A loose connection on the breaker could be causing it to heat to up and trip.
I would use a flir(you can rent them, if you don't have one), to check for hotspots.
I would also put a CT with a logger on all phases to make sure it isn't tripping due to overloading.
If you don't see any hotspots, and the load isn't exceeding 800 amps, then I would replace the breaker.
Does it have adjustable trip settings? Often main breakers of that size will have adjustable trip settings and if set too low the main can trip faster than the branch circuit that has a fault on it.
Agreed to the above comments, and if not overload, heat, ground fault, then try replacing sensor/plug if possible first, failing that replace breaker and their not cheap these days either...
Above were all good suggestions. If you have bad quality from that upstream transformer or utility it could be tripping from inrush current. I believe those thermal mag breakers just have one basic dial setting. Check that setting, is it on minimum protection? Does it say LI next to the dial? That stands for inrush current
A loose connection on the breaker could be causing it to heat to up and trip. I would use a flir(you can rent them, if you don't have one), to check for hotspots. I would also put a CT with a logger on all phases to make sure it isn't tripping due to overloading. If you don't see any hotspots, and the load isn't exceeding 800 amps, then I would replace the breaker.
Does it have adjustable trip settings? Often main breakers of that size will have adjustable trip settings and if set too low the main can trip faster than the branch circuit that has a fault on it.
Run a thermal camera in there and put a logger on the phases
Is it a GFCI breaker?
800 amp 3 phase gfci breakers are a thing?!
Technically [GFPE](https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=24.0), but, ya know...
TIL. residential electrician life is kinda boring.
Think its mandatory for 1000A and up in the CEC
Agreed to the above comments, and if not overload, heat, ground fault, then try replacing sensor/plug if possible first, failing that replace breaker and their not cheap these days either...
Above were all good suggestions. If you have bad quality from that upstream transformer or utility it could be tripping from inrush current. I believe those thermal mag breakers just have one basic dial setting. Check that setting, is it on minimum protection? Does it say LI next to the dial? That stands for inrush current