SKM ANSI fault study multiplier
SKM ANSI fault study multiplier
(OP)
Hi everyone,
I'm conducting an ANSI (A_Fault) study using SKM Power Tools and have a question about the multiplier used for the crest kA it "spits out."
I have figured out from the ANSI/IEEE standards that the multiplier for momentary current (i.e. 1.6) is for the half-cycle current, but where does the crest current multiplier (i.e. 2.7) come from?
This value of current seems like it would be very siginificant in the application of breakers/fuses since it's larger in magnitude than the momentary value.
Any advice would be appreiciated, thanks.
Richard Byrne.
I'm conducting an ANSI (A_Fault) study using SKM Power Tools and have a question about the multiplier used for the crest kA it "spits out."
I have figured out from the ANSI/IEEE standards that the multiplier for momentary current (i.e. 1.6) is for the half-cycle current, but where does the crest current multiplier (i.e. 2.7) come from?
This value of current seems like it would be very siginificant in the application of breakers/fuses since it's larger in magnitude than the momentary value.
Any advice would be appreiciated, thanks.
Richard Byrne.






RE: SKM ANSI fault study multiplier
Just as in the calculation of the multiplier for the rms case, the peak (crest) multiplier implicitly assumes a fault point X/R ratio of 25 and that the maximum value occurs at 0.5 cycles into the fault current cycle.
Any confirmation/contradication is appreiciated, but this is the thinking I'll use for now.
Cheers, Richard.
RE: SKM ANSI fault study multiplier
I could attach an excel spreadsheet ( if I knew how ) to show that if I"k = 1, and X/R = 15, then i peak = 2.56 and I asym rms = 1.52
RE: SKM ANSI fault study multiplier
RE: SKM ANSI fault study multiplier
The 'first cycle asymmetrical rating' is the rms of the current over the first 0.5 cycle following fault inception.
The attached spreadsheet shows that if I"k = 1, and X/R = 15, then i peak (crest) = 2.56 and I asym rms = 1.52
I asym rms is also called "close and latch" or "making" capability, previously known as "momentary" capability.