Answer / nilesh kumar singh

Selection of Circuit breaker
CURRENT RATING:
Transformer Rating (X)kVA
%age impedance of transformer - Z%
I Full load = X*1000/Sq. root 3*VL
Where X = Transformer rating in kVA
VL = Line Voltage
BREAKING CAPACITY:
Breaking Capacity of the breaker> I Fault Current

Fault Current (Level) = 1.1*VL/Sq. root 3*Z
Where Z = Impedance of Transformer
VL = Line Voltage

Z= %Z*10*(kV)*(kV)/KVA
MAKING CAPACITY:
Making Capacity depends upon the following:
a) Power Factor
b) Instant of switching ratio 'n' between required rate s/c
making capacity and rated s/c breaking capacity.

Answer / kapil

this depends on the application type where you are using
this circuit breaker. actually we will have to know whether
we require some delay in tripping after the overcurrent or a
frequent operation is needed. if a delay is needed and we
will have to bear some over temperature conditions then we
use thermal relay which has some delay and low trip current
at high temp. when we require the frequent operation then we
use magnetic type relay. also we will have to mind the
operating environment, nominal current rating and max peak
current bearing capacity and delay time of the circuit breaker.

Answer / sandipan gandhi

CB is selected on the basis of it's Breaking capacity,
Making capacity, operating voltage, operating
current,system frequency, insulation level & type of use
i.e indoor or outdoor.

Answer / mahesh.s

Selection of circuit breaker mainly depends on the voltage
capacity for which its is connected in the circuit like
3.3kv,6.6kv,11kv,33kv,...etc and the breaking capacity of
that breaker means at wht current it will trip the ckt...
and making capacity,
Thermal capacity,
application like indoor or outdoor usage.

Answer / ammar

we have to calculate the rated current of the load
connected to the circuit breaker from the power equation
p=√3.V.I.cosθ
then we can decide which is the suitable rating for the CB.

Answer / gurbir singh

a ciruit breaker depend upon the total capacity of load

Answer / magesh

We use circuit breaker for short circuit and overload
protection
For OL -2.5 times of the rated I
For Short circuit-12times of I

According to this calculations breakers are selected

Answer / andy

people you all are crazy, can you answer simple question
and give simple example ?

Answer / rk

Rateed voltage
Normal current
breaking curren i.e. fault current fault mva
aux supply volatge
C-O-c seq for auto recl function

Answer / majid amoudi

direct technical answer:-
suppose if d interviewver asks u
"for 11kva transformer what size of d ckt breaker shud b used"
answer-
11kva= 11,000va

11,000 divided by 220v(depending on various countries voltage rating)=50A
50A is d current passing thru d ckt...according to NEC(national electrical code) current shud be 125%...
so, 50* 1.25=62.5A
d breaker will trip if d current passes more dan 62.5A & d ckt breaker size shud be selected 4 times d nominal ckt breaker current i.e 62.5*4=250A.
so..a MCCB(moulded case circuit breaker) of 250A shud be used for 11kva T/F.

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