Amnesia

[pridelion]

Whats the formula for that AIC rating for a main breaker based on the impedance of the supply transformer?

 

[bphgravity]

Re: Amnesia

KVA / Voltage / xfrmr Z = Available fault current

 

[jtester]

Re: Amnesia

I agree with Bryan for a single phase transformer, but not for a three phase transformer. The way I remember it is to calculate the full load current of the transformer and then divide that by the impedance as a decimal, not in percent.

Jim T

 

[bphgravity]

Re: Amnesia

"Z" is the symbol for impedance not "%". Even so, 5% for example is the same as .05 in decimal form.

 

[rattus]

Re: Amnesia

"Z" in this case does not carry the units of impedance. It is per unit impedance, Zpu, expressed as a decimal fraction. The formula computes the rated current from the KVA rating and then multiplies the rated current by the inverse of the per unit impedance. That is:

Zpu = Ir(A) x Z(Ohms) / Vr(V); by definition,

1 = Iss(A) x Z(Ohms) / Vr(V); short circuit condition,

Dividing one equation by the other yields,

Iss(A) = Ir(A)/Zpu = (KVA/Vr)/Zpu

Where Iss is the short circuit current, Ir is the rated current, and Vr is the rated voltage.

OK?

[ September 12, 2005, 09:16 PM: Message edited by: rattus ]