SKM ANSI fault study multiplier

[rbyrne15]

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.

 

[rbyrne15]

So I did some number crunching after a small revelation I just had - it seems like the number comes from the expression for the assymetrical peak current (which is based on the ac and dc components).

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.

 

[EddyWirbelstrom]

The Crest is the peak of the waveform half a cycle after fault inception, assuming worst-case where fault occurs at voltage zero and current lags voltage by 90°.
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

 

[gepman]

Attachments are made on Step 3 of your reply, just above "Preview Post".

 

[EddyWirbelstrom]

The Crest is the peak of the waveform half a cycle after fault inception, assuming worst-case where fault occurs at voltage zero and current lags voltage by 90°.

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.