breaker trip investigation

[mshields]

A client of ours had a 4000A Insulated Case, 4000A circuit breaker trip for no apparent reason. The square D equipment is only 10 years old. The client's maintenance guy reset the breaker without looking to see what target it tripped on (this is now lost to eternity and I did check with the vendor). We did a quick coordination study of the board. i.e. just wanted to show that the feeders (all of which have ground fault), coordintated properly - and they do.

And I'm wondering what else I can do. Injection testing of the 4000A breaker is one thing I suppose to make sure it's functioning per spec. But what if anything after that.

Now, this is Surgery building in a hospital. If it was anything else, I'd say, call me if it happens again and this time make a note of the trip targets (not that would narrow it down much - I'm assuming it's a GF anyway). But because of the nature of the facility, I don't want to leave any stone unturned. Any suggestions?

Thanks,

Mike

 

[zog]

There is always a reason, just being a cheap insulated case is reason for suspicion. LOL.

Seriously, only 10% of faults are self clearing on 480V systems, could have been a rat, snake, Yankee fan, etc. But I would start with breaker testing and meggering the load.

Is this a Masterpact? Lots of issues with those that could be a clue. Post the details of the breaker and trip unit. What sort of loads?

 

[mshields]

additional info

additional info

thanks for the input.

Yes, it's a Masterpact NW40H with a Micrologic 6.0 trip unit.

As for the load, they are as follows:

1600 MCCB breaker to an MCC
4 - 800A frame breakers to ATS's
1-250A MCCB to a parking garage.
1 800A MCCB to a DP (don't know what's on it but I think it distributes to other panels in the facility)
1-800A MCCB feeding another small administrative building.

All of these MCCB's have full function trip units, all with LT, ST, I and GF.

So as you can see, there are lots of suspects. Would you megger each individual feeder?

 

[mshields]

By the way

By the way

By the way, nice touch with the Yankee fan comment. Always a well received line of humor in this neck of the woods!

 

[rcwilson]

If it is a 3-phase 4-wire service take a look at the neutral CT input to the breaker's ground fault/ trip unit. If the neutral CT is backwards or the wires are crossed the sensor will treat neutral load current as fault current. Instead of subtracting neutral current from the sum of the phase CT's, the reversed CT adds it. With a 1000 A setting, the unti will trip on 500 amps neutral current.

The reversed CT can go unnoticed until neutral current due to unbalanced loads or harmonics gets large enough to trip or a downstream fault occurs.

Best way to test the neutral CT is by primary injection in place, but that takes an outage on the switchgear.

Another possibility is poor coordination of the short time function with motor inrush. At a hopspital is it possible that the trip occurred when or just after an ATS transferred load back to utility from the generator? A short utiltiy outage or voltage dip might have the same effect: motors still turning and acting like induction generators, then power returns and the inrush current is high enough to activate the short time protection.

I've seen both of these scenarios cause nusiance trips.

Or it could just be a bad trip unit.

 

[jim dungar]

Breakers like this should be maintained yearly (at a minimum cycled on and off). They should be 'performance tested' every couple of years (3-5?).

Why does it seem that 'critical operations' never have time and money for proper preventative maintenance, but always seem to be able to accomodate an emergency?

 

[hillbilly1]

If it is a 3-phase 4-wire service take a look at the neutral CT input to the breaker's ground fault/ trip unit. If the neutral CT is backwards or the wires are crossed the sensor will treat neutral load current as fault current. Instead of subtracting neutral current from the sum of the phase CT's, the reversed CT adds it. With a 1000 A setting, the unti will trip on 500 amps neutral current.

The reversed CT can go unnoticed until neutral current due to unbalanced loads or harmonics gets large enough to trip or a downstream fault occurs.

Best way to test the neutral CT is by primary injection in place, but that takes an outage on the switchgear.

Another possibility is poor coordination of the short time function with motor inrush. At a hopspital is it possible that the trip occurred when or just after an ATS transferred load back to utility from the generator? A short utiltiy outage or voltage dip might have the same effect: motors still turning and acting like induction generators, then power returns and the inrush current is high enough to activate the short time protection.

I've seen both of these scenarios cause nusiance trips.

Or it could just be a bad trip unit.

Yes, all of the above are good points, but be aware the CT will be backwards compared to a phase monitoring CT if it monitors only the neutral (the phase CT's are built into the breaker, unless it's a window CT that captures all of the conductors), also look for load neutrals terminated on the wrong side of the CT.

 

[zog]

If it is a 3-phase 4-wire service take a look at the neutral CT input to the breaker's ground fault/ trip unit. If the neutral CT is backwards or the wires are crossed the sensor will treat neutral load current as fault current. Instead of subtracting neutral current from the sum of the phase CT's, the reversed CT adds it. With a 1000 A setting, the unti will trip on 500 amps neutral current.

The reversed CT can go unnoticed until neutral current due to unbalanced loads or harmonics gets large enough to trip or a downstream fault occurs.

Best way to test the neutral CT is by primary injection in place, but that takes an outage on the switchgear.

All true but I think this would have shown up in 10 years. Maybe it has?

Another possibility is poor coordination of the short time function with motor inrush. At a hopspital is it possible that the trip occurred when or just after an ATS transferred load back to utility from the generator? A short utiltiy outage or voltage dip might have the same effect: motors still turning and acting like induction generators, then power returns and the inrush current is high enough to activate the short time protection.

I've seen both of these scenarios cause nusiance trips.

Very possible

Or it could just be a bad trip unit.

Yep, lots of micrologic issues.