On Mon, 10 Jul 2017 13:20:22 -0700 (PDT), TimR
wrote:

On Monday, July 10, 2017 at 1:18:30 PM UTC-4, wrote:
OK - Euro system. Different problems. No "load balance" issues. 2
"main breakers" in the building. One never trips. The other trips when
it is wet outside. You have external lighting circuits on that main.
You most likely have leakage in those 2 circuits, not enough to trip
the branch breakers, but enough to load the main heavily enough that
the rest of the loads, combined with the circuit leakage, overloads
the main. Find out what is on the circuits in question and get those
circuits checked and fixed - or permanently disconnected.

This.

But just for completeness, breakers go open for three reasons that I know of, probably more.

You turn them off. They can stand a lot of this. Some are rated to be used this way, but even nonrated ones can do a lot of cycles.

They overload slightly. They can do a lot of these cycles too, but not nearly as many before getting too sensitive or failing. I think it's a factor of 10.

They see a dead short. This is bad. They're only rated to do that twice, and sometimes once is enough. That's okay, they did the job and kept your house from burning down, but now you need a new one.

Rainy weather is probably an overload rather than a dead short. Who knows?
Fine line between a heavy overload and a dead short.
If the main is tripping and the branch is not, you can be pretty sure
it is NOT a dead short on one of the branch circuits - - - -