Worse case or not ?

[maintenance man]

There are two mcc's with an opened tie breaker between them, each mcc being feed by a seperate cb from the main switchgear. The mcc's each have a main cb. One mcc is listed at a 3 and the other is listed as a 1 after an arc flash study. Should for any reason the tie is used how would the mcc be rated 3 or 1 ? I thought the study used the worse case possible which would have been a 3 for both mcc. Should each mcc have its own arc flash level label before the tie is used ? Does the tie ever have a label off its own ?
The mcc's are 480volt and 400amps each.

 

[augie47]

Ignoring your arc-flash question for a moment, if I may.
Is there a "kirk key" or similar arrangement that prevents both panels to be energized by
their normal feeds from MSB and the tie breaker being closed also ?

 

[jim dungar]

There is no set of official, or even standard, rules for how to label equipment based 'worst case'.

Each company need to create their own set of work rules, an Electrical Safe Work Practices program, which addresses the method they use to determine appropriate PPE.

In general, I believe most companies want the equipment label to be the 'worst case during normal conditions' with exceptions, both good and bad, handled by the 'live work permit' process.

for example:
Data center, worst case is when facility is running on generator power. Company policy is to not allow energized work while running on generators, so installed labels reflect worst case utility values. Permitted emergency work would require PPE to be 'looked up' in the AF report.

Manufacturing plant with key operated Tie breaker which is used for closed transition. Installed label for Tie breaker is higher than those on the rest of the equipment. The authorized switching procedure requires a live work permit which addresses the inherent problems of closed transition switching.

 

[maintenance man]

Yes there is a kirk key system in place. Only tie cb can close when one of the main cbs is open and locked out .

 

[maintenance man]

Jim , first thank you for your reply. Reading your last sentence about a permit for live work . Are you saying operating these cb,s require a energized work permit ? I guess we would have to open switchgear cb or is the mains in the mcc ok to eliminate the energized permit.

 

[jim dungar]

Are you saying operating these cb,s require a energized work permit ?

That is up to your company's Electrical Safe Work Practices program.
What does it say about procedures for operating key interlocked equipment?

It is too bad that many companies seem to feel compliance to NFPA70E is done once they acquire some Arc Flash labels and PPE.
I believe, the 'authors' of NFPA 70E intended that its grey areas might be best addressed on a company by company basis, as opposed to thinking there is a single way to be safe.