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INTRODUCTION
Low Voltage Systems
Depending on the application, they include equipment for switching, protecting,
conversion, control, regulation, monitoring and measurement of electrical equipment
such as motor pumps, fans, lighting and emergency system.
The dimensions and terms of low voltage switchgear assemblies have been standardized.
Also, barriers and partition, such as internal segregation of functional areas, i.e. between
the busbars, switching apparatus, cables and as protection for the operating personnel
have been considered.
Depending on the type, switchgear apparatus can be used for single or multiple switching
tasks. Switching tasks can also be conducted by a combination of several switchgear units
of fixed or draw-out installation type and can be actuated by different types of protection
as in case of motor control centers (MCC). The main switching apparatus are:
 Circuit breakers (CB)
 Contactors
 Motor starter
 Fuse combination unit
 Miniature circuit breaker (MCB)
In most cases of power plant low voltage switchgear, several over current protection
devices are connected in series between the current source and apparatus to be protected
in case of fault. These devices must operate selectively to limit a fault to the place of its
origin as far as possible. Redundancies in the switchgear design ensure the availability of
energy supply.
Separated Extra Low Voltage (SELV)
A Separated Extra Low Voltage (SELV) circuit must be
 safely separated from other circuits that carry higher voltages;
 isolated from earth (ground) and from the protective earth conductors of other
circuits.
The safety of an SELV circuit is provided by
 the extra low voltage;
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2.  the low risk of accidental contact with a higher voltage;
 the lack of a return path through earth (ground) that a current could take in case of
contact with a human body.
The design of an SELV circuit typically involves an insulation transformer, guaranteed
minimum distances between conductors, and insulation barriers. The connectors of SELV
circuits should be designed such that they do not mate with connectors commonly used
for non-SELV circuits.
Protected Extra Low Voltage (PELV)
In contrast to an SELV circuit, a Protected Extra Low Voltage (PELV) circuit has a
protective earth (ground) connection. A PELV circuit, just as with SELV, requires a
design that guarantees a low risk of accidental contact with a higher voltage. For a
transformer, this can mean that the primary and secondary windings must be separated by
an extra insulation barrier or by a conductive shield with a protective earth connection.
Functional Extra Low Voltage (FELV)
The term Functional Extra Low Voltage (FELV) describes any other extra low voltage
circuit that does not fulfill the requirements for an SELV or PELV circuit. Although the
FELV part of a circuit uses an extra low voltage, it is not adequately protected from
accidental contact with higher voltages in other parts of the circuit. Therefore the
protection requirements for the higher voltage have to be applied to the entire circuit.
Examples for FELV circuits include those that generate an extra low voltage through a
semiconductor device or a potentiometer.
1. Make a overview drawing of the installation with: 60 / 10kV transformer station
and connection to T , TR.T. , HT , UT , L1 , L2 , L3.Determinate the point of supply.
2.1 Give a theoretically description of the dimension purpose and methods for the
overload protection and current value calculation Iz.
Current Carrying Capacity of Copper Conductors
2

3. Current carrying capacity is defined as the amperage a conductor can carry
before melting either the conductor or the insulation.
Heat, caused by an electrical current flowing through a conductor, will determine
the amount of current a wire will handle. Theoretically, the amount of current that can be
passed through a single bare copper wire can be increased until the heat generated
reaches the melting temperature of the copper.
There are many factors which will limit the amount of current that can be passed
through a wire. These major determining factors are:
Conductor Size:
The larger the circular mil area, the greater the current capacity.
Table 4.6 - Current ratings and volt drops for unsheathed single core p.v.c. insulated cables
In In
Cross conduit conduit In In
Clipped Clipped
sectional in in conduit conduit Volt drop Volt drop
direct direct
area thermal thermal on wall on wall
insulation insulation
(mm²) (A) (A) (A) (A) (A) (A) (mV/A/m) (mV/A/m)
3 or 4 2 3 or 4 2 3 or 4 3 or 4
- 2 cables 2 cables
cables cables cables cables cables cables
1.0 11.0 10.5 13.5 12.0 15.5 14.0 44.0 38.0
1.5 14.5 13.5 17.5 15.5 20.0 18.0 29.0 25.0
2.5 19.5 18.0 24.0 21.0 27.0 25.0 18..0 15.0
4.0 26.0 24.0 32.0 28.0 37.0 33.0 11.0 9.5
6.0 34.0 31.0 41.0 36.0 47.0 43.0 7.3 6.4
10.0 46.0 42.0 57.0 50.0 65.0 59.0 4.4 3.8
16.0 61.0 56.0 76.0 68.0 87.0 79.0 2.8 2.4
Insulation:
The amount of heat generated should never exceed the maximum temperature rating,
of the insulation material.
Table 4.5 - Derating factors (CI) for cables up to 10mm² in cross-sectional
---------------- area buried in thermal insulation.
Length in insulation (mm) Derating factor (CI)
50 0.89
100 0.81
200 0.68
400 0.55
3

4. 500 or more 0.50
If a cable is totally surrounded by thermal insulation for only a short length (for example,
where a cable passes through an insulated wall), the heating effect on the insulation will
not be so great because heat will be conducted from the short high-temperature length
through the cable conductor. Clearly, the longer the length of cable enclosed in the
insulation the greater will be the derating effect. {Table 4.5} shows the derating factors
for lengths in insulation of up to 400 mm and applies to cables having cross-sectional
area up to 10 mm²
Ambient Temperature:
The higher the ambient temperature, the less heat required to reach the maximum
temperature rating of the insulation.
Table 4.3 Correction factors to current rating for ambient temperature
-------------- (Ca) (from [Tables 4C1 and 4C2] of BS 7671: 1992)
Ambient
Type of insulation
temperature
(°C) 70°C p.v.c 85°C rubber 70°C m.i 105°C m.i
25 1.03 (1.03) 1.02 (1.02) 1.03 (1.03) 1.02 (1.02)
30 1.00 (1.00) 1.00 (1.00) 1.00 (1.00) 1.00 (1.00)
35 0.94 (0.97) 0.95 (0.97) 0.93 (0.96) 0.96 (0.98)
40 0.87 (0.94) 0.90 (0.95) 0.85 (0.93) 0.92 (0.96)
45 0.79 (0.91) 0.85 (0.93) 0.77 (0.89) 0.88 (0.93)
50 0.71 (0.97) 0.80 (0.91) 0.67 (0.86) 0.84 (0.91)
For example, if a cable has a rating of 24 A and an ambient temperature correction factor
of 0.77, the new current rating becomes 24 x 0.77 or 18.5 A.
Conductor Number:
Heat dissipation is lessened as the number of individually insulated conductors,
bundled together, is increased.
Installation Conditions:
Restricting the heat dissipation by installing the conductors in conduit, duct, trays or
raceways lessens the current carrying capacity. This restriction can be alleviated
somewhat by using proper ventilation methods, forced air cooling, etc.
Taking into account all the variables involved, no simple chart of current ratings can be
developed
and used as the final word when designing a system where amperage ratings can become
critical.
4

5. The chart shows the current required to raise the temperatures of single insulated
conductor in
free air (30°C ambient) to the limits of various insulation types. The following table gives
a derating
factor to be used when the conductors are bundled. These charts should only be used as a
guide
when attempting to establish current ratings on conductor and cable.
4.1 Give a theoretically description of the dimension purpose and methods for the
protection against direct and indirect contact. The installation is a TN-C-S- system.
Direct contact protection
The methods of preventing direct contact are mainly concerned with making sure
that people cannot touch live conductors. These methods include:
1-The insulation of live parts - this is the standard method. The insulated conductors
should be further protected by sheathing, conduit, etc.
2-The provision of barriers, obstacles or enclosures to prevent touching (IP2X). Where
surfaces are horizontal and accessible, IP4X protection (solid objects wider than 1 mm
are excluded - see {Table 2.4}), applies
Table 2.4 - Numbers in the I P system
First Mechanical Second Water protection
Number protection against Number against
0 Not protected 0 Not protected
Solid objects
1 1 Dripping water
exceeding 50mm
Solid objects Dripping water when
2 2
exceeding 12mm tilted up to 15
Solid objects
3 3 Spraying water
exceeding 2.5mm
Solid objects
4 4 Splashing Water
exceeding 1.0mm
5 Dust protected 5 Water jets
6 Dust tight 6 Heavy seas
- - 7 Effects of immersion
- - 8 Submersion
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6. 3-placing out of reach or the provision of obstacles to prevent people from reaching live
parts
4-the provision of residual current devices (RCDs) provides supplementary protection
{5.9.1} but only when contact is from a live part to an earthed part.
Indirect contact protection
There are three methods of providing protection from shock after contact with a
conductor which would not normally be live:
1-making sure that when a fault occurs and makes the parts live, it results in the supply
being cut off within a safe time. In practice, this involves limitation of earth fault loop
impedance, a subject dealt with in greater detail in {5.3.1}.
2-cutting off the supply before a fatal shock can be received using a residual current
device {5.9.1}.
3-applying local supplementary equipotential bonding which will ensure that the
resistance between parts which can be touched simultaneously is so low that it is
impossible for a dangerous potential difference to exist between them. It is important to
stress that whilst this course of action will eliminate the danger of indirect contact, it will
still be necessary to provide disconnection of the supply to guard against other faults,
such as overheating.
It is important to appreciate that in some cases a dangerous voltage may be maintained if
an un-interruptible power supply (UPS) or a standby generator with automatic starting is
in use.
5.3.1 – Principle
The path followed by fault current as the result of a low impedance occurring between
the phase conductor and earthed metal is called the earth fault loop. Current is driven
through the loop impedance by the supply voltage.
The extent of the earth fault loop for a TT system is shown in {Fig 5.7}, and is made up
of the following labeled parts.
Fig 5.7 The earth fault loop
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7. l. - The phase conductor from the transformer to the installation
2- The protective device(s) in the installation
3- The installation phase conductors from the intake position to the fault
4 - The fault itself (usually assumed to have zero impedance)
5 - The protective conductor system
6 - The main earthing terminal
7 - The earthing conductor
8- The installation earth electrode
9- The general mass of earth
10- The Supply Company's earth electrode
11- The Supply Company's earthing conductor
12- The secondary winding of the supply transformer
For a TN-S system (where the Electricity Supply Company provides an earth terminal),
items 8 to 10 are replaced by the PE conductor, which usually takes the form of the
armoring (and sheath if there is one) of the underground supply cable.
For a TN-C-S system (protective multiple earthing) items 8 to 11 are replaced by the
combined neutral and earth conductor.
For a TN-C system (earthed concentric wiring), items 5 to 11 are replaced by the
combined neutral and earth wiring of both the installation and of the supply.
It is readily apparent that the impedance of the loop will probably be a good deal higher
for the TT system, where the loop includes the resistance of two earth electrodes as well
as an earth path, than for the other methods where the complete loop consists of metallic
conductors
5.9.1 - Why do we need residual current devices?
{5.3.1} has stressed that the standard method of protection is to make sure that an earth
fault results in a fault current high enough to operate the protective device quickly so that
fatal shock is prevented. However, there are cases where the impedance of the earth-fault
loop, or the impedance of the fault itself, are too high to enable enough fault current to
flow. In such a case, either:
1. - current will continue to flow to earth, perhaps generating enough heat to start a fire,
or
2. - metalwork which is open to touch may be at a high potential relative to earth,
resulting in severe shock danger.
Either or both of these possibilities can be removed by the installation of a residual
current device (RCD).
In recent years there has been an enormous increase in the use of initials for residual
current devices of all kinds. The following list, which is not exhaustive, may be helpful to
readers:
RCD residual current device
RCCD residual current operated circuit breaker
SRCD socket outlet incorporating an RCD
PRCD portable RCD, usually an RCD incorporated into a plug
RCBO an RCCD which includes overcurrent protection
7

8. SRCBO a socket outlet incorporating an RCBO
TN-C-S system
In this system, the installation is TN-S, with separate neutral and protective conductors.
The supply, however, uses a common conductor for both the neutral and the earth. This
combined earth and neutral system is sometimes called the 'protective and neutral
conductor' (PEN) or the 'combined neutral and earth' conductor (CNE). The system,
which is shown diagrammatically in Fig, is most usually called the protective multiple
earth (PME) system, which will be considered in greater detail in fig.
Fig - TN-C-S earthing system - protective multiple earthing
TN-C-S System: The neutral and earth terminals are combined, but separated just
outside the consumer's installation.
Protective Devices
All electrical circuits must be protected against over current therefore a protective
device has to be installed in order to isolate the fault from the supply so as to protect the
equipment and appliances from being damaged.
Over current is caused by:
Two or more live conductors touching each
1) Short circuit
other
8

9. Adding loads greater than that of the rated
2) Overload
value
A phase conductor touching the protective
3) Earth fault conductor by means of direct or indirect
contact
If the electricity passes through a human body, the person will suffer electric shock and
burns. It could also cost damage to properties.
The main causes of electrical accidents are:
A) Lack of maintenance
B) Failure or lack of earthing
C) Unsafe and carelessness operating procedures
D) Electrical wiring and equipment's physical form could have been damaged
E) Incorrectly connected wires and other mistakes are usually caused by ignorance or
negligence
Direct and indirect contact
There are two possible ways one can get electric shock. They are direct contact
and indirect contact.
The severity of an electric shock is determined by the amount of current flowing
through the body. The effects of the electric shock can be very dangerous. It may even
prove fatal.
Here are the results:
~ 1mA - 2mA No harmful effects
~ 5mA - 10mA Painful and burning sensation
~ 10mA - 15mA Muscular contraction
~ 20mA - 30mA Impairs breathing or having breathing difficulties
~ 40mA and above Ventricular fibrillation or death
5.2 Describe the principal building of the 10kV switchboard and one single cubicle.
12kV ZS1 SWITCHBOARD
ZS1: simple, robust construction
The factory-assembled, typetested switchgear panels of type ZS1 tor 12 kV to 24 kV
rated voltage are metalclad, airinsulated and subdivided into part-compartments for
busbars, circuit-breaker, cable termination connections and low voltage equipment. ZS
switchgear panels contain withdrawable parts to accommodate the relevant switching
devices. These can be fitted either with vacuum circuit-breakers type
VD4 and VM1, SF6 circuitbreaker type or with vacuum contactors up to 12 kV.
ZS1: quickly installed
9

10. The factory-assembled switchgear panels are erected side-by-side at site to form
switchgears. A baseframe laid in or on the floor is recommended for speedier and easier
alignment. The new type of busbar system can be installed in a very short time.
10

11. 12kV ZS1 SWITCHBOARD
STRUCTURE OF ONE SINGLE CUBICLE
11

12. 2.3 Dimensions and weights
12

13. 2. Technical data
2.1 Electrical data
2.1.1 Main parameters for panels with circuit breakers
2.1.2 Dimensions and weights of 12/17.5 kV units
Cable connection of 12/17.5 kV units:
In the 550 and 650 mm wide panel, up to three parallel plastic cables can be connected
with singlecore cable protection and push-on sealing ends with a maximum cross-section
of 630 mm2.
In the 800 or 1000 mm wide panel, up to six parallel plastic cables can be connected with
single-core cable protection and push-on sealing ends with a maximum cross-section of
630 mm2.
Customer requests regarding connections to bars, three-core cables, special cables or
sealing ends
of different types must be considered during the order-planning stage
13

14. Cable connection in the panels for 12kV, 17,5 kV and 24 kV:
In the panel with switch-disconnector 1 plastic one-core cable can be connected on each
phase with the cross-section up to 240 mm2 as standard.
2.1.3 Operating conditions
2.1.4 Normal operating conditions
The switchgears are basically suitable for normal operating conditions for indoor
switchgears and switchboards in accordance with IEC 60694. The following limit values,
among others, apply:
Ambient temperature:
Maximum + 40 0 C
Maximum 24 h average + 35 0 C
Minimum (according to “minus 5 indoor class”) - 5 0 C
The maximum site altitude is 1000 m above sea level.
5.3 Describe the line protection equipment.
Circuit breaker
A circuit breaker is an automatically-operated electrical switch which is designed to
protect an electrical circuit from damage caused by overload or short circuit. Unlike a
fuse which operates once and then has to be replaced, a circuit breaker can be reset
(either manually or automatically) to resume normal operation. Circuit breakers are made
in varying sizes, from small devices which protect an individual household appliance up
to large switchgear designed to protect high voltage circuits feeding an entire city.
Some circuit breakers are implemented using a solenoid (electromagnet) whose pulling
force increases as the current increases. The circuit breaker's contacts are held closed by a
latch and, as the current in the solenoid increases, the solenoid's pull releases the latch
which then allows the contacts to open by spring action. Another method of sensing
current is with a bimetallic strip, which heats and bends with increased current, and is
similarly arranged to release the latch. Some circuit breakers incorporate both techniques,
with the electromagnet responding to short, large surges in current (short circuits) and the
bimetallic strip responding to less extreme but longer-term overcurrent conditions.
Circuit breakers for larger currents are usually arranged with pilot devices to sense a fault
current and to operate the trip opening mechanism.
Under short-circuit conditions a current of many times greater than normal can flow (see
maximum prospective short circuit current). When electrical contacts open to interrupt a
large current, there is a tendency for an arc to form between the opened contacts, which
would allow the flow of current to continue. Therefore, circuit breakers must incorporate
various features to divide and extinguish the arc. In air-insulated and miniature breakers
an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc,
and blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those
14

15. used in electrical power distribution may use vacuum, an inert gas such as sulfur
hexafluoride or have contacts immersed in oil to suppress the arc.
The maximum short-circuit current that a breaker can interrupt is determined by testing.
Application of a breaker in a circuit with a prospective short-circuit current higher than
the breaker's rating may result in failure of the breaker to safely interrupt a fault.
Small circuit breakers are either installed directly in equipment, or are arranged in a
breaker panel. Power circuit breakers are built into switchgear cabinets. High-voltage
breakers may be free-standing outdoor equipment or a component of a gas-insulated
switchgear line-up.
Miniature circuit breaker (MCB)
The advantages of a miniature circuit breaker (MCB) are:
1) Shorter tripping time
2) Can be reused
3) Easy to reset
4) Has a switch that can isolate the equipment
The disadvantages of a miniature circuit breaker (MCB) are:
1) The most expensive protection device for home use
2) Slow tripping time due to aging
3) Surrounding temperature may affect the MCB
FUSES
The fuse is a piece of wire which can carry a stated current. If the current rises above this
value it will melt.
If the fuse melts (blows) then there is an open circuit and no current can then flow thus
protecting the equipment by isolating it from the power supply.
The fuse must be able to carry slightly more than the normal operating current of the
equipment to allow for tolerances and small current surges.
With some equipment there is a very large surge of current for a short time at switch on.
If a fuse is fitted to withstand this large current there would be no protection against
faults which cause the current to rise slightly above the normal value.
Therefore special antisurge fuses are fitted. These can stand 10 times the rated current for
10 milliseconds. If the surge lasts longer than this the fuse will blow.
Always find out why the fuse blew before replacing it. Occasionly they grow tired and
fail. If the fuse is black and silvery then it is likely that there is a dead short (very low
resistance) somewhere.
What is the Difference Between a Fuse and a Circuit Breaker?
Fuses and circuit breakers are two different ways of protecting against suddenly large
overloads of electrical flow. Large power overloads are dangerous, potentially destroying
electrical equipment or causing a fire. Both fuses and circuit breakers will automatically
block against an incoming surge of electrical power past a certain safety limit. But while
they both accomplish the same task, each uses different technology in the way that it
stops the flow of electricity.
15

16. Fuses are typically small objects that plug into a fusebox or other central location. They
are an early technology, dating back to the 19th century. Inside the fuse is a small piece
of metal, across which the electricity must pass. During normal flow of electricity, the
fuse permits the power to pass unobstructed. But during an unsafe overload, the small
piece of metal melts, stopping the flow of electricity. When a fuse is tripped, it should be
thrown away and replaced with a new fuse. As there are many varities of fuses available
that handle different capacities of electricity, care should be taken when choosing
replacement fuses.
Circuit breakers are a more recent invention and improve on fuse technology. Circuit
breakers are switches that are tripped when the electrical flow passes a safe limit. The
excess of electricity typically triggers an electromagnet, which trips the circuit breaker
when an unsafe limit is reached. Once tripped, the switches simply turn off. That stops
the flow of electricity, which will remain off until the switch is reset. To reset the flow of
electricity after the problem is resolved, the switch can simply be turned back on. Circuit
breakers are often located in a cabinet of individual switches, typically inside of an
apartment or other central place. While often used in homes, circuit breakers can be used
for much larger industrial applications as well.
Fuses and circuit breakers have unique advantages and disadvantages. One advantage of
fuses is that they are cheap and can be purchased from any hardware store, but they have
the drawback of needing to be replaced once they stop an overload. That can be
challenging in a darkened room. Alternatively, circuit breakers can simply be reset with a
flip of a switch after an overload. However, the technology can be more expensive than a
fusebox. Electricians are best qualified to determine whether fuses or circuit breakers are
better for a particular electrical installation.
5.4 Describe the10kV switchboard protection equipment.
SWITCHGEAR PROTECTION – 12 KV
Circuit Breakers
i) The circuit breakers shall be of standard design and construction conforming to IEC 60056 : 1987. The
interrupting medium of the circuit breaker shall be of Vacuum.
ii) The control mechanism of the circuit breakers shall be of spring assisted trip free type with remote /
local control selector switch and manual operational facility.
iii) The circuit breaker shall have a mechanical counter to register the number of circuit breaker operations.
iv) Characteristics of the Circuit Breakers shall be as follows;
a) Number of poles -3
b) Type - Indoor
c) Rated voltage kV - 12
d) Rated frequency Hz - 50
e) Rated insulation level
i) Impulse (1.2/50µs) withstand voltage (peak) kV - 75
ii) Power frequency withstand voltage (rms) kV - 28 /1min.
f) Rated normal current A - 630/800
g)Rated shortcircuit breaking current (rms ) kA - 20
h) Rated short circuit making current kA - 50 rms
i) First pole to clear factor - 1.5
j) Duty cycle `O’ - 3 min. - `CO’ - 3 min. -`CO’
16

17. k)Tripping supply voltage VDC - 24
Protection
i) The programmable type numerical protection relays, conforming to IEC 60255, shall
be used. The relays used shall only be from the following manufactures of the countries
indicated.
a) ABB Relays AB, Sweden/Switzerland/Germany
b) Alsthom, England
c) Toshiba Corporation, Japan
d) Siemens AG, Germany
e) Group Schneider, England/France/Italy
ii) Necessary software package and the hand held programming unit shall be supplied
with the numerical protection relays to set the protection relays as required. For every
five panels one number of handheld programming unit shall be supplied. Software shall
be compatible with Windows 98 or upper versions.
iii) The numerical relay unit shall be suitable for use in the tropical climatic conditions as
given under the Clause 3.0 Service
iv) It shall be possible to select required type of over current and earth fault protection of
IDMT characteristics.
v) The numerical relay unit shall have provision for accommodating RTU for
incorporating SCADA system in the future.
Earthing
i) The copper earthing conductor shall be provided along the entire length of the panel.
The cross sectional area of earthing conductor shall be such that the current density shall
not exceed 200 A/mm² under the specified earth fault conditions.
5CEB STANDARD 025 : 2001
ii) It shall be possible to earth the cable with facility for padlocking, and mechanical
interlocking shall be provided to prevent earthing the busbar.
iii) All metallic parts of the functional units intended to be earthed shall be bonded to the
earthing conductor of the panel.
5.5 Describe the 60 / 10kV transformer protection equipment.
Transformer Protection
Transformers–600V or Less
17

18. If secondary fuse protection is not provided then the primary fuses must not be
sized larger than as shown below.
Individual transformer primary fuses are not necessary where the primary circuit fuse
provides this protection.
Primary Fuse (600V or less) and Secondary Fuse (600V or less). If secondary
(600V or less) fuses are sized not greater than 125% of transformer secondary current,
individual transformer fuses are not required in the primary (600V or less) provided the
primary feeder fuses are not larger than 250% of the transformer rated primary current.
Note: Transformer overload protection will be sacrificed by using overcurrent protective
devices sized much greater than the transformer F.L.A. The limits of 150%, 167%, 250%
and 300% may not adequately protect transformers. It is suggested that for the highest
degree of transformer overload protection the fuse size should be within 125% of the
transformer full-load amperes.
There is a wide fuse ampere rating range available to properly protect
transformers. FUSETRON® Class RK5 and LOW-PEAK® YELLOW Class RK1 dual-
18

19. element fuses can be sized on the transformer primary and/or secondary rated at 125% of
the transformer F.L.A. These dual-element fuses have sufficient time-delay to withstand
the high magnetizing inrush currents of transformers. There is a wide ampere rating
selection in the 0 to 15 ampere range for these dual-element fuses to provide protection
for even small control transformers. The required secondary protection may be satisfied
with multiple overcurrent devices that protect feeders fed from the transformer
secondary. The total ampere rating of these multiple devices may not exceed the allowed
value of a single secondary overcurrent device. If this method is chosen, dual-element,
timedelay fuse protection offers much greater flexibility. Note the following examples:
Design 1: Utilizes a single secondary overcurrent device. It provides the greatest degree
of selective coordination, transformer protection, secondary cable protection, and
switchboard/panelboard/ load center protection. The transformer cannot be overloaded
to a significant degree if future loads are added (improperly). With this arrangement the
transformer’s full capacity is utilized.
Design 2: In this case the single secondary overcurrent device is eliminated, much of the
protection described in Design 1 will be reduced.
19

20. If dual-element fuses are utilized as branch circuit protection, the transformer can
continue to be loaded with the five 83 amp motors because 5 ≈ 110 = 550 amps, (less
than the maximum 600 amps). If additional loads are improperly added in the future,
overload protecton will be lost because the primary device can be sized
at 250%.
Design 3: If the single secondary overcurrent device is eliminated and MCP’s are utilized
as branch circuit protection, the transformer will be seriously under-utilized because only
one motor can be connected.
20

21. For one motor, 1 ≈ 700% of 83 = 581 amps. For two motors, 2 ≈ 700% of 83 =
1162 amps. Since the sum of the devices cannot exceed 600 amps, only one motor can be
connected when the motor circuit is protected by an MCP.
Design 4: Using the same procedure, if the single secondary main is eliminated and
thermal magnetic circuit breakers are utilized as branch circuit protection per 430.52,
only three of the motors can be connected because the thermal magnetic breakers will
have been sized at approximately 250% of the motor F.L.A. (83 ≈ 250% = 207.5A.)
Using a 200 ampere circuit breaker would allow only three (600 ÷ 200) motors to be
connected. To add two additional motors of the same type as shown in Design 1 and
Design 2 requires a larger transformer - one that would have a 1000 ampere or more
secondary capability. A 300 KVA 208V transformer has a 830 ampere secondary rating
which is not sufficient. Therefore, the next standard size 3Ø transformer is a 400 KVA
with a 1110 ampere capacity to meet the new rule.
6.1 Give a theoretically description of selectivity considerations.
A theoretical outline of selectivity
Selection of the protection system of the electrical installation is fundamental both to
guarantee correct economical and functional service of the whole installation and to
reduce the problems caused by abnormal service conditions or actual faults to a
minimum.
Within the sphere of this analysis, the coordination between the various devices dedicated
to protection of sections of installation or specific components is studied in order to:
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22. – guarantee safety of the installation and of people in all cases;
– rapidly identify and exclude just the area involved in the problem, without
indiscriminate trips which reduce the availability of energy in areas not involved in the
fault;
– reduce the effects of the fault on other integral parts of the installation (reduction in the
voltage value, and loss of stability in rotating machines);
– reduce the stress on components and damage to the area involved;
– guarantee service continuity with good quality power supply voltage;
– guarantee adequate support in the case of malfunction of the protection delegated to
opening;
– provide the personnel in charge of maintenance and the management system with the
information needed to restore service to the rest of the network as rapidly as possible and
with the least interference;
– achieve a good compromise between reliability, simplicity and cost-effectiveness.
In detail, a good protection system must be able to:
– perceive what has happened and where, discriminating between abnormal but tolerable
situations and fault situations within its zone of competence, avoiding unwanted trips
which cause unjustified stoppage of a sound part of the installation;
– act as rapidly as possible to limit the damage (destruction, accelerated ageing, etc.),
safeguarding
power supply continuity and stability.
The solutions come from a compromise between these two antithetic requirements –
precise identification of the fault and rapid tripping - and are defined according to which
requirement is privileged.
Selectivity techniques
This section describes the different selectivity techniques and their area of application.
In the overload zone with the protections in play, time-current type selectivity is usually
realised.
In the short-circuit zone with the protections in play, various selectivity techniques can be
used. In particular, the following will be illustrated in the paragraphs below:
Current selectivity
Time selectivity
Energy selectivity
Zone selectivity.
After an initial theoretical description of the different selectivity techniques, the
selectivity technique which can be used appropriately for the different types of circuit-
breakers will then be analysed.
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23. Time-current selectivity
In general, the protections against overload have a definite time characteristic, whether
they are made by means of a thermal release or by means of function L of an electronic
release. A definite time characteristic is intended as a trip characteristic where, as the
current increases, the trip time of the circuit-breaker decreases. When there are
protections with characteristics of this type, the selectivity technique used is time-current
selectivity.
Time-current selectivity makes trip selectivity by adjusting the protections so that the
load-side protection, for all possible overcurrent values, trips more rapidly than the
supply-side circuit-breaker.
When the trip times of the two circuit-breakers are analyzed, it is necessary to consider:
- The tolerances over the thresholds and trip times
- The real currents circulating in the circuit-breakers.
1.05 x I1 of the supply-side circuit-breaker
Assuming IA =1.05xI1, with reference to what has been said about
the real currents which circulate in the circuit-breakers, the IB current
is obtained on the load side.
The trip times of the two devices are obtained from the time-current
curves.
1.20XI3 (or I2) of the load-side circuit-breaker
Assuming IB = 1.20XI3 (or I2), the IA current is obtained in the same
way on the supply side and, from the time-current curves, the trip
times of the two devices are obtained.
If the following is true for both the points considered:
tA>tB
then selectivity in the overload zone is guaranteed.
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24. Current selectivity
This type of selectivity is based on the observation that the closer the fault point is to the
power supply of the installation, the higher the short-circuit current is. It is therefore
possible to discriminate the zone the fault occurred in by setting the instantaneous
protections to different current values.
Total selectivity can normally be achieved in specific cases only where the fault current is
not high and where there is a component with high impedance interposed between the
two protections (transformer, very long cable or a cable with reduced cross-section, etc.)
and therefore a great difference between the short-circuit current values.
However:
– The ultimate selectivity current is usually low and therefore selectivity is often only
partial;
– The setting level of the protections against overcurrents rises rapidly;
– Redundancy of the protections, which guarantees elimination of the fault (rapidly) in
the case of one of them not operating, is not possible.
– The protection against short-circuit of supply-side circuit-breaker A will be set to a
value which means it
does not trip for faults which occur on the load side of protection B. (In the example in
the figure I3minA >
1kA)
– The protection of load-side circuit-breaker B will be set so as not to trip for faults
which occur on its load
side. (In the example in the figure I3MaxB < 1kA)
Obviously the setting of the protections must take into account the real currents
circulating in the circuitbreakers.
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25. The ultimate selectivity value which can be obtained is equal to the instantaneous trip
threshold of the supply-side protection less any tolerance.
Is = I3minA
Time selectivity
This type of selectivity is an evolution of the previous one. In this type of coordination,
apart from the trip threshold in terms of current, a trip time is also defined: a certain
current value will make the protections trip after a defined time delay, suitable for
allowing any protections placed closer to the fault to trip, excluding the area which is the
seat of the fault.
The setting strategy is therefore to progressively increase the current thresholds and the
trip delays as one gets closer to the power supply sources (level of setting directly
correlated to the hierarchical level). The delayed trip thresholds must take into account
the tolerances of the two protection devices and the effective currents which circulate in
them.
As in the case of current selectivity, the study is made by comparing the time-current trip
curves of the protection devices.
Generally this type of coordination:
- is easy to study and realize;
- is not very costly with regard to the protection system;
- allows even high selectivity limit values to be obtained (if Icw is high);
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26. - allows redundancy of the protection functions.
However:
- the trip times and energy levels let through by the protections, especially by those close
to the sources, are high.
It is a type of selectivity which can also be made between circuit-breakers of the same
size, equipped with electronic releases with delayed protection against short-circuit.
The protections against short-circuit of the two circuit-breakers will be set:
- with the I2 trip thresholds against delayed short-circuit adjusted so as not to create trip
overlapping,
taking into consideration the tolerances and the real currents circulating in the circuit-
breakers.
- with t2 trip times adjusted so that the load-side circuit-breaker B extinguishes the fault
whereas the
supply-side circuit-breaker A, still in the timing phase, manages to “see” the extinction of
the current
and therefore remains closed.
The ultimate selectivity limit which is obtained is equal:
– to the instantaneous trip threshold of the supply-side protection,
if this function is enabled, less any tolerance:
Is = I3minA
– to the value of Icw for supply-side air circuit-breakers when the instantaneous
protection function is set to OFF.
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27. Energy selectivity
Coordination of energy type is a particular type of selectivity which exploits the current-
limiting
characteristics of moulded-case circuit-breakers. It is pointed out that a current-limiting
circuit-breaker is “a circuit-breaker with a sufficiently short trip time to prevent the short-
circuit current from reaching the peak value which would otherwise be reached” (IEC
60947-2). In practice, all the ABB SACE moulded-case circuitbreakers of the Isomax and
Tmax series, the modular circuit-breakers and the E2L E3L air current-limiting circuit-
breakers have more or less marked current-limiting characteristics.
The protections against short-circuit of the two circuit-breakers must respect the
conditions given below.
- Supply-side release of thermomagnetic type
the magnetic trip thresholds must be such so as not to create trip overlapping, taking into
consideration the tolerances and the real currents circulating in the circuit-breakers; the
magnetic threshold of the supply-side circuit-breaker must be equal to or higher
than10xIn or set to the maximum value when it is adjustable.
- Supply-side release of electronic type
any protections against delayed short-circuit S must be adjusted following the same
indications as time selectivity;
the instantaneous protection function I of the supply-side circuit-breakers must be set to
off
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28. The Is ultimate selectivity limit obtained is the one given in the tables
Zone selectivity
This type of coordination is an evolution of time coordination. In general, zone selectivity
is made by means of dialogue between the current measuring devices which, once the
setting threshold has been detected as having been exceeded, allows just the fault zone to
be identified correctly and the power supply to it to be cut off.
It can be realised in two ways:
– the measuring devices send the information linked to
the current setting threshold having been exceeded to a
supervision system and the latter identifies which
protection has to intervene;
– when there are current values higher than their setting,
each protection sends a lock signal by means of a direct
connection or a bus to the hierarchically higher level
protection (on the supply side in relation to the power
flow direction) and, before intervening, checks that a
similar lock signal has not arrived from the load-side
protection. In this way only the protection immediately
to the supply side of the fault intervenes.
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29. Compared with coordination of time type, zone selectivity allows:
- reduction of the trip times (these can be lower than hundred milliseconds);
- reduction both of the damage caused by the fault and of interferences to the power
supply system;
- reduction of the thermal and dynamic stresses on the components of the installation;
- a very high number of selectivity levels to be obtained. However:
- it is more burdensome both in terms of cost and of complexity of the installation
- it requires an auxiliary supply.
This is a type of selectivity which can be realised:
- between Emax air circuit-breakers equipped with PR122 and PR123 releases.
The ultimate selectivity limit which can be obtained is equal to the Icw Is = Icw
- between Tmax T4L,T5L and T6L moulded-case circuit-breakers equipped with PR223
EF releases.
The ultimate selectivity limit which can be obtained is 100kA Is = 100kA
Then, by means of the additional IM210 module, it is possible to make a chain of zone
selectivity between Tmax and Emax.
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30. FINAL CONCLUSION
In this report, we had some knowledge about low voltage installations. At the
same time, this report includes protection techniques. We learnt that in TNCS systems,
fuses and circuit breakers are very important for protecting from direct and indirect
contact ( and overload, short circuit) . Furthermore, we tried to describe principal
building of the 12 KV switchboard, one single cubicle and protection equipment. We saw
that selectivity considerations are quiet important for installation. And it is not easy to
select devices, because it is about safety.
Of course this report will not satisfy you, but we tried to do our best. We tried to
find all theoretical information about this report.
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