One of our accounts has a 3 pole 400 Amp brkr tripping. One of our guys went to get the numbers off the brkr and ordered a new one, looped it at 320 Amps. I am going there Sat to install it, not even sure if breaker is bad, but there seems to be no fault or overload and we need to do something fast, as the plants offices are part of the load served.

Have you seen breakers just go bad so they trip well below their rating?

It's a GE breaker, 208 volt. I always scoffed at people's description of a "weak breaker", but this sounds like the best way to describe this one. It's tripping 3-4 times daily.

There is no heat coming off the breaker? I've seen breakers internally go bad with a loose connection and then the overload mechanism takes in heat and eventually trips. But when this happened the breaker itself was HOT to the touch. The load was well within acceptable range. Was an old ITE if I remember correctly. Obsololete and cost $750. Now that I remember.

I don't know what you will pay for the brk you are going to use to replace the one you think might be defective but you might think about renting a Multi Amp Model SR-76 Relay Test Set. It's a bit of over kill but you can determine if the breaker is tripping early and how early. Breakers wear and begin to trip early, usually because they have been burdened an excessive number of times or are used in place of a snap switch to turn the circuit on and off under load.
If the Brk isn't the problem you could install a (Drantz 626) line disturbance analyzer to determine what the voltage and current characteristics are when the brk trips.

I'll ask the guy who was there if it was hot...I haven't had the pleasure to go there yet, just am the only one available Sat to install new brkr.

What is your company going to do if changing the breaker does not help?

Before we went that route we would install a data logger or considering it is tripping 3 - 4 times daily just use an amp clamp with a memory.

If this breaker is running 320 amps continuously a large motor starting may be enough to trip it out.

Hot weather and it may be a AC unit that is starting up or is simply has an internal problem.

I have not found to many larger breakers that have worn out or gotten weak. No one that I know uses a 400 amp breaker as a light switch.

I would make a rough list of the loads on this breaker and look for equipment that starts 3 - 4 times a day.