Elimination of Incomer Breaker in 600V MCC
Elimination of Incomer Breaker in 600V MCC
(OP)
We have a 4.16kV Primary --Step down to 600V via a Xmer and a 600V MCC in the same S/S. A prposal has been received to eliminate the MCC Incomer and rely on upstream fused 4.16kV contactor to clear any fault in the 600V bus. Reflected short circuit current in the primary side is 4.5kA approx and is within the max. short circuit breaking capacity of the contactor. Is this arrangement feasible? Proposal also suggests installing an o/c relay on the 600V side to trip the contactor. Are any codes ( CEC ) violated here?






RE: Elimination of Incomer Breaker in 600V MCC
I am not sure of your location, but if in the U.S. and I understand your situation correctly, you need to be concerned with NFPA-70E - specifically the issue of arc flash protection. Removing the main breaker can cause the arc flash hazard on the MCC bus to rise due to the slow response of the primary fuse. Hope this helps.
RE: Elimination of Incomer Breaker in 600V MCC
RE: Elimination of Incomer Breaker in 600V MCC
The 4160 V contactor itself is useless for a 600 V fault unless 480 V relaying is installed. The 4160 V fuses will be next to useless.
Arc-flash levels for the entire MCC will generally be extremely high if there is no main breaker.
Installing relays to open the contactor might work, but you have to make sure that the fault current will be in the contactor's interrupting range. Also, most new MV contactors have a built-in time delay on opening to give the fuse a chance to clear the fault before the contactor opens. This would significantly increase arc-flash levels.
Also, consider maintenance and troubleshooting issues with no low side main device.
RE: Elimination of Incomer Breaker in 600V MCC
RE: Elimination of Incomer Breaker in 600V MCC
RE: Elimination of Incomer Breaker in 600V MCC
A very bad idea IMHO. It is a sepaprately derived system and should be treated as a service entrance. At the best you have to stick to 6-disonncet rule as mentioned by dpc. But the starter disconnects may not qualify for that.
The primary side OCPD does not proectect the seconday side conductors/bus at any rate and so recognized by Code.
Most electrical inspectors will not accept this so you better check with them too.
RE: Elimination of Incomer Breaker in 600V MCC
RE: Elimination of Incomer Breaker in 600V MCC
RE: Elimination of Incomer Breaker in 600V MCC
Application of this rule is generally a hallmark of design and construction done to the lowest possible cost.
RE: Elimination of Incomer Breaker in 600V MCC
That should really read
Rarely over the life of the facility will it actually be the lowest cost solution. Use of the six main rule has resulted in some very expensive modifications that would not have been necessary if a main had originally been installed.
RE: Elimination of Incomer Breaker in 600V MCC
A Close/Trip control switch mounted near the MCC would allow local on/off control, similar to a main breaker.
Are there other disadvantages of this cost-effective (cheap) approach?
RE: Elimination of Incomer Breaker in 600V MCC
Off the top of my head, the only one I can think of is knowing what tripped the 4.16kV contactor. Was it a transformer fault or the relay on the 600V system. This could complicate fault determination.
Anyone else ahve any thoughts?
RE: Elimination of Incomer Breaker in 600V MCC
Also, most modern contactors have a built-in time delay on de-energization to make sure the fuse clears any serious high current faults. This would increase the clearing time and arc-flash energy.
Also, the contactor might not be considered a local disconnecting means at the bus.
RE: Elimination of Incomer Breaker in 600V MCC