All,

Just checking to see if there are any understanding that MCCB ratings are not to consider L-L-G available fault levels. As most of you know this number can get quite larger than bolted three phase faults with short runs from Y side of delta/Y XFMR since no fault impedance (Zf = 0) is assumed in nominal calcs.

The simple understanding I have is that in general L-N fault levels are tested at lower values than marked current for UL489 listing, which I think are based off the understanding that Zf is not zero in real world conditions. Yet I don't see a test for L-L-N in UL489 (per IEEE 1015).

Please should L-L-G values be the Available Fault Current (if it is the largest current from all the fault conductor combinations) shown on panels that utilize only Molded case breakers?

Thoughts please, and I apologize if this has been covered before.

Thank you much

I'll take a stab at this but am eager to hear what others say.

I looked at IEEE 551 to see what it says about double line to ground faults. What I gather is, since this is an unbalanced fault you really need to know the impedance of the return path to calculate accurately. At that point it is no longer a bolted fault. As you pointed out the bolted LLG fault current can be higher than the bolted 3 phase fault only when it is very close to the source with an assumed impedance of zero from a bolted fault condition.

The question then becomes, under what scenario would / are you likely to have a bolted LLG fault in a solidly grounded system very close to the transformer? IEEE 551 seems to say that is extremely unlikely.

Here are a couple of excerpts from IEEE 551 for reference:

"Line-to-line-to-ground faults, Figure 2-10(c), are typically line-to-ground faults that have escalated to include a second phase conductor. This is an unbalanced fault. The magnitudes of double line-to-ground fault currents are usually greater than those of line-to-line faults, but are less than those of three-phase faults. Calculation of double line-to-ground fault currents requires the use of symmetrical components analysis. The impedance of the ground return path will affect the result, and should be obtained if possible."

"The most likely cause of an interruption of a line-to-line short circuit by one pole of a circuit breaker is a double line-to-ground short circuit in a three-phase system that is not solidly wye grounded, such as an ungrounded system, high resistance grounded system, or corner of the delta grounded system. Full line-to-line recovery voltage can occur across a single interrupting pole when one phase is grounded on the source side of a circuit breaker and another phase is grounded simultaneously on the load side. For a corner of the delta grounded system, this might be a common occurrence. The situation is less likely to occur in high resistance grounded or ungrounded systems where operating procedures require the first ground to be removed as soon as practical."

Bbaumer,

thank you for the time and consideration. Those definitely are some balancing thoughts to consider.

Cable libraries in ETAP/SKM have zero sequence impedance values in them. My current thought that this captures a simple return path impedance of the raceway and earth(it assumes the raceway attached to earth at both ends thus earth is involved). Please is my understanding way off or does anyone know how the zero sequence values are developed for these cable libraries. ETAP can calculate cable impedance and it changes with an aux neutral conductor is added (which I think means the return path is parallel combination of raceway/earth/neutral in that situation)? Appreciate any consideration again.

Thank you much.

Suggest reading the definition of "Fault Current, Available" in the NEC. Since it includes the word "largest", I take that to mean look at the worst of all fault types. Also see the informational note and diagram.

Have you actually run a study where the LLG currently was higher than the LLL current and it caused the bus or protective device ti fail the evaluation so you had to go to the next higher standard withstand or AIC rating or look for a series combo?

Example - LLL - 63,500 amps; LLG - 65,300 amps

Pass LLL, fail LLG so had to go to 100K rated instead of 65K

Thank you again

Yep the LLG is about 1.5 KA greater than LLL and it is approaching the KAIC of the breaker marked rating 25KAIC. Just don't want to cause an issue if is not accepted practice to use LLG on molded case breaker ratings for 3ph/4W Wye systems. If I use 1.04 pu instead 1.0 for prefault, I can exceed the rating on LLG fault levels.

Thank you again.

If you're that close I'd be looking to see if you already have a series combination rating.

Yep I am just performing the calcs for this effort. It's an old panel (Siemens) and just installed upstream switchboard (GE) feeding it. So probably no ready series combo that I am aware of for different manufacturers.

Thanks again.

I'm not aware of any cross manufacturer tested series breaker/breaker combos either. Fuse/breaker, yes. Breaker/breaker no.

Might have to replace some breakers. Also, double check your feeder length just to be sure. Sometimes I enter 1'0" when I'm building a model when I don't yet know the length and forget to go back and update. That's where using the Data State in SKM comes in handy.

Good luck.

Thank you much. It definitely sounds like an option to look into.