According to KT :
Hi. We've got a couple of electric baseboard heaters that appear to
be connected to a relay that apparently runs to a wall mounted
thermostat. We're trying to track down a power problem. Service
disconnect on the circuit box trips soon after baseboards start
heating.

_Service_ disconnect? Really? Takes out the whole house? Are you sure
you don't mean branch circuit breaker? (probably a pair of breakers tied
together).

If it's really the service disconnect, we need more detail. Ie: how big
are the main breakers? What is the combined wattage of the heaters?

Do you have a whole-house ground fault detector?

I can't help thinking that you're really going to need an electrician
for these.

If you have a whole-house ground fault detector, it's probably trying
to tell you that one of the baseboards has a wireing fault (short to ground).

It could be that you're on a relatively small service, and that running
the set of heaters all at once are too much for it. In which case you'll
need to upgrade your service (or use something other than electric heat).

No trouble when their breakers are turned off. Can anyone
tell me what the symptoms of a faulty heater relay would be? Could it
allow to much current to pass? Thank you!

Unlikely to be the relay. Relays either "doesn't open" (won't stop heating),
"doesn't close" (won't start heating) or makes the breaker go "bang" real
fast (short). If the relay short is causing the main breaker to trip, you
have big problems. If you're getting heat from the heaters, it's probably
not the relay.

If it's the branch breakers:

Two main possibilities:

1) One of the baseboards is going bad and drawing too much current.
2) The breaker is "getting tired" and is tripping at too low a current.

If you could borrow an amprobe, that'd be the best way to test the
current on each of the heaters.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.