480V SWGR MCB TRIP?

[Isaiah]

Last night, we had three MCBs trip at the same time on 480V switchgear located in different buildings. The protective relays, ie SEL-751, showed no indication of spikes or surges (also no SPDs are installed at the 480V).
Any ideas what could have caused the MCBS to open?


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[masterinbama]

Are they all on the same primary feed?

 

[Isaiah]

Are they all on the same primary feed?

They come from 3 different transformers 13.8kV to 480V. Our Project Mgr thinks they were intentionally tripped, but I personally doubt this.


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[jim dungar]

Opening is not the same as tripping.
Your relays have a history file. You need to look at it.
Investigate the tripping logic equation.

For some reason when there are site wide power losses the POCO initially claims they didn't see anything. They seem to first want you to prove it wasn't you before they look at their system.

 

[hillbilly1]

Do they have phase loss relays in them? I have seen gear with that option in older buildings. The utility may have lost a phase, causing the main to trip protecting the three phase loads.

 

[jim dungar]

Do they have phase loss relays in them? I have seen gear with that option in older buildings. The utility may have lost a phase, causing the main to trip protecting the three phase loads.

The OP relays have almost every function you could want for main/feeder protection. Their trip equation defines which functions actually trip the breaker and when.

 

[Jraef]

Your relays have a history file. You need to look at it.

Yes, SEL-751s keep a log of the last 8 to 44 trip events, depending on how log (in cycles) the even lasted. In the manual, it's called a "Sequential Event Recorder".

 

[Isaiah]

Yes, SEL-751s keep a log of the last 8 to 44 trip events, depending on how log (in cycles) the even lasted. In the manual, it's called a "Sequential Event Recorder".

Jraef would it make sense to “extend” the cycles to allow more ride through during surges and spikes?


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[jim dungar]

Jraef would it make sense to “extend” the cycles to allow more ride through during surges and spikes?


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Only if you know you are tripping on these events.
What does your trip equation say? Are the surge and spikes events which are acted on?

The cycles Jraef was talking about, deal with how much memory is being used by the events recorder not the tripping. Most events occur in just a few cycles, so no changes are needed. There are some events, like swells and sags, which occur over a longer period and so need more recorder memory.