potential transformers (P.T.) are used. In order to find any fault in any
of the links of the star connected capacitor banks. As a safety measure and
precaution nobody should go near the capacitors unless ten minutes have elapsed
on the safe side after cutting out of the capacitor bank. though with discharge P T
in circuit, it takes about 2 to3 cycles to completely discharge the capacitor banks.
Further, using hot stock, ground individual terminals of the capacitor to discharge
it completely. The short circuiting grounding pumpers should be left attached
while work is being done on the capacitor. In all modern capacitors chlorinated
Aromatic Hydrocarbon Impregnating Compound known as ASKARELS are used
as the dielectric, which is very toxic. They are harmful if taken internally or if in
prolonged contact with skin. Hence such compounds should be handled with great
care and recommendations given by the manufacturer should be strictly adhered
to.
5.6 GENERATORS/EXCITERS/SYNCHRONOUSCONDENSERS :
Excessive overloading for longer periods for machine is to be avoided as it is vital
and expensive equipment a station, especially if it is of very much high rating.
The following are the common problems encountered on machine.
Dust, moisture overload etc. Though modern machine are totally enclosed and
hydrogen cooled, many are open and dust settles, on the winding, slip rings,
commentators, etc. On winding dust acts as a layer of insulation that confines the
heat until it may a dangerous point. Dust also plugs ventilation spaces and further
interferes with the cooling. On sleep ring and commutators, dust acts both as an
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abrasive and as an insulators which increases wear and blocks full passage of
current. Not all electrical insulation’s acts as a perfect barrier to moisture. Some
become porous with age and absorb moisture and resulting in short circuit, fire or
complete breakdown. If the machine is out for overhaul and the resistance is
found to be at a dangerous point. It should dried out first before taking into
service. If the machine is over loaded, i.e. current exceeds its rated value, heating
may increase as much as the square of the current increases and insulation may be
damaged, soldered connections melted, bearing burnt out etc.
5.7 INSULATORS :
Insulators are widely used in overhead transmission network and at the power
station for supporting bus bars and conductors. They are also used on circuit
breakers. Disconnects, transformers and many other electrical high-tension
equipments for insulating purposes. Failure of insulators is very common
especially in dusty and polluted atmosphere. Dust with moisture forms a
conducting medium and then flash over takes place, which results in equipment
going out of service. In order to avoid this, insulators should be periodically
cleaned, by taking the outage of requirements. If the equipment outage is very
difficult then hot line washing with hot line tools should be carried out with most
care.
6.0 DIAGNOSTIC TESTING :
Most electrical equipments are designed for safe operation under limited overload
conditions for specified periods. Operators should be thoroughly familiar with the
limitations of their equipments and should be trained to observe and report
abnormal conditions. Continued overload may introduce additional operating
hazards by causing short circuits, fire, circuit failure, and machine failure.
Trough various diagnostic testing of electrical equipments many of these
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conditions can be detected before they go out of control and cause damage,. One
of the most important functions of diagnostic testing is the detection of incipient
faults, location of their faults and gauging their severity. Proper measures can
then be initiated before the fault develops resulting into a failure and in hazardous
condition.
7.0 INSPECTION ANDMAINTENANCE :
Equipment should be de-energised by the operator. It should always be assumed
that equipment is live until it is proved dead. Therefore, disconnected switches
shall be checked to see that they are open and isolated. Tests to be done to
determine that the part of the equipment to be worked on are dead. As an
additional safeguard, disconnects switches should be locked open and tagged so
that they cannot be energised even inadvertently.
Before doing any work on dead lines or equipments, where there is a possibility
of their becoming energised should be safeguarded and grounded between the
location of work and all possible sources of energy. Temporary grounding cables
shall be flexible stranded copper not less than 0.1 sq. in. and shall be equipped
with proper clamps at each end. Use of chains and defective grounding device
should be avoided. When grounding lines or equipment, the connection to the
ground shall be made first and that to the lines or equipment last in removing
ground, first remove the connection to the lines or equipment and then remove the
ground connection. Insulated hot sticks should be used in making the ground
connection to the lines or equipments. Grounds shall be placed on all phases even
if work is to be carried out on the phase only. Persons should not work in
cramped places, unless space or outlet should be available for freedom of
movement in case of emergency. Especially while working on the circuit breaker,
bus bars side and feeder side live portion should be covered with insulated barrier
as far as possible.
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In case of generators driven by prime movers, the control or starting valves, or
other means of controlling the energy to the driving part of the unit should be
locked and tagged. While retating machines are in operation, no attempts should
be made to clean or polish step rings and commutators. Whenever persons are
working on an equipment, no hi-pot test, current injection test or any other tests
should be permitted which is likely to inject or induce high voltage or dangerous
electric charge. Contact for testing insulation resistance should be made
whenever possible with an exposed conductor in the circuit to be tested.
Punctuating of insulation is not recommended. If it is done, then the place
punctured should be taped or reinsulated. As an additional safety insulation test
should be made of the taped portion. Pliers, screwdrivers, testing lights and other
tools used in electrical maintenance work should be insulated.
8.0 GROUNDING
It is necessary to provide earthing in the Generating/Receiving Sub-station for
safety of personnel and equipments. Earthing is making a connection to the
general mass of earth. Earthing has the following functions :-
A) Providing the ground connection for grounded neutral systems.
B) Providing the discharges path for lightning arrestors.
C) Ensuring that non-current carrying parts, such as equipment frames, are
always safely at ground potential even though insulation fails.
D) Providing the means of discharging/de-energising equipment before
releasing it for maintenance.
8.1 The following are the equipments, which are to be grounded for safety :-
A) The neutral point of equipment is earthed to the station-grounding grid
either directly or through reactor or through grounding transformers.
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B) Apparatus, framework and other non-current carrying metal work
associated with power system e.g. generators, transformers, circuit
breakers, control panel frame etc. are grounded directly.
C) Metal framework not associated with the power system e.g. Boundary
fence, steel structure, sheaths of communication cables etc. are grounded
directly. This is required to provide ground path for any induction,
leakage, direct lightning flashover etc.
D) Lightning arrestor shall have independent ground free of points, so that
high discharge current is passed to earth.
9.0 FACTORS WHICH CAUSE DETERIORATION OF ELECTRICAL
EQUIPMENT,WHICHIS,HAZARDOUS :-
9.1 ELECTRICAL
Discharge is voids leading to erosion, normal and transient over voltages,
switching and lightning surges, over-fluxing, under excitation, over/under voltage
operations.
9.2 MECHANICAL :
Vibration and differential expansion leading to insulation damage, external short
circuits and through faults, switching and on-line starting stresses.
9.3 THERMAL
Overloading, thermal cycles, degradation of insulating oil, insulating mica/resin
bonds, causing de-laminated or embattlement.
9.4 CHEMICAL:
Oxidation of organic insulation, electro-chemical deterioration.
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9.5 ENVIRONMENT:
Pollution, humidity, moisture ingress, high ambient temperature,
chemical/corrosive atmosphere.
10.0 PREVENTATIONOFELECTRICAL HAZARDS:
10.1 PLANTSEFTYRULES:
They must be practical, based on sound judgement and experience and kept
up-to-date. New problems appear as often as new techniques are developed.
We must revise safety-rules in according with the developments/changes in
the plant.
More often victims of accidents are workers who have not observed the
safety rules. It is for the employers to provide for safety equipment, rules
and method of work and for the employees to use that equipment correctly,
to follow the rules strictly and to adopt the safe methods.
10.2 CERTIFICATIONOFPERSONAL :
The section of workers for electrical work is highly important. Their training
and their attitude towards the responsibility involved are no less important.
As in any hazardous job, method employed to work on electrical equipment
must be of a type that will conform to rules and time tested work practices.
10.3 OPERATING INSPECTIONFOR “LIVE LINE” AND“DEADLINE”
WORKING
Specific work rules and instructions must be provided. Appropriated tools
and protective equipment must be made available to the workers and be
used by them. Men who are working on live line must take all requisite
precautions as specified in the areas of work on the live lines must be
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covered to protect the workmen from the hazards. The practice of working
on live installations has some advantages. In particular, it renders
unnecessary the excessive haste with repairs following a shut down are
frequently undertaken, while ensuring that service is uninterrupted. As a
rule, such live line work should not be undertaken in inclement weather.
When work is to be done on dead lines, appropriate tests must be made to
ensure that the line is actually dead and the line must be also be earthen . The
protective earth points must be situated so as to be visible from the working
area.
10.4 OPERATION OFELECTRICALINSTALLATIONANDLOCKING
PROCEDURES:
When circuits are taken out of service, it is imperative that effective steps be
taken to insure that effective steps be taken to ensure that the line or
equipment cannot be accidentally re-energised. Visible break in the circuit
must be maintained and it must be tagged to prevent accidental reclosure. The
safety rules must be strictly observed and no violation may be permitted
under any circumstances.
SAFETYFEATURE
FOR
ELECTRICALCONTROLANDSWITCHGEAR
With the rapid increase in demand of electrical energy in our country there has
also been an increase in the requirement for switchgear and allied control
equipment. With the inflow of sophisticated technology and introduction of new
systems and features, the aspect of safety has also assumed greatly importance. It
is disheartening to note that inspite of growing awareness on part of all concerned
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and various efforts by institutions like the National Safety Council, deaths due to
electrical faults and damage to costly equipment are not common in our country.
Achieving the maximum possible safety must form a basic and fundamental
aspect of the professional responsibility of electrical engineers and technical
personnel employed in all fields of industry.
Since the subject of safety in the area of electrical controls and switchgear is vast
one it will only be possible to cover certain relevant topics during the course of
this talk.
The principal aspects are:
Cabling:
Care should be taken to properly lay cables without sharp bends to avoid undue
stress on the cables, which could lead to cable insulation failure. The cable sizes
should be adequately chosen taking into consideration derating actual site
conditions. Armoured cables are to be used when mechanical strength is
necessary. Proper cabling is vital for plant safety as very often accidents are
caused by improper cabling.
SwitchboardConstruction:
Incorporation of safety features in Switchboard is very much dependent upon the
skill and experience of the operating and maintenance personnel. In low voltage
and medium voltage switchboard panels there are two types of executions in
practice: a) draw and out b) non draw out. Whereas sub-distribution, lightning
distribution are generally non-draw out one has the copies of both for the main
distribution system. Draw out systems for circuit breakers, switches and motor
feeders as in low voltage MCCs and for circuit breakers in medium voltage
switchgear panels ensure maintenance and repair in the isolated position of the
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equipment. Such systems have features like door interlocking which prevent
opening the enclosure door when the main switch is “on” as well as distract test,
service and isolated positions. Reputed manufacturers provide threading of
incoming live parts as well as shrouding of outgoing cable connections to prevent
inadvertent touching of live parts. It is often noticed that even in
compartmentalized systems such as MCCs even if a particular feeder is isolated
during the cabling in the common cable chamber, there is the danger of tools
dropping and shorting terminals of other live feeders. Care should be taken to
prevent this by shrouding live terminals suitably.
WiringofSwitchgearandControl gearinPanels :
Proper wiring is an important aspect of the safety of electrical installations.
Power wiring should be done with different colours to distinguish different
phases. Since this is not feasible in modern factories, single coloured wires are
generally used and coloured sleeves are used at the ends for distinction. As per
BIS 5578 the dependent local and making system should be used for switchboard
wiring of control/auxiliary circuits. This is a system of marking, where the
marking of a conductor and is the same as that of the terminal to which it is
connected. The marking is done with printed ferrules provided at termination
points. Ferruling ensures that wires are reconnected correctly after equipment is
removed and replaced. This prevents short circuits and failures due to wrong
connection.
Proper lungs (pin type, ring type or tongue type) and washers (spring, curved, flat
etc.) are to be used for various connections depending upon the equipment
terminals. Flexible wire connections between fixed and movable parts like doors
etc of panel boards should be done through flexible PVC sleeving or flexible PVC
conduits. Both ends of the flexible connection should be clamped properly with
metal or hard plastic clamps. Also when inter-panel wiring is carried out care
should be taken to use rubber grommets or backline bushings to prevent abrasion
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of wires with sharp edges. Incoming wires from the main bus to feeders are not
sufficiently protected and hence should not be bunched, together. As far as
possible wires should not be bent at right angles but should be curved. This will
prevent damage to insulation and conductors. Wiring should be neatly carried out
and unnecessary rounding and shaping should be avoided to prevent unnecessary
heating. The termination of wires should not cause undue stress at equipment
terminals.
BusbarJointing:
Bus bar joints used in switchgear boards are i) Aluminium to aluminium; ii)
Copper to Copper/silver plated copper; iii) Aluminium to copper or silver plated
copper. One should check whether any manufacturing defects like bur, bulging
etc. exist on the contact surface. The contact surfaces should then be prepared by
brushing and greasing then the jointing carried out immediately thereafter. For
the tightening of the connections Belleville washers and high tensile bolts/nuts
(BIS 1367 Class B) should be used. For smaller sections plain and spring washers
with normal bolts should be used. Since a torque of 55mm is required ring
spanners are necessary and with flat spanners a maximum torque of only about
35mm can be achieved. In case of aluminium to silver plated copper joints only
the aluminium surface should be brushed and a cupal sheet has to be inserted
between the contact surface.
It is essential that bus bar joints are done properly as incorrect jointing leads to
greater contact resistance due to which heating and flash overs can take place.
Maintenance andOperation
As mentioned earlier most accidents originate from improper handling during
operation and maintenance. Operation and Maintenance should be carried out
only by competent personnel and very great care is to be exercised when carrying
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out work on sections of boards, which are live. Main switches are to be
padlocked in the off position when work is being carried out and switched on only
after all work is completed. Switchboard doors are to be kept closed during
normal course. This will not only prevent entry of dust and vermin but also
possibility of unauthorized approach to live parts.
CrepageandClearances:
It is needless to say that during the planning and design of electrical installations
all factors of safety stipulated as per the Indian electricity rules, Indian standards
etc. have to be adhered to. For out door switchyards and substations, proper
planning is necessary when deciding the layouts. Fire protection walls, fire exits,
clearances between phases and earth, creapage distances as per site conditions
(normal or pollute atmosphere) and the entire earthing and lightning protection
system have to be properly designed. The control systems designed for
interlocking should be fail safe to reduce the possibility of maloperation and
danger to personnel. Inflammable materials like oil for transformers and
breakers should be properly stored and oil and soak pits for transformers designed
to load away used oil.
Earthing
This is perhaps one of the most important requirements in an electrical system.
Be it on electrical appliance or a power transformer, the earthing constitutes a
vital part of the safety considerations. Earthing of all electrical equipment should
conform the BIS 3043 and no concession should be made in this regard. It may
be pointed out that improper earthing is the major cause of electrical shocks every
where.
ProtectiveEquipment
Protection in electrical systems is used for saving costly equipment from damage
and preventing fire and other hazards to people. Selection of protection
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equipment which range from fuses, bimetal relays to sophisticated static relays
should be carried out by experienced and specialised engineers. Very often it is
noticed that improper fuse selection of replacement of fuses by overrated ones
cause fires due to burning of conductors or equipment on short circuit.
There have been several discussions on the aspect of safety in electrical
installations in both national and international forums. It has been identified that
whereas a lot of importance is laid on equipment specifications like short circuit
strength, mechanical and electrical life. Etc. little is done to increase the safety
features. More testing is necessary for the various materials used by the electrical
industry and the inflammability of insulators should be ensured. One can only
conclude by stressing once again that safety is a vital aspect in the design and
operation of electrical installations hence all possible measure are to be adopted
this regard.
SAFETY PROCEDURES
IN
ELECTRICALMAINTENANCE
1.0 INTRODUCTION
Safety first and always” should be the motto of every industry, be it a power
generating industry. Public utility or any manufacturing complex. In long run,
this approach helps indirectly in realising tangible savings for the organisation as
it prevents accidents, which normally result in loss of man-hours, damage to
equipment and ultimately loss of production. In Tattoo Electric Cos., we have
laid down a policy of priorities in companies’ objectives to ensure continuous,
high quality power supply at a cheaper rate to the consumers. Safety of personnel
and equipment is of paramount importance. The priorities are given below in the
order:
a) Safety of personnel – Self, collegues and public.
b) Safety of equipment
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c) Continuity and high quality of power supply.
d) Economic generation, transmission & distribution.
Safety procedures take care of the first two item and this in turn also helps in
achieving the last two items indirectly.
This handout covers the safety procedures required to be followed during
electrical maintenance activities.
2.0 FUNDAMENTALSOFSAFETYPROCEDURESINELECTRICAL
MAINTENANCE
All lines and equipment must be considered energised unless the conditions are
designated by placing some kind of Tags.
All voltages must be considered dangerous even though voltage may not be high
enough to produce serious shock.
3.0 STATUTORY REGULATIONS
It is the duty of all persons who may be concerned with installation, operation and
maintenance of electrical lines or equipment to make themselves thoroughly
conversant with the regulations and safety rules concerning the work.
Generation, transmission, distribution and utilisation of electrical energy in our
country is governed by:
a) Indian Electricity Act, 1910 and rules made thereunder i.e.
Indian Electricity rules, 1956.
The electricity supply undertakings are required to comply with the following
statutory Acts beside those given above:
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a) Factory Act, 1948
b) Workmen’s Compensation Act, 1923 and
c) Payment of Wages Act.
In addition to above statutory regulations electrical organisations usually lay
down safety rules, instructions and standing orders for guidance of staff for their
operations and maintenance, which should be strictly complied with at all times.
It shall be the responsibility of the person-in-charge to interpret and explain
correctly the rules and regulations to all staff.
4.0 CLASSIFICATION OFSAFETY PROCEDURE :
Safety procedure to be followed in electrical maintenance may be divided in the
following four main sections:
a) Prior to commencement of works i.e. safety in planning and operation.
b) While carrying out maintenance and testing.
c) During commissioning or energising of an equipment.
d) General.
5.0 PERMIT-TO-WORKSYSTEM(SAFETY TAGGING SYSTEM)
All major work on electrical installation shall be carried out under “Permit to
Work System” unless standing instructions are issued by the competent
authorities to follow other procedures.
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