High current MCCBs in parallel

[jraef]

I see in the NEC (US National Electric Code) that they have a reference to running circuit breakers in parallel, but only if it is a factory assembled unit listed for that application.

Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.

My question is, has anyone ever seen this done? It makes no sense to me, I can't see how you would avoid nuisance tripping. Your parallel circuit would have to be nats-on perfect to prevent one breaker from seeing more than it's share of load and tripping. Even a slight additional resistance in one termination would cause a shift in conduction to the other side. I can't see how 2 breakers can effectively share a load in parallel.

The reason it came up is that I am proposing some 1600A MCCBs on an inverter application, my friendly competitor is proposing to the end user that they can do this with 2x 800A MCCBs in parallel (probably because he stops at 1200A) feeding 2 800A contactors, which are both tied together again downstream. I cried BS, they countered with using that NEC reference to say it is legit. Mind you, I have now also cried fowl on the fact that I doubt they have a "factory assembled and listed" offering, but the entire exercise has me wondering. Does anyone really do this?

It also seems to me that if this were to possibly work, the only way it would is if the breakers had their line and load sides connected right at the terminals, then again on the contactors' line and load sides. To have them separate as they enter the contactors would be inviting the risk of damage if the contactors closed at even very slightly different times, a likely scenario. I also see problems in getting them to trip together. The only way I see to do it would be with UVRs and mutually series circuits from aux contacts in each breaker, but then I wonder how you would even get them to close the first time.

Any comments would be appreciated.

 

[waross]

I have seen this in catalogues a few times. The breakers are tied together with bus straps at both the line and load side, and share a common operating mechanism.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SteveFehr]

Are these two 800A MCCBs feeding two inverters, or are they feeding the same 1600A inverter?

Parallel feeders like this are common in UPS applications where the device that's fed offers regulation of the current and parallel redundancy is standard practice. AFAIK, NEC prohibits straight-up parallel feeds of one bus to another, though there are circumstances where it is generally allowed; maintenance bypass switches and closed transition transfer switches, for instance. I can't recall ever seeing codes that specifically speak to this besides 240.8, though.

Every 240.8 device I've ever seen used a single yoke to tie all the breakers together. If the circuit is balanced and both feeds are the same wire, same distance, same connectors, etc, both feeds will carry the exact same current in theory and in practice. In a fault or overload situation, one breaker will naturally trip first, dumping all the load to the other, which would quickly follow suit; the conductors will remain protected. There will be more current availible for the fault, though, as the path to a fault in either cable can be traced through the tie point, though this is really no worse than if the conductors had been oversized. With this respect, it's not so much a safety hazard as simply poor engineering practice, and one that could be eliminated by using the proper components.

You also could point out to your customer than running in series exponentially inreases the chance of failure over a single unit

 

[dpc]

I've never seen it done with breakers. It's pretty common with medium voltage current-limiting fuses.

Seems like a a jury-rigged solution, since the larger breakers are readily available.

Two thumbs down!

 

[Zogzog]

Sq-D does this now with larger Masterpact breakers

 

[davidbeach]

What Square D is doing, from what I've seen, is putting two poles in parallel on each phase within a single breaker package.

 

[jraef]

It is feeding one 1600A inverter, so no, this is not a backup parallel power path. The mechanical linkage would solve the tripping issue I suppose, and if both side were directly bussed together I suppose it could work, but I just don't see the reason to do it when there is an alternative.

I'm feeling stronger that they are not doing the right thing here, probably mislead by the salesman who had nothing else to sell. Sad really...

 

[Zogzog]

David, you are correct, and you described it better, but I think that would qualifiy as an example of a "Factory assembled unit listed for that application"

Jref, I think you are right, this guy is just trying to make a sale and is in over his head. Why not propose 16 x 100A breakers in parrell, that would be even cheaper

I have no idea how you would coordinate the trip units, seems to me like it could be a real mess and not protect the equipment.

Zog

http://www.cbsnuclear.com

 

[burnt2x]

My experience with fuses in parallel were fuses protecting SCRs used in a shipunloader dynamic braking control. The main fuse is in parallel with a smaller fuse with a spring that pushes-out an indicator when that phase/line experiences fault! I guess that's one of the "factory assembled unit" referred to by the NEC.