Questions about potential issues with 1) N.O. Breaker and 2) VFD/Disc. Switch order

[fandi]

Hello all,
Could you please help me with these questions:
1) Currently there are two 4000A 480V switchboard fed from two utility transformers. These two switchboards (both were designed same time from same manufacturer) share a N.O. breaker (same size as the two main breakers).
I think the intend is if one Tx was down, the N.O breaker will be closed so that two switchboards will be fed from the other Tx.
My question is what happens if the N.O. breaker become N.C. so that both switchboards will be fed from 2 Tx at the same time? In the event of fault current somewhere in this hypothetical system, both switchboards have enough AIC ratings to withstand the combined fault current created by the two Tx.
On the other hand, with the N.C. breaker, two swbds essentially become one bus. There's nothing wrong with two incoming power sources feeding one bus and its downstream loads. We see this setup with PV, micro turbine all the time.
Please advise me.
2) I have a SLD like this order: a MCC (no starter) - a feeder breaker - a VFD - a disconnect switch - a pump. My questions is: if the technician turns off the disc. sw (next to the pump) without locking/tagging out the feeder breaker, the upstream vfd doesn't know this and will continue to run without load. If this situation prolongs, would the vfd be damaged?
Usually I swap the order between a VFD and a disc. sw so that if the technician turn off the disc. sw, it will turn off the VFD also. But there are times when I have to put the vfd ahead of the disc. sw. due to various reasons.

I really appreciate your input. Thanks a lot.

 

[Aleman]

On your first issue I would think you need some kind of mechanical interlock. But I am unfamiliar with situations like yours I haven't personally
encountered a parallel setup like this.

On the 2nd issue you need an aux contact on your local disconnect for the VFD motor. Tie the aux NO contact into the VFD enable circuit, so if disco is off
VFD is disabled. That's what we do on our VFD's.

 

[big john]

What you're describing is a "main-tie-main" or "double-ended" switchboard and it's a very common method of building some redundancy into the system. I don't know that I've ever seen such a setup that wasn't mechanically and/or electrically interlocked, however, and at a minimum you would need:

The OCPDs have to be rated to interrupt the combined available fault current.
The switchboard itself would have to be rated to withstand the combined available fault current, and switchboards have a lower SCCR than switchgear to begin with, so this would be a definite area of concern.
Both transformers would need to be capable of being paralleled, meaning they would likely have to be fed from the same primary source in order to prevent a backfeed in the event of an outage.

Regarding the VFD I agree a make-before-break aux switch in the local disconnect is the way to go, as opening the output of a VFD under load can cause transistor damage.

 

[Pharon]

First, that tie breaker should be kirk-keyed with the other two mains so that only two of the three breakers can ever be closed at the same time. Alternatively, you could make the breakers motorized so that the transfer is automatic. But it should probably be break before make.

If the utility company allows the two transformers to be paralleled, at a bare minimum you would need network protectors on the load side of each transformer to protect each line from backfeeds.

Under no circumstance should that tie be normally closed without the utility company's knowledge, no matter what the AIC rating of the breakers is.

 

[fandi]

Thank you so much for your answers.