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Short Circuit Rating
2

Short Circuit Rating

Short Circuit Rating

(OP)

Hi all,

I saw the short circuit rating of a distribution system as following

1. for 27.6/16.0 kV    16kA
2. for 13.8/8.0 kV     34kA
3. for 2.4/4.16kV      56kA


Why short circuit rating is higher when system voltage is small compare to higher system voltage ?

Is not higher short circuit rating in higher voltage system ?

I will appreciate for reply.

RE: Short Circuit Rating

2
For the same MVA, if the secondary voltage is low, the FLA (VA/(1.732 X V))is higher. Since Isc is more or less equal to FLA/Zpu, then Isc will be greater:
Example: 10 MVA trafo; %Z = 5.79% or 0.0579 pu. Vs1 = 2400; and Vs2 = 4160:
Isc1    = 10,000/(1.732 X 2400 X 0.0579) = 41.5 kA.
But Isc2= 10,000/(1.732 X 4160 X 0.0579) = 23.97 kA.

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