MLO MCCs protected by LVPCB

[wroggent]

I was out visiting one of my company's plants recently and found some MCCs that are main lug only. The protective device for each MCC is a low voltage power circuit breaker in a nearby switchgear. I know switchgear is built to have a 30 cycle withstand rating, and the LVPCBs can be selected without instantaneous trips. I'm not really familiar with the standards that MCCs are built to, but I don't think they have a 30 cycle withstand rating. Thus, I'm thinking it's probably not a good idea to protect the MCCs with LVPCBs. I suppose it might be ok so long as an instantaneous trip is used. I am looking for advice on the acceptability of this application.

I read this thread but I'm not sure what the conclusion was.

http://forums.mikeholt.com/showthread.php?t=139942

The thread made it sound like it's fairly common to protect MCCs with LVPCBs out of a coordination concern. I've only seen MCCs protected by MCCBs in other installations.

Also, the focus of that thread seemed to be on the additional arc flash hazards associated with no instantaneous trip vs. the hazard that might be associated with shutting down an entire MCC. I think this was the focus because of the assumption that branch circuit breakers will interrupt faults outside of the MCC. I don't see how this will always be the case - the branch circuit breaker may fail to open.

 

[wroggent]

Meant to put this thread in Electric Power & T&D. Sorry.

 

[dpc]

Not quite sure on your question, but it is very common to use LVPCBs in 480 V and 600 V switchgear to feed downstream MCCs. In these situations, coordination will be much better if there is no MCC main breaker, since it will almost always be a molded case circuit breaker.

With LVPCBs, the instantaneous trip is optional. With no instantaneous trip, the LVPCB will generally be completely selective with the downstream MCCBs in the MCC. MCCBs must have an instantaneous trip, so in general, complete selectivity between two MCCBs in series is not always possible.

Absent an instantaneous trip, the short-time trip will be slower causing a higher arc-flash incident energy. The trade-off is between vastly improved coordination and lower arc-flash energy at the MCC bus.

 

[wroggent]

Will the MCC be damaged if it is allowed to experience a fault for more than 4 cycles or so? What would this damage be? Melted bus bars?

 

[jraef]

If I remember correctly, NEMA MCC bus fault bracing is for 3 seconds. So as long as the fault current is below the bracing level, that should not be a concern. That does not preclude proper coordination of course.

"Will work for (the memory of) salami"

 

[wroggent]

Jeff, would that be in ICS 18-2001 (R2007)? If so, I took a look but didn't see anything about the duration of a short circuit test.

It does say "The short-circuit tests shall be conducted in accordance with the procedures and requirements of UL 845." UL 845 is over $700

 

[dpc]

I believe the bare bones NEMA requirement for MCC bus bracing is 3 cycles. You can probably specify a 30-cycle rating. But in practice, if the fault is downstream of a feeder device or starter, this device will have instantaneous trip. If it is an MCC bus fault, the bus is damaged already.

 

[dpc]

Additional info here:

http://www.scribd.com/doc/98453850/Selective-Coordination

 

[jraef]

Eh, 3seconds, 3cycles, I was close

"Will work for (the memory of) salami"