400A breaker heating up and tripping

[GoldDigger]

The only thing that I can blue sky as a cause of heating which would not show up on a power analyzer would be spikes or high frequency components which are for some reason not being counted by the power analyzer. But if the analyzer is designed to detect transients and spikes, and has a decent frequency response, that seems highly unlikely.
The other guess is that the power analyzer was not in fact recording any time that the breaker tripped. Which says that something intermittent, but long duration is involved.
Complicated by the possibility that a fault on a single motor can only cause a trip if there is additional load in parallel!

 

[Jraef]

Any chance that you are using extra flexible (high strand count) conductors, such as DLO / RHH/RHW? Most breaker lugs are not rated for high strand count conductors, so no matter how tight you get them, there is always a heating problem, usually leading to failure.

 

[GoldDigger]

Any chance that you are using extra flexible (high strand count) conductors, such as DLO / RHH/RHW? Most breaker lugs are not rated for high strand count conductors, so no matter how tight you get them, there is always a heating problem, usually leading to failure.

Looking at it another way, when you kept swapping breakers and positions to test, did you continue to use the same (or similar) conductors?

 

[A1b5c0pe1984]

Any chance that you are using extra flexible (high strand count) conductors, such as DLO / RHH/RHW? Most breaker lugs are not rated for high strand count conductors, so no matter how tight you get them, there is always a heating problem, usually leading to failure.

No, just copper 500s.

 

[A1b5c0pe1984]

Looking at it another way, when you kept swapping breakers and positions to test, did you continue to use the same (or similar) conductors?

Yes, the same wires are used. But they have been tested and are good.

 

[A1b5c0pe1984]

Update. We had the power company monitor their end of it to essentially cross it off the list of potential problems. I didn't think it was the problem, but like I said we're grasping at straws. Their end seems good, they were able to show the voltage when we had our breaker event, and showed other events where they thought there might be some disturbance on the line. I have bought a different breaker since I last posted, but it has not come in yet. Also we're down to 4 potential problem motor starter circuits. So the next time it trips will be the last time we have to trouble shoot the problem. Just want to say thanks for all of the input and ideas. I really appreciate it.

 

[A1b5c0pe1984]

Just wanted to give an update because I think I found the solution. I think the 400A breakers installed were not a good fit for what we are using them for. Originally we had the hjxd63b400 siemens breakers feeding every 400A circuit coming off the main panel. We bought one SHJD69400 and installed it in place of the hjxd63b400 and have had no issues after we did. Now, the other breaker is tripping. My guess is these just heat up and trip, and when I was moving motor starters back and forth it really didn't matter, as they would just heat up independently of one another and trip whenever. We're going to install the SHJD69400s on each and hopefully this will solve our issue. Just wanted to say thanks to everyone who posted and gave me advice.

Never would have thought this would have been the problem with less than half the allowed amperage on the breaker. But, sometimes the easiest solution is the correct one.. and hopefully this is the case here.

 

[Flicker Index]

That's a magnetic/thermal combo fuse.

I've seen an automation industrial power supply that would randomly trip the breaker upon power up. It had a setup of capacitors on the input with a series TRIAC. The input is connected through a high wattage resistor to limit inrush current, then the TRIAC would short across the resistor once capacitors are charged. The TRIAC failed for some reason, so that particular unit would power up across the line. Depending on the phase angle of the line when the switch was turned on, it would trip the breaker on magnetic trip.

If the breaker doesn't have a trip cause indicator, you can't tell if it was a magnetic/instantaneous trip or thermal/overload trip. If it was a Limitron fuse, a test lab can cut it open and determine if it's a overload element or the short circuit element that opened. On a much smaller scale, a blown glass tube fuse would look like two wires with balled up ends if it opened by an overload whereas a short circuit overload blacken the glass and look like something exploded inside.

Just knowing whether the cause is a magnetic vs thermal is a huge step in figuring out the cause. Meggers can always detect an intermittent short.

 

[ptonsparky]

.... can't always detect...