Answer / sobhuza, za

For those who this my answer (Answer 2) is incorrect. It is
important to remember that HT circuit breaker has two
current ratings. One is the continuous current rating. The
other one is the maximum fault breaking rating that the
breaker can carry for about say 2 seconds. The former is
what the question is about. The latter is in kA range
typicaly 25kA or so. Therefore 1250A is just a normal load
current that the circuit breaker can carry.

Answer / rahul

bcoz CBs are supposed to interrupt fault current which can
be several times larger than the continous load current.
( for further clarification see CB specifications)

Answer / shan80

there is no necessery to use 1250 vcb for 50A u can use
630A vcb for that load.1250A is only manifaturing capicity
630A vcbs also available and these rating r standard. 1250A
rating is only normal current rating not shot circuit rating

Answer / vikash kumar

HT breaker is sized based upon the fault current. Here 1250A is the fault current. 50A is the normal current and overload protection is provided by CT and over current relay.

Answer / arjun

it is coz the voltage at the HT side is much high & so is the pressure of the voltage is also much high, therefor the selection of the HT breaker depends on various other factors such as pole distance(distance between the poles of the breaker ) ,voltage (The voltage rating of the circuit breaker must be at least equal to the circuit voltage.), ambient temperature,overload,continuous current rating & max current it can handle before tripping .so even if the current though a HT circuit is low we need a breaker of much higher ratting

Answer / v.v.j.rao

The rating of the breaker current 1250 means it can carry
1250 amps after that % over loaad depends on O/Load setting
of 50%,75% or 100% only.If you put this breaker at 50 amps
oad this will not trip under any O/L condition of that
particular load.

Answer / sobhuza, za

It's because the equipment is rated based on the capacity
levels and not at loading levels. CB ratings are
standardised, as it may happen that the loading on one
circuit is 50A, but on another it can be 900A. So for
rating purposes it is better to standardise at say 1250A so
for ease of procurement. Remember that when the load
increases over time and you won't have to change CBs due to
loading increase.

Answer / ajay

I think it is according to its circuit breaking capacity at
fault, that how much current it can bear durring breaking
of curcuit

Answer / sbb

It is because we cannot measure the high current directly.So
for this we will keep CT's to measure high currents.In
primary side it will be 1250A and in secondary side it will
be 50A,this is nothing but CT Ratio.

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