Single Phasing circuit breaker

[kntx]

I'm attempting to understand a problem we had several weeks ago. I'm a mechanical engineer, so I probably don't have some of the terminology straight either.

We had several failures in the circuit breakers on equipment fed from our 5B 480V bus during a startup on our power plant. In one of the breakers a control transformer was burned up, some relays powered from the 5B bus were melted. We found in the circuit breaker from the 5B station service transformer to the 5B 480 Volt bus, the "b" phase "stab" was bent down and was apparently not making good contact with the b phase "fingers" on the back of the breaker.

An electrician bent the stab back up into line with the other two and we were able to rack it back in with no problems. He said it was "single phasing". how does this relate to burned up control transformer and relays?

Is it recommended to inspect other equipment which is powered from this bus.

We had no problems on any of the other 480v busses.

Thanks in advance for your replies.

 

[buzzp]

I dont see that being an issue with relays and transformers. More than likely, the origination of the problem was due to the arcing and sparking occurring due to the less than adequate connection on the b-phase. There is virtually no way a single phase condition would ever burn up a relay or a control transformer (these are single phase of course). It may help burn up a three phase transformer if its loaded because the other phases will have to supply that much more current.

 

[waross]

If by relay you mean a small contactor, then some of these can be burned out by low voltage. If the voltage is too low to pick-up and seal-in the contactor it will draw excessive current and often fail. That in turn can overload a small control transformer and burn it out. Your poor connection may have caused a low voltage condition. Single phasing may have caused a low voltage condition. It's hard to say exactly without seeing the equipment and knowing the exact connections but it is possible.
yours

 

[jraef]

I agree. Bad connection = high resistance = voltage drop. Voltage drop through induction devices such as transformers and relay coils = excessive heat = failure. Quite possible.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[buzzp]

The poster asked about relays and control transformers.
The control power should of been fused so it should not have burned up any control power transformer if it had the correct protection. The relays would have had the same issue as a contactor mentioned above. However, the control power transformer fuse should have protected them as well if the theory of excessive current draw as a result of low voltage is valid, and I think it is but the fuse should of popped.

I can see some of those things happening but not all of them. I guess if I could see it, rather than rely on a few paragraphs, I would have a better idea and may agree 100% that it would be possible. Stil can't see the CPT burning. This is one of them things you have to see to evaluate.

 

[advidana]

I would check and test if possible all 3 ph motors connected to this bus for damage and burnt areas on winding.