# What determines what type of short-circuit current is highest?

I'm doing a short-circuit analysis of a system, and started wondering about the following: What determines which type of fault current is the highest (and lowest)?

The screenshot below shows the result for 4 different switchboards. As can be seen, for the two first switchboards, an LG-fault results in the highest current, while LLG and 3P-faults result in the highest currents for the two last switchboards.

I know that in an isolated system, the LG-current will be (close to) zero, but other than that, I'm not really sure what makes up the differences. The results below is for a solidly grounded system, where the zero-sequence impedances are 3 times the positive sequence. None of the switchboards are connected (only through a high voltage distribution bus).

Does anyone know why it differs from switchboard to switchboard? I would think that for a solidly grounded system, one of the types would always be the largest. Are there any system characteristics that could tell me (before I run an analysis), which of the types is most likely to be highest?

Thanks!

# Update:

As Li-aung Yip (thanks) points out, the zero sequence impedance is normally a bit lower than the positive sequence in a transformer. I see the question text above is not completely correct: The zero sequence impedance is 3 times the positive sequence for lines, while for transformers, it looks something like this (this data is used when achieving the results above):

• More of these kind of questions, please! :) – Li-aung Yip Dec 4 '13 at 8:50

A power transformer will commonly have a lower zero-sequence impedance than positive sequence impedance. See the J&P Transformer Book. Typically a transformer will have $0.8 Z_1 \leq Z_0 \leq 1.0 Z_1$.