Lv switchgear & lv cable sizing

LV Switchgear & LV Cable sizing
Asif Eqbal

INTRODUCTION
• Area of application
• Leading manufacturers
• Business size of LV Switchgear Industry
LOW VOLTAGE SWITCHGEAR
• Governing standards
• System parameters
• Construction
• Busbars and Other components
• Wiring and Schematics
• Layout aspect & EHS ASPECT
LV CABLE
• Sizing criteria
CONTENTS

All Industrial LV load distribution
• LV Motor (up to 200kW) – For pump, grinder, crushers etc…..
• VFD – For application requiring speed control
• Auxiliary & control supply of control & protection equipments
Domestic load distribution- For residential consumers
• Lighting
• Small power
• HVAC
Commercial consumers
Printer & Xerox machine Vending machines
Scanner Baggage handling system
Elevators and escalators Fire detection and alarm system
Public address system Illumination
Area of Application
INTRODUCTION
Why use VFD?

LOW VOLTAGE SWITCGEAR
Governing standards
IEC 60947, 61439,
60694
IS 13947
IS 10118, 8623,
11353
ANSI/IEEE
C37.20.1.1987,
C37.010 &
C37.013
Switchgears with rated voltage up to 1000V
AC and 1500V DC are termed as low voltage
Adopted from IEC 60947
Low voltage circuit breakers
Any difference
in ANSI and
IEC?

Definitions
Switchgear
PCC &
MCC
Metal
enclosed &
Metal clad
A general term covering switching devices and their combination with
associated control, measuring, protective and regulating equipments
enclosed inside a enclosure.
Any switchboard directly connected to transformer and which usually feeds other
downstream switchboards is technically called PCC (power control center).
A switchboard, which does not feed any downstream boards and directly feeds to motor or
other loads, is MCC (Motor control center).
A switchboard which feeds both downstream boards and motor loads or one incomer
connected to emergency DG than they are called PMCC.
As per ANSI/IEEE:
Completely enclosed on all sides and top with sheet metal except for ventilating opening.
In addition to above requirement if the main switching and interrupting device is draw
out. Metal clad is applicable term for medium voltage. For LV application metal enclosed
+ compartmentalized is metal clad
Check point:
Specification needs to
be checked if it contains
the term metal clad

System Parameters
Design Ambient
Altitude
Humidity
Main system/Frequency
Rated operational voltage
Power frequency
withstand voltage dry
Rated current
Fault level & duration
50C
<1000Mtr MSLD
<100%RH non condensing
3Ph, 4W, 50Hz
415V ± 10%
2.5kV for power circuit
Rated current of switchgear is bus bar rating
Fault level is normally 50kA for 1 Sec
humidity?
Why
humidity?
Why non
condensing?
Check point: Design
ambient and fault level
are specification check
points

Construction
 In the pioneering days of electrical
distribution, switching medium & low
voltage supplies was a challenge.
 Since then there have been major
changes to switchgear.
 Fig shows old style open switchgear

Construction
Fixed type
Draw out
Fixed or draw out type is decided taking into
account a function a switchgear has to
perform, Safety, Criticality of load and
Overall cost of ownership.
In a fixed construction, all the feeders in the
switchboard, feeding the various load points, are
securely mounted in the assembly and rigidly
connected to the main bus.
In this construction each feeder is mounted on a
separate withdrawable chassis. Circuit breaker is
installed on a carriage which can be pulled out.
Disconnecting of main incomer is not required
The modules of identical types can also be easily
interchanged and defective modules replaced by
spare modules in the event of a fault.
A draw-out assembly can be designed only in a
cubicle construction and is totally
compartmentalized.

Construction
Semi-draw-out type In this design the incoming and outgoing power contacts
are of the draw-out type, but the control terminals are the plug-in type. The
control terminals are to be disengaged manually first, when the trolley is to be
drawn out.

Construction
Fully draw-out type In this construction the control terminals are of the
sliding
type. The moving contacts are mounted on the trolley while the fixed matching
contacts are mounted on the panel frame. These contacts engage or disengage
automatically when the trolley is racked-in or racked-out of the module
respectively.

Construction
Single front
Double front
Mounting
Single sided arrangement of feeders.
Two sided feeder arrangement on front as well as rear side. Panel depth
increases
Floor mounting with integral base frame.
.
.
Check point: Draw-
out, fixed type and
Single front, double
front type are
specification check
points

Construction
Cross sectional view of double front panel
with common horizontal bus and separate
vertical bus (Vertical bus not visible)
Common
horizont
al bus
 Maintenance on horizontal bus can be done by
opening top cover or door of bus bar
compartment
 Point on vertical bus where male contact of bus
meets with female contact is most vulnerable to
fault and same can maintained through
breaker compartment or cable alley
 A busbar alley is required after every feeder to
access the vertical busbar

Construction
Draw out requires fully
compartmentalized
arrangement
EDO: Electrical
draw out
MDO:
Mechanical draw
out
Fixed type can be non
compartmentalized also
depending upon feeder rating
Compartmentalized
Non
Compartmentalized
(for MCB feeders)
Summary
Single
front
Double
front
Single
front
Double
front
Single
front
Double
front
Single
front
Double
front

Construction
Sheet steel thickness of
various parts
Load bearing members-2.0mm CRCA
Non load bearing members & Doors -1.6mm CRCA
.
What are load and non load
bearing members?
What is cold rolled?

Construction
Degree of
protection
IP-54 < 1600A and IP-42 for ≥ 1600A. Except, switchboard where soft
starter panels or APFC capacitor banks are present wherein IP-40 will be applicable.
Developed by the European Committee for Electro Technical Standardization
(CENELEC) (NEMA IEC 60529 Degrees of Protection Provided by Enclosures - IP
Code), specifying the environmental protection the enclosure provides.
The IP rating normally has two (or three) numbers:
Protection from solid objects or materials
Protection from liquids (water)
Protection against mechanical impacts (commonly omitted, the third number is not
a part of IEC 60529)
An "X" can used for one of the digits if there is only one class of protection, i.e. IPX1
which addresses protection against vertically falling drops of water e.g. condensation
.

Construction
IP Third number - Protection against mechanical impacts (commonly omitted, the third number is not a part of IEC
60529).
IEC 60529 does not specify degrees of protection against mechanical damage of equipment.

Construction
Older enclosure specifications are sometimes seen with an optional third IP digit
denoting impact resistance. German standard DIN 40050‐9 extends the IEC
60529 rating systems with an IPK rating. Newer products are likely to be given
an IK rating instead.

Construction
NEMA Rating (enclosures)
The NEMA Standard for Enclosures for Electrical Equipment does test for
environmental conditions such as corrosion, rust, icing, oil, and coolants.
Apart from NEMA 6P, NEMA 12 & 13 are also present

Bus bars and other components
Bus bars
NEMA Phase
arrangement
A bus is a low impedance conductor or set of conductors that serves as a common
connection for two or more circuits.
NEC Article 408 requires three-phase bus bars to have phases in sequence R, Y and B
vertical left right when viewed from front and horizontal right to left.
The order of the busbars is usually N, L1, L2 and L3:
- from the front to the rear of the switchboard,
- from the left to the right of the switchboard.

Main bus bar or
Horizontal bus bar
Vertical or distribution
busbars
Main busbar that distributes power horizontally between the various switchboard columns. It may
be installed on the top, middle or bottom of the switchboard depending on the type of switchboard,
customer specifications and/or local practices.
Vertical or distribution busbars are connected to the main busbar. They provide power to outgoing
devices

Bus bars material
Sizing criteria
Copper or Aluminium
Bus bar is sized for its cross section by correlation of electrical and
thermodynamic equations.
Step-1 is Input data
required Ambient Temp 400C
Initial Busbars Temp before Fault 900C
Permissible Final Temp after Fault 1000C
Busbars Material AL E91E
Conductivity 31.9m/Ω.mm
Details of Busbars Support :
Material SMC
Tensile Strength 750Kg./sq.cm
Flexual Strength 1650Kg./sq.cm
Compressive Strength 1900Kg./sq.cm
Di-electric Strength 10KV/mm

Step-2: Minimum
cross section
requirement as per
fault level

where
S =cross sectional area,
I=fault level,
t=fault withstanding time,
k=constant.
For formula reference &
value of constant k refer IS
3043 or INDAL Al BUSBAR
HAND BOOK

Step-3: Finding ampacity of
minimum cross section area
2
Heat generated in
electrical terms
1 2
Where
Q = Heat dissipated in thermodynamic terms as per Stephen
Boltzmann’s law
Q1 Heat generated by radiation 36.8 x 10‐12 x Emissivity Factor x Final
Temperature 4‐ Ambient Temperature 4
2
0.0022 x Air Pressure 0.5 x Temperature Rise above new
ambient 1.25 / Height of cross section of busbar mounted along
thickness 0.25
Is Q2 in still
air?
Ambient as per
IEC 60694 is
40oC. If project
specific
ambient is
used than no
temperature
derating
required at
later stage

Electrical and thermodynamic equation can be equated (because of rate of heat
generated = rate of heat dissipated in steady state) to give following expression
for ampacity.
{24.9(θ− θn) 0.61. S0.5. P0.39} √ρ [1 + α (θ−20)]
This ampacity expression is called Melson & Both equation
l : permissible current expressed in amperes (A)
θn : ambient temperature 40 °C
(θ - θn) : permissible temperature rise as per IEC 439 50 °C
S : busbar cross-section/bar in Sq centimeter as obtained in
step-2
p : busbar perimeter/bar in centimeter
ρ : resistivity of the conductor at ambient temperature.
Copper: 1.83 μΩ cm
α : temperature coefficient for the resistivity Copper 0.004 at 20
degree Celsius.

Can this current
value be used for
selection of bus bar?
Derate the busbar as per the derating factor, K which is
product of 6 other factors k1, K2, k3, k4, k5
Hence k = k1xk2xk3xk4xk5
Coefficient k1 is a function of the number of bar strips per phase for:
1 Bar k1= 1 (This derating is due to proximity effect)
For 2 or 3 Bars, see table below:
e / a

Coefficient K2 is a function of the surface condition of the bars:
bare: k2 = 1
painted: k2 = 1.15
Painted bus bars are
uprated?
Colored tape at
suitable locations
as per IS 11353
Heat loss by
radiation is directly
proportional to
emissivity
Emissivity depends
upon surface
condition of bus bars
that is painted or
bare
Painting is done to
improve emissivity
and radiation losses
Paint the bus
bars for better
heat dissipation
and possibly
increase the
current carrying
capacity
Pain layer acts as an
insulator reducing
the efficiency of
convection process
and possibly reduce
the current carrying
capacity
Radiationheatloss
Convectionheatloss
Painting may be worthwhile for very wide
bars (where convection is less effective)
operating at large temperature rises (where
radiation is more effective).

Coefficient k3(optional) is a function of the busbar position:
edge-mounted busbars: k3 = 1
1 bar flat-mounted: k3 = 0.95
several flat-mounted bars: k3 = 0.75
Coefficient k4 is a function of where the bars are installed:
Calm indoor atmosphere: k4 = 1
Calm outdoor atmosphere: k4 = 1.2
bars in non-ventilated ducting: k4 = 0.80
yp
Insulator
type?
Check point:
Insulator type is a
specification check
points

SMC (Sheet molding compounds) for the busbar mounting supports.
They are basically fiber- or glass-reinforced thermosetting plastics (FRP or GRP) and possess good physical and thermal
stability, high mechanical strength and excellent. The improved properties, particularly its strength, over DMC is a result
of reduced degradation of the glass and the ability to use longer fiber. In DMC, this is usually 6-25 mm, while in
SMC it is about 25-50 mm.

Coefficient k5 is a function of the artificial ventilation, busbar area & enclosure
area:
Enclosure factor = Busbar area/ Enclosure area

Mechanical criteria to be
checked:
Why
mechanical
check?
We have to check if the selected bus bars can withstand the
electrodynamic forces
The electrodynamic forces following a short-circuit current
are given by the equation:
F1 = (2 L/d) x Idyn
2 x 10-8
F1 : Force expressed in daN
Idyn : is the peak value of short-circuit expressed in A
L : distance between insulators on the same phase cm
d : phase to phase distance cm
What is peak
value?

checked:
Indicates direction of short circuit current Idyn
Indicates phase to phase distance d in cm
Indicates F1 force expressed in daN
Distance L between insulators on the same phase
cm
Force at head of support:
F = F1 (H+h)/H
H = Insulator height
h = Insulator top from bus bar CG
F = This force is deflection force and should
be ≤ insulator bending resistance

checked:
Since bus bar are rigidly fixed to insulator and insulator has sufficient cantilever strength
to resist the deflection so bus bar will be subjected to bending moment whose resultant
strain is:
∏ = F1xLxn/12
∏ is Resultant strain
F1 is Electrodynamic force
L is distance between insulators on the same phase
n is modulus of inertia of given bus bar size and arrangement arrangement
∏ should be ≤ Permissible strain of bus bar material
In case of failure of any of the above two condition than insulator
type or support span or type or both.

What is current density?
What is role of current
density in bus bar sizing?
Current density is a measure of the density of an electric
current. It is defined as a vector whose magnitude is the
electric current per cross-sectional area. In SI units, the
current density is measured in amperes per square
metrer

To save time and avoid the multiple repetition of above process a very
approximate starting point is to assume an average current density of 2 A/mm² in
still air for Copper and iterate either up or down. The more popular thumb rule
being followed in India is to assume current density of 1.0 Amps / Sq.mm for
Aluminium and 1.6 Amps for Copper for any standard rectangular conductor
profile.
Current density is indicative of distribution of current
over surface and shape of a conductor.
You need to determine the minimum cross sectional
area first, then find its resistance and then correlate
the thermodynamic heat loss by convection and
radiation at (allowable temperature rise) with
electrical heat loss.

Bus bars and other components: Forms of separation inside switchboard
There are several reasons for partitioning a switchboard:
Partitioning rules are defined in standard IEC 61439-2. This definition is subject to agreement between
the switchboard manufacturer and the end user.
Form 2: Partition of
bus bars and functional
unit inside switchboard

Form 3: Each
functional unit
is partitioned
from another.
Draw-out type
boards possible
with form 3
and higher

Form 4: Each
functional unit as
well as there
terminal connecters
is partitioned from
another. Draw-out
type boards with
cable alley is
possible with form
4

Bus bars and other components: Sleeving or bare
Bus bars can be bare or sleeved. For safety to personnel during maintenance and to protect the live system
from lizards and rodents the busbars may be covered with PVC tape or heat shrinkable PVC sleeve.
The joints and the tap-offs can be protected through FRP shrouds as shown in picture.
It is possible that one may not be able to provide a true
skin-fit sleeve through the length of the busbars, which
may affect its cooling.
At certain places, it may have air bubbles from where it
will provide a reduced heat dissipation.
Disadvantages of Sleeving a bus bar
For higher rating systems, say 2500 A and above, Sleeving
is normally not used.
L&T and Schneider can offer both sleeved and bare bus bars with type tested design.
Siemens has sleeved bus bar as only type tested design.
If panel is type tested with sleeved bus bar than bare bars cannot be offered but reverse is
possible
Check point: Bare or
sleeved bus bar is
specification check point

Bus bars and other components: ACB, MCCB, MPCB, MCB, ELCB
Circuit-breaker: a mechanical switching
device capable of making, carrying and
breaking currents under normal circuit
conditions and also making, carrying for a
specified time, and breaking currents
under specified abnormal circuit
conditions such as those of short-circuit
(IEC 60947-1 def. 2.2.11)
Current-limiting circuit-breaker: a circuit-
breaker with a break-time short enough to
prevent the short-circuit current reaching
its peak value (IEC 60947-2 def. 2.3)
L&T H range ACB from 800A to
4000A
L&T C Range ACB from 800A to
6300A

ACB:
Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.
When a live circuit is interrupted, an arc is formed between the parting contacts. the intensity
and magnitude of which would depend upon the quantum and the quality (p.f.) of the current
being interrupted.
The insulating material ( may be fluid or air) used in circuit breaker should serve two
important functions. It should provide sufficient insulation between the contacts when
circuit breaker opens. It should extinguish the arc occurring between the contacts when
circuit breaker opens
At low voltage level di electric strength of air provides sufficient insulation between the
contacts as well as arc quenching medium

ACB:
Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.
During interruption, the arc is formed producing N2 (80%) and O2 (20%) and metallic vapors
Figure shows the dielectric properties of different
mediums at different contact gaps
done.
There are two methods by which interruption is
done.
High resistance
method.
Low resistance
method or zero
interruption method.

ACB:
In high resistance method we can increase the electrical resistance many times to such a high value that it
forces the current to reach to zero and thus restricting the possibility of arc being restruck. Proper steps
must be taken in order to ensure that the rate at which the resistance is increased or decreased is not
abnormal because it may lead to generation of harmful induced voltages in the system. The arc resistance
can be increased by various methods like lengthening or cooling of the arc etc.
This method is
advantageous in dc
power circuit breaker,
where there is no natural
current zero so forcing
the current to reach zero
is achieved through this
method. This is also
employed in low rating
AC breakers.
During arc discharge
most of the energy is
received by CB itself.
Hence circuit breaker
require more mechanical
endurance/ mechanical
strength to withstand
sudden change of large
energy.

ACB:
Restriking voltage: Voltage at the terminal of CB, after the circuit interruption. Restriking voltage is
what appears across the contacts at current zero during arc period. The current interruption in the circuit
depends upon restriking voltage
Here v = restriking voltage.
V = value of voltage at the instant of interruption.
L and C are series inductor and shunt capacitance up to fault
point.
Thus from above equation we can see that lower the value of
product of L and C, higher the value of restriking voltage.
Recovery voltage: It is the normal frequency (50 Hz) r.m.s voltage that appears across the contacts
of the circuit breaker after final arc extinction.

ACB:
Rate of Rise of Restriking Voltage (RRRV): It is defined as the ratio of peak value of restriking
voltage to time taken to reach to peak value. It is one of the most important parameter as if the rate at
which the dielectric strength developed between the contacts is greater than RRRV, then the arc will be
extinguishes.

ACB:
Low resistance method is applicable only for ac circuit and it is possible there because of presence of
natural zero of current. The arc gets extinguished at the natural zero of the ac wave and is prevented from
restricting again by rapid building of dielectric strength of the contact space.
There are two theories which explains the phenomenon of arc extinction:
Energy Balance Theory: When the contact of circuit breaker are about to open, hence generated
heat would be zero and when the contacts are fully open there is no production of heat. So
maximum generated heat is lying between these two cases, this theory is based on the fact that the
rate of generation of heat between the contacts of circuit breaker is lower than the rate at which
heat between the contact is dissipated. Thus if it is possible to remove the generated heat at a high
rate than the generation, than arc can be extinguished.
Voltage Race Theory : The arc is due to the ionization of the gap between the contact of the
circuit breaker. As the contact separates the resistance starts increasing. If we remove ions at the
initial stage either by recombining them into neutral molecules or inserting insulation at a rate
faster than the rate of ionization, the arc can be interrupted.

ACB: Rating & Sizing
The rated currents of the circuit-breakers, the cross-sectional areas of cables, the transformer
power and the various other characteristic quantities of electrical equipment are represented
by a series of numbers 10, 12.5, 16, 20, 25, 32, ...630, 800 not immediately understandable.
Charles Renard (1847-1905), a French
army engineer
French army engineer Charles Renard invented Renard series which are geometric progressions with
common difference n
Start with 100. Multiply that by 1.2589. Rating is 125.89. Round that to
125.
Multiply that by 1.2589. rating is 158.48. Round that to 160.
Multiply that by 1.2589. Rating is 199.52. Round that to 200.
Multiply that by 1.2589. Rating is 251.19 Round that to 250
And so on…….
User wants a ‘fine’ production in order to find
always the product suitable for his own
requirements, whereas the manufacturer
tends to rationalize the production in a
‘discrete’ way. Renard invented his series to
meet both the requirements.

As per clause number 4.3.2.3 of IS 13947 part-2, Rated current for circuit-breakers, is the
rated uninterrupted current and is equal to the conventional free-air thermal current
(Ith).
Further IS 13947 part-1 clause number 4.3.2.2 says that this current (Thermal current) is
not a rating and is not mandatorily marked on the equipment.
As per IS 13947 part-1, clause number 4.3.2.2, the value of the conventional enclosed thermal current
shall be at least equal to the maximum value of the rated operational current of the enclosed equipment
in eight-hour duty. The conventional enclosed thermal current is the value of current stated by the
manufacturer to be used for the temperature-rise tests of the equipment when mounted in a specified
enclosure.
What is
thermal
current?
Why free air thermal
current, enclosed thermal
current and rated current? rating or free air rating?
Specification check
point: Is it in panel
rating or free air rating?

 Transformer size and Circuit
breaker size are considered
based on maximum demand
and not connected load.
Hence demand factor can be
used.
 For the load calculation, load
factors to be considered shall
be 0.9 for continuous loads,
0.4 for intermittent loads.
 Diversity in operation of loads
can be considered in sizing of
transformer but not circuit
breaker. Continuous de-rated
current of ACB/MCCB due to
enclosure shall be more than
transformer current rating.
Any margin required on
transformer full load
current?

ACB: Rating & Sizing, Icu, Ics & Icw
 Icu - Ultimate short-circuit breaking capacity. Icu is subject to an O-t-CO sequence.
 As per duty cycle if the breaker happens to break its full rated ultimate short circuit breaking
current,"Icu", twice within a time interval of 3 minutes, the breaker cannot be re-used again, even
for carrying its normal rated continuous operating current and must be replaced immediately.
 Ics - Service short-circuit breaking capacity. Ics is subject to an O-t-CO-t-CO sequence. The breaker
is then subject to both dielectric withstand and temperature rise tests.
 This means that the breaker can break its rated service short circuit breaking current three times
within a gap of 6 minutes. And, even after this, the breaker can be used as a switch, capable of
making, carrying & breaking its normal rated continuous current and overload currents. Only, it
would not be able to break any further short circuit currents. As such, the breaker can be continued
in service, albeit, with a short circuit protective device back-up, until the replacement arrives.
 Icw - Rated short-time withstand current. Circuit breakers may be subject to through fault which they
are not intended to clear. While not clearing these faults, the breaker will still need to withstand the
thermal and mechanical stress imposed by the fault current.
 The more ‘Icw’, that switchgear has got, the more co-ordination interval that one has got.
 Many switchgear may have breaking capacities of, say, 50kA, but their one-minute ‘Icw’ may be lower
say, 35kA only. So, this point must be paid attention to, while specifying switchboard.

ACB: Rating & Sizing, Icu, Ics & Icw
 The Ics rating of all LV circuit breakers should at least be equal to short circuit current at the point of
installation. This will ensure that in worst case, the breaker is able to clear the full magnitude of fault
current (full 3 phase ‘bolted’ short circuits at the CB terminals, which is rare of the rarest fault) three
times and remains operational.
 Majority of faults tend to be towards the load end and cable impedance substantially reduce the fault
level. The faults are more likely to be a single phase rather than a three phase thereby reducing the
fault further.
Icw = Ics = 100% Icu will suffice the
requirement

ACB: Rating & Sizing, Short circuit making current of CB
Short circuit making current = 2.5 x
short circuit breaking current
RMS Value
of Short
circuit
current
cosΦ n
I ≤ 5 kA 0.7 1.5
5kA <I ≤ 10
kA
0.5 1.7
10kA <I ≤ 20
kA
0.3 2
20kA <I ≤ 50
kA
0.25 2.1
50kA <I 0.2 2.2
As per Indian standard 8623-part-1 for low
voltage switchgear and controlgear assembly
peak value multiplier is as per table above

ACB: Rating & Sizing, TP, TPN & 4P CB
1. TP Triple Pole
2. TPN Triple Pole and Neutral
3. 4P Four Pole
R Y B NR Y B R Y B N
TP TPN 4P
How to decide between
4P, 3P and TPN CB?
Specification check
point: What type of CB
is asked in specification?
Neutral link

Where to use 4 Pole CB instead of TPN CB
In case of parallel operation of DG (For DG and mains or two DG: IS 3043 clause number 23.2.2)
In case of multiple incomer to a switchboard:
 DG and transformer
 Two transformer
If one DG is failed and other DG sets are in running condition to feed the loads, and there is some
unbalance in loads then depending on degree of unbalance, there will be flow of current through
neutral. During this time, if any technician is attending on failed DG incomer and if he touches the
neutral conductors (which is earthed) he will get electric shock depending on the potential rise in
common neutral due to flow of current through neutral conductor.
This problem will also be there if there is standby DG with transformer and their location is different.
Three pole breaker with direct neutral earthed at their respective end will cause hazard during
maintenance being done at any one of them with other in running condition.

Where to use 4 Pole CB instead of TPN CB
3. If unrestricted ground fault protection is fitted to the transformer neutral, then the bus section circuit
breaker should have 4-poles and preferably incomer circuit breakers should also have 4-poles.
What is unrestricted
earth fault protection?
Ground fault located at the load side of a feeder have two return
paths. As shown above, a ground fault on a feeder at the bus section
“A” will have a current return path in both the incomers, thus tripping
both Bus. The sensitivity of the unrestricted ground fault relay is
reduced due to the split current paths.

ACB: Auxiliary supply
ACB Spring charge motor : 240V AC, Source - Internally tapped from incomer before breaker
ACB Closing & tripping & relays : 220V DC, Source - External
Panel illumination & space heater : 240V AC, Source - Internally tapped from incomer before breaker
Meters & Transducers : 240V AC, Source - Internally tapped from incomer before breaker
Control circuit of O/G feeders : 240V AC, Source - Internally tapped from incomer before breaker
Motor space heater : 240V AC, Source tapped from respective feeder after breaker
Specification check point:
What type of control and
auxiliary supply is asked
in specification?
Selection of control and auxiliary supply inside switchboard should be done carefully as it
will have direct impact on:
1. Size of battery and chargers
2. Amount of cabling
3. Requirement of control transformer

ACB: Protection through relay or integral release
Protection in LV network can either be achieved through external protection relays or integral releases in
CB
There are two types of release:
1. Thermal Magnetic Release - Thermal element for over current and magnetic element for short
circuit.
2. Microprocessor Release - The Microprocessor release works on monitoring of current True R.M.S
value. It is simulated and calculated from peak values, which installed microprocessor, can detect.
1. Integral releases consume lesser wiring, thus saving on cost, complexity
and vulnerability to failures.
2. Operating times of releases are lesser when compared to external relays.
3. There are more components - Main Protection Relays, Aux. Relays, Master
Trip Relays, DC Sources, Shunt Trip Coils, wiring & connections between
them, etc. - that can go wrong in arrangement with external relays.
Whereas in arrangement with integral releases, these are minimized to a
great extent.
4. Energy savings - Coil consumption wattage of relays, coils, etc. reduces
thereby reducing the capacity requirement of the DC Source.

MCCB
What is an MCCB?
“A mechanical switching device, capable of making, carrying and breaking
currents under normal circuit conditions and also making, carrying for a specified
time and breaking currents under specified abnormal circuit condition such as
those of short circuits.”
Same definition as given in ACB in slide no-43 than what is the difference?

MCCB
MCCB differs with ACB on account of following:
Construction- Housing of the contacts structure are contained within a molded
case.
Application- At what level of distribution system, intended CB is to be applied as MCCB.
Governing standards- For ACB: 60947 & For MCCB: IEC 60898, BS EN 60 947-1 General Rules,
BS EN 60 947-2 Circuit Breakers & BS EN 60 947-3 Switch Disconnector
The principle of operation of ACBs & MCCBs have common features.
A contact system with arc-quenching, a mechanism to operate the
breaker, a system to provide a means of protection, control and
indication. However, there are some fundamental differences in
application that should be considered.

MCCB
Where to use MCCB?
This image cannot currently be displayed.
distribution network.
 A normal low voltage installation for an
industrial or commercial application
would consist of an ACB incomer
connecting the low voltage side of the
distribution transformer to the main
switchboard.
 The area known as final distribution would
consist of MCBs feeding the loads directly.
 The area in between is normally where an
MCCB would be located within the
distribution network.
Why intermediate level is MCCB and not ACB?

MCCB
Selection between ACB & MCCB?
Difference between
ACB & MCCB?
Application wise
difference
Where to use MCCB?
Selection between
ACB & MCCB
What are application
wise difference
between ACB &
MCCB?

MCCB
Load
Fault level
Discrimination

MCCB
Category A
This is for MCCBs with no intentional time delay and are therefore not specifically intended for high
selectivity applications, which tend to be a thermal magnetic MCCB. These MCCBs would not have a
Icw rating.
Category B
These MCCBs have a time delay which makes them more suited to applications that demand higher
selectivity .The majority (but not all) of microprocessor MCCBs have a short time withstand Icw
rating. For example a 1250A MCCB may have an Icw of 15ka rms for 300msec

MCCB
Thermal-magnetic circuit breaker –
Contains a thermal element to trip
the circuit breaker for overloads and a
faster magnetic instantaneous element
to trip the circuit breaker for short
circuits.
Electronic trip circuit
breaker – Contains a solid-state
adjustable trip unit. These circuit
breakers are extremely flexible in
coordination with other devices.

MCCB
MCCB is constructed in five major components:
1. Frame (Molded Case)
2. Contacts
3. Arc Chute Assembly
4. Operating Mechanism
5. Trip Unit
 MCCB do not have Icw rating.
 As practice used upto a rating of 630-800A
 Draw-out feature is only possible in
microprocessor based MCCB
The frame provides an insulated
housing to mount the circuit breaker
components. The construction
material is usually a thermal set plastic
such as glass-polymer.

MCCB: Contacts
MCCB use a straight-through contact as well as blow apart contact arrangement. The electrical path
through the contacts is a straight line.
 The two contact arms are positioned parallel to each other as shown.
 Because the current flow in one arm is opposite in direction to the current flow in
the other arm, the two magnetic fields oppose each other.
 During normal current conditions, the magnetic field is not strong enough to
 force the contacts apart.

MCCB: Contacts
 When a fault develops, current increases which increases the
strength of the magnetic field.
 The increased strength of the opposing magnetic fields open the
contacts faster by forcing them apart.
 In comparison, the blow-apart contact design helps to open the
contacts faster than the straight-through arrangement.
 I2T is greatly reduced since arc extinguishment in less than 4
milliseconds is common with blow-apart contacts.
 Electrical equipment is exposed to less heat over a shorter period
of time.

MCCB: Arc chute assembly
 As the contacts open a live circuit,
current continues to flow for a short time
by jumping the air space between the
contacts.
 This forms arc and if isn’t extinguished
quickly the pressure from the ionized
gases could cause the molded case to
rupture.
 An arc chute assembly is used to quench
the arc.
 This assembly is made up of several “U”
shaped steel plates that surround the
contacts.

MCCB: Operating handle
 An operating handle is provided to manually open and close the
contacts.
 Molded case circuit breakers (MCCBs) are trip free, meaning
that they cannot be prevented from tripping by holding or
blocking the operating handle in the “ON” position.
 There are three positions of the operating handle: “ON”
(contacts closed), “OFF” (contacts open), and “TRIPPED”
(mechanism in tripped position).
 The circuit breaker is reset after a trip by moving the handle to
the “OFF” position and then to the “ON” position.
Can this operating
handle remotely
operated?

MCCB: Operating Mechanism
The operating handle is connected to the moveable contact arm through an operating mechanism.
 Operating handle
is moved from
the “OFF” to the
“ON” position.
 In this process a
spring begins to
apply tension to
the mechanism.
 When the handle
is directly over
the center the
tension in the
spring is strong
enough to snap
the contacts
closed.
To open the contacts,
the operating handle is
moved from the
“ON” to the “OFF”
position.

MCCB: Trip Unit for thermal magnetic release type
The trip unit is the “brain” of the circuit breaker. It consists of components that will automatically trip
the circuit breaker when it senses an overload or short circuit.
 MCCBs are Compact. They save considerable panel
space
 MCCBs minimize downtime. Unlike in a fuse-based
system, there’s no searching for a replacement fuse.
MCCBs can be Reset & Switched On immediately
after clearing the fault that caused the tripping.
 MCCBs minimize inventory. Unlike fuses, they are
not "consumables" and hence there is no need to
stock MCCBs the way fuses have been stocked.
Erosion of contact buttons results in higher contact
resistance and subsequent overheating/nuisance tripping
of the MCCBs. To evaluate the extent of contact erosion,
milli volt drop should be measured across the contacts. In
case these values exceed the limit suggested by the
manufacturer, the MCCB needs to be replaced.

MCCB: Type of Trip Unit
Trip Unit or Release type
Electromechanical
Thermal
magnetic
Magnetic
Electronics
Static
Digital µp
based
Numerical
Difference
between digital &
numerical?
Cl f Al t
guide
Cl 7.5 of Alstom
network protection
& automation
guide

MCCB: Type of Trip Unit
Thermal magnetic
For Power feeders having
basic protection requirement
of OC & SC
Magnetic
For motor feeders having
separate motor over load
relay
Specification check point:Specification check point:
What type of release is
asked for power & motor
feeders in specification?

MCCB: Metering Module of releases
Specification check point:
Metering through release
or meter or HMI?
Is it possible transmit metering data like current, voltage and kW from a numerical or
digital release MCCB? Or Can any MCCB display metering data and save external MFM?
Option-1 Option-2 Option-3
Individual meter Common HMI in B/C
compartment
Door display through Schneider
FDM 121 or L&T MTX3.0
Release
Most cost effective
for non ACB feeders
Cost effective compared to
individual HMI however not
used in switchgears mostly
in UPS or process plant
switchboard
Cost effective for ACB feeders

MCCB: Type of Trip
Unit

MCCB: Type of Trip
UnitOL trip between 25 sec &
175 sec at 600 amps with a
40°C ambient temperature
This circuit breaker has an
adjustable instantaneous trip
point from 900 A to 2000 A.
If the trip point adjustment is
set to minimum & a fault
current of 900A or greater
occurs, the breaker will trip
within 1 cycle (16.8ms). If the
trip point setting is set to
maximum & a fault current of
900A occurs, the breaker will
trip between approximately
12 & 55 seconds.

MCCB: Type of Trip
Unit
Frame Size – The term Frame size is applied to a group of circuit breakers
of similar configuration. Frame size is expressed in amperes and corresponds
to the largest ampere rating available in that group.
Interchangeable Trip - The user does not have access to the trip unit on some
Circuit Breakers. This means the trip unit cannot be changed with another.
Interchangeable trip is a design feature that is available on some thermal-magnetic
and solid state breakers. The advantage of a breaker with an interchangeable trip
unit is that the user can change the continuous current rating of the breaker
without replacing the breaker. This is done by replacing the trip unit with one of a
different rating.
GE Frame Sizes: three basic frame sizes up-to 800A. First frame covers 3.2A to
160 A, Second in 200 / 250 / 400A and third in 400 / 630 / 800A.
Specification check point:Specification check point:
Interchangeable trip units
asked in specification or
not?

MCCB
guidesGuidesMCCB Tech detailsL&T MCCB_Technical details.xlsx
guidesGuidesMCCB Tech detailsSchneider MCCB_Technical details.xlsx
Fuse based system:
In this system MCB & MCCB is not used at all. Till 630 Ampere fuse is used and above 630
Ampere ACB is used.
• In any switchgear with two incomer and buscoupler, If fuse is used than changeover has to be manual,
because with fuse you cannot attain automatic changeover. where as if MCCB is used you can attain
automatic changeover.
• Whenever we say fuse it means we are supposed to use SFU or SDF that is switch fuse unit or switch
disconnect fuse.
• In case of MCCB you get spare contacts for remote indications and if you use MCCB with shunt trip coil
you can even use it for remote tripping and operation with relays. Same is exactly not possible with fuse
• In case of SFU of say 132 Ampere please note that the rating should be indicated as 132/132 or 132/150
ampere where numerator indicates fuse rating and denominator denotes switch rating. Generally switch
rating is next higher side to fuse rating but even if it is same no issues, it should be left to vendor as
different manufacturer have different standards.
Comparison between Fuse based and fuse less system:
Broadly because of following advantages fuse based system is preferred:
• Fuse has high rupturing capacity.
• Fuse has shortest operating time.
• Because of quick operating time of fuse we do not need to check the SC criteria at all for LT power cables
• Thermel magnetic MCCB is to be used below 160 Ampere which has operational history of nuisance
tripping.
• Only disadvantage of fuse is that once blown it has to be replaced so spare should be available.

MCB
Construction wise MCB is similar to MCCB with exception that it is
available in lower kA rating hence it is of small frame size and hence
the name miniature circuit breaker
 As practice used up-to a rating of 125A
 Motorized operation not possible because of small size hence not
available with shunt trip coils for remote operation
 Available with auxiliary contacts
Trippin
g class
Application
B Fast Acting. Instantaneous trip response
of 3 - 5X rated current (In). Used for sensitive
equipment and purely resistive loads.
C Normal acting. Instantaneous trip response of 5 - 10x
rated current (In). Used for control circuits and mixed
loads
D Slow Acting. Instantaneous trip response of 10 - 20x
rated current (In). Used for motor, transformer and
solenoid applications.

MPCB
This can match closely the thermal characteristics of a motor. In this case one MPCB will be
sufficient to replace the HRC fuses and the thermal relay and a separate OCR may not be necessary.
Motor overload protection, Thermal
protection has a delayed response, to allow
the high inrush currents when a motor
starts. However, if the motor is unable to
start for some reason, thermal protection
will trip in response to the extended inrush
current.
Protection against phase unbalance and
phase loss. MPCB will disconnect the
motor in either case as soon as the fault is
detected.
Thermal delay to prevent the motor from
being turned back on immediately after an
overload, giving the motor time to cool
down.

ELCB
Difference between Earth Fault and Earth Leakage. As per IS Earth leakage current is flow of
current to earth through extraneous conductive parts of any installation which is in normal condition
electrically sound that is either isolated or non-current carrying.
According to IEC 60947-2, Annex B, Earth fault current is the current flowing to earth due to
insulation fault and Earth leakage current is the current flowing from the live parts of the
installation to earth in the absence of an insulation fault.
In earth fault, a phase conductor insulation is completely damage at point of fault, thus allowing large
current (limited by earth resistance plus circuit resistance up-to fault location)
 Earth leakage current can cause safety hazard by direct or indirect contact.
 Protection against direct and indirect contact is governed by IS 732 Cl no 5.0.

ELCB
 Protection against direct contact electric shock is based on normal common sense
measures such as insulation of live parts, use of barriers or enclosures, protection by
obstacles or protection by placing live parts out of reach. As a result, under normal
conditions it is not possible to touch the live parts of the installation or equipment
inadvertently.
 IS 732 says that protection against direct contact electric shock:
 By preventing the current from passing through the body. Live parts shall be inside
enclosures or behind barriers providing at least the degree of protection IP 2X
 Limiting the body current to a safe value

ELCB
Fault current = Phase to ground
voltage/(Earth resistance + Equipment
resistance up-to frame)
I∆ = V/(Re+Ri)
I∆ = 240/(1.5+20) = 11.2Amp
Voltage across man touching the leaked
machine = I∆ x Re = 11.2x1.5 = 16.75V
Physical body current = 16.75/2000 =
8mA
240V
 IS 732 says that protection against indirect contact electric shock:
 By preventing the current from passing through the body
 Limiting the body current to a safe value
 Automatic disconnection from the supply mains (by RCD). The basis of RCD protection is
to ensure that any voltage, exceeding 50V that arises due to earth leakage currents, is
immediately disconnected. This is calculated by using a simple formula given in BS 7671
Regulation 411.5.3.

ELCB
Internal impedances of the human body
Number indicates the percentage of internal
impedance of human body with respect to
hand to foot impedance.
In order to calculate the total body
impedance Zt for a given current path, the
internal impedance of all parts of the body in
path of current to be added as well as
impedance of skin of contact area.
Electrocution should not be viewed in terms of “current” alone, but in terms of
“contact voltage”. A person gets electrocuted by coming in contact with an object
that has a different potential from his/her own. The difference in potential causes
the current to flow through the body.
The human body has known limits:
- Under normal dry conditions, voltage limit = 50V
- In damp surroundings, voltage limit = 25V

ELCB

ELCB
Range
16A - 63A
Sensitivity
30mA, 100mA, 300mA
Execution
Double Pole (2P)
Four Pole (4P)
Specification
IEC 61008-1/IS 12640 - 1:2000 / EN
61008-1
Residual current circuit breaker (RCCB), also popularly known as Earth Leakage Circuit Breaker (ELCB).
IEC 61008/ IS 12640 part 1: Residual Current Operated Circuit Breaker without Integral
Overcurrent Protection (RCCBs)
IEC 61009 / IS 12640 Part 2: Residual Current Operated Circuit Breaker with Integral
Overcurrent Protection (RCBOs)
RCBO has kA rating in addition to rated current In and
sensitivity

ELCB
RCDs may be distinguished by their technology, as follows.
Voltage Independent RCDs. These RCDs rely on the energy of the residual current to activate the RCD.
These devices are sometimes referred to as Electromechanical RCDs, and are voltage-independent in
operation.
Voltage Dependent RCDs. These RCDs use the mains supply voltage to power an electronic circuit and
the tripping mechanism to activate the RCD. These devices are sometimes referred to as Electronic RCDs
and are voltage-dependent in operation.

ELCB
ELCB + MCB in incomer
or one single RCBO is
optimum design.
Placing ELCB + MCB in
each branch circuit is not
viable
Specification check
point: IS ELCB or
leakage protection asked
in wiring installation?

Wiring Schemes & type of feeders
Different terminology in schemes
Power circuit: It consists of power equipments for switching and protection of the downstream
equipment e.g. ACB, MCCB, fuse switch, power contactor, relays, CTs, etc. Power circuit is generally
indicated by the use of thick lines in the scheme drawing. Such that, it can be easily differentiated from
the control circuit.
Control circuit: The control circuit consists of metering, protection and indication devices with
necessary switches and interlocks. It includes the coil circuit of contactors, ACBs, their auxiliary contacts,
shunt trips of MCCBs and all indicating lamp circuits. The supply for control circuit is either tapped from
one of the phases and neutral of the feeder directly (or through a control transformer) or from external
supplies. Control fuses are used for protection of control circuit. The rating of control fuses is selected
according to the equipments connected in the control circuit.
Cross-reference: For reading convenience, the drawing page on which scheme is drawn is divided into
equal parts. The scheme drawing consists of various different equipments like contactors, relays, ACBs,
MCCBs, etc. These devices consist of power contacts & auxiliary contacts. A specific tag number denotes
each device & its terminals used in the scheme. The family of a device along with its contacts & terminals
are drawn in one of the sections of the drawing page & the section where each terminal is used is also
written alongside. Now the section in which a specified terminal is used also bears a notation alongside,
which indicates the section in which the family of that specified terminal exists & the number of the
device to which the terminal belongs.guidesGuidesMCCB Tech detailsDOL Reference Scheme.pdf

Contacts / Push Buttons: The push buttons are the switches, which can close or open the circuit
whenever it is pressed. The contact is a part which is actually responsible for opening or closing of the
circuit. The contacts are basically of two types. power contacts & the auxiliary contacts. The power
contacts act in the power circuit, whereas the auxiliary contacts act in the control circuit.
NO/NC contacts: The auxiliary contacts are further classified into two parts, namely NO (Normally
Open) & NC (Normally Closed). This implies that these contacts are opened or closed respectively when
the coil is de-energised or is in the OFF condition.
Stay-put Push Button: This type is a special version of push buttons. As its name suggests these
buttons once pressed stay or remain in the same position. Turning the button in the clockwise direction
can open these buttons. Another version of this button can be turned to open only by using its own
specific key to prevent any unauthorized use of it.
Specification check
and control circuit?
Specification check
point: Wiring size for
CT, PT, Annunciation
and control circuit?

Interposing relay: In auto mode, only the auto start command (coming from central control room
through PLC/DCS, etc.) will be able to start the motor. In such cases, the contact coming in the circuit
should be capable to carry the pick-up current of the power contactor coil. This needs special attention
since the contact comes from an electronic circuit. In case the contact is not rated for that much current,
we have to use an auxiliary relay to multiply this command. The contact of the auxiliary relay is then used
in the circuit of the power contactor coil. The auxiliary relay in such cases is known as Interposing relay

Potential free contacts (also called "dry contacts") are simply contacts which are physically operated
with the main device, but not electrically connected to it. For example a motor contactor often has
auxiliary contacts that are operated by the main coil and open and close at the same time as the main
contacts but are not used for control of the motor starter. If they are connected to an outside circuit to
indicate the status of the starter without being powered by the motor supply they would be considered
potential free contacts.
DCS Side Swb Side

Concept of DI/DO/AI/AO/RO
What is Signal?
Electricity (AC or DC) has many uses. Electrical engineer use
electricity for power and energy. Similarly electronics
engineer or telecommunication engineer use electricity in
different sense that is using electricity as a tool/medium to
convey information which is nothing but signal.
A pneumatic (air signal) level "transmitter" device set up to measure
height of water (the "process variable") in a storage tank would output
a low air pressure when the tank was empty, a medium pressure when
the tank was partially full, and a high pressure when the tank was
completely full. This is pneumatic signaling system where air pressure
signals are transmitted using tubes, easily measured (with mechanical
pressure gauges), and are easily manipulated by mechanical devices
using bellows, diaphragms, valves, and other pneumatic devices.

Concept of DI/DO/AI/AO/RO
DI: Digital Input- is potential free contact from any other control circuit. (Hard wired)
DO: Digital output- is potential free output from controller (Hard wired)
AI: Analog input- 4-20mAmp (by paired cable) or any other continuous signal like Voltage,
kVA, Current etc…into controller (Hard wired)
AO: Analog output- 4-20mAmp (By paired cable) or any other continuous signal like
Voltage, kVA, Current etc…as output of controller (Hard wired)
RO: Relay output- Hardwired voltage driven signal with some current rating. Usually 240V
AC, 10Amp. Potential free relay outputs are also available.

Basic Schemes 1: DOL Feeder

Basic Schemes 2: DOL Feeder with local remote selection and remote
start

 Basic Schemes 3: DOL Feeder with star delta starter
 Basic Schemes 4: DOL Feeder with Local remote selector switch + Auto
manual selector switch
 Basic scheme 5: RDOL Feeder with Local remote selection + Auto manual
selection
Process for scheme preparation
• Identification of
type of drives,
Motorized valve,
pumps etc….
Based on process
requirement & kW
rating
• Drive control
philosophy
Based on type of
control & Signal
exchanges • Field and DCS I/O
list finalization
• Logic diagram for
respective breaker
operation
Electrical control
schematics can be
prepared
Specification check
specification?
Specification check
point: Drive control
philosophy, I/O list
should be checked in
specification?

Layout aspect
Clearance criteria:
1. Section 51 of IE rule
2. CPWD norms
3. NEC rule 10
4. TNEB guidelines
5. Tariff advisory committee guidelines
6. CEA guidelines
Points to check:
1. Bus duct entry or cable entry
2. Bottom entry or top entry of bus duct
3. Single front or double front panel
4. Fixed type or draw out type
5. Future space required or not
6. Phase sequence of transformer and switchgear incomer to be checked.
7. In case of space constrained possibility of making L shaped panel or placing two single front back to
back can be checked
8. There should not be any doorway or cupboard behind the switch board. Board should not block the
doorway.

LOW VOLTAGE CABLES
SIZING CRITERIA
The following three criteria apply for the sizing of cables for circuit breaker
controlled feeders:
I. SHORT CIRCUIT CURRENT WITHSTAND CAPACITY
This criteria is applied to determine the minimum cross section area of the cable, so that cable can
withstand the short circuit current.
II. CONTINUOUS CURRENT CARRYING CAPACITY
This criteria is applied so that cross section of the cable can carry the required load current
continuously at the designed ambient temperature and laying condition.
III. STARTING AND RUNNING VOLTAGE DROPS IN CABLE
This criteria is applied to make sure that the cross sectional area of the cable is sufficient to keep the
voltage drop (due to impedance of cable conductor) within the specified limit so that the equipment
which is being supplied power through that cable gets at least the minimum required voltage at its
power supply input terminal during starting and running condition both.

LOW VOLTAGE CABLES
SIZING CRITERIA
Minimum conductor size is given by the following
formula:
A = (Isc x √t)/K
Isc = RMS Short Circuit current Value in Ampere
K = Constant value is 94
A = Minimum required cross section area in mm2
t = Duration of short circuit in seconds
The fault clearing time (tc) of the breakers/fuses per ANSI/IEEE C37.010, C37.013, and UL 489 are:
For medium voltage system (4.16 kV) breakers, use 5-8 cycles
For starters with current limiting fuses, use ½ cycle
For low voltage breakers with intermediate/short time delay, use 10 cycles
For low voltage breakers with instantaneous trips, use 1 cycle
Si.
No.
Parameters Time in Mili
Seconds
Source/Back up
1 Relay sensing/pickup time 20 SIEMENS 7SJ61 technical data
2 Tolerance/Delay time 10 SIEMENS 7SJ61 technical data
3
Breaker operating time
40
L&T make C-Power breaker have typical opening
time of 40 ms and closing time of 60ms)
4 Relay overshoot 20 GEC handbook “Network Protection & automation
Guide”
5 Safety Margin
30
TOTAL TIME IN MILI SECONDS 120

LOW VOLTAGE CABLES
SIZING CRITERIA
Cable selected for a circuit
breaker feeder in 415V or
400V switchgear shall be
suitable to withstand the
maximum rated fault
current for at least
120msec. However taking
allowance of 40 Mili
seconds in the opening
time of circuit breaker due
to various reasons.
Specification check
point: Any short circuit
withstand time given in
specification?

LOW VOLTAGE CABLES
SIZING CRITERIA
Standard ampacity tables are available for a variety of cable types and cable
installation methods and can be used for determining the current carrying
capacity of a cable for a particular application.
Ampacity Deration factor = Product of applicable multiplying factors
among 1 to 4 listed above.
K = K1 X K2 X K3 X K4
K1= Variation in ambient air temperature for cables laid
in air / ground temperature for cables laid
underground.
K2 = Cable laying arrangement.
K3 = Depth of laying for cables laid direct in ground.
K4 = Variation in thermal resistivity of soil.
What if the cable is
laid in two different
type of installations?

LOW VOLTAGE CABLES
SIZING CRITERIA
IEEE standard 525 annexure C,
clause number C3 mentions- An
acceptable voltage drop is
determined based on an overall
knowledge of the system. Typical
limits are 3% from source to load
center, 3% from load center to load,
and 5% total from source to load.
These values are indicated
diagrammatically below.
When voltage
tolerance is 10% why
allowable drop is 5%? Specification check
point: Any voltage drop
criteria mentioned in
specification?

LOW VOLTAGE CABLES
SIZING CRITERIA
Vd = VS + (IRCosф + IX Sinф) - VS2 – √(IXCosф – IRSinф)2
Vd = IRCosф + IXSinф
Recommended Practice for Electric Power
Systems in Commercial Buildings, clause
number 3.6.1 and IEEE-141, Recommended
Practice for
Electric Power Distribution for Industrial
Plants, clause number 3.11.1
X component of voltage drop:
= Vdx = AE = AD + DE = AD + BG
= IRCosф + IX Sinф (Equation-1)
Y Component of voltage drop:
= Vdy = CE = CG-EG
= CG-BD
= IXCosф – IRSinф (Equation-2)
X component of VS:
VSx = OE = √ (OC2 –CE2)
VSx = √ VS2 – Vdy2 (Equation-3)
V = OE –AE = VSx – Vdx (Equation-4)
Now Voltage drop Vd is:
Vd = VS – V = VS – (VSx –Vdx) (Putting the value of V from equation-4)
Vd = VS + Vdx – VSx
Vd = VS + Vdx – VS2 – Vdy2 Equation -5 (Putting the value of VSx from
equation-3)
Now substituting the values of Vdy and Vdx from equation-2 and
equation-1 respectively:
Vd = VS + (IRCosф + IX Sinф) – √ (VS2 – (IXCosф – IRSinф)2 (Equation -
6)

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