This is, I am sure, an extremely naive question. I hope that's okay (and I hope it can be answered at a level I'll understand).
The background: My swimming pool has underwater light fixtures, on GFCI circuits. I recently replaced the bulb in one of these fixtures, reassembled the fixture incorrectly, and re-installed it. About 48 hours later the circuit breaker (the one inside the house, in the breaker box, not the GFCI) tripped. I removed the fixture, which was full of water, realized my mistake, reassembled it correctly, re-installed it, and it's now been fine for several weeks.
Question 1: In my naivete, I'd have thought that the water in the fixture would trip the GFCI immediately, so that the circuit breaker in the house would never have tripped. But the circuit breaker in the house did trip. So was my expectation wrong, or does this mean my GFCI is not working?
Question 2: After I re-installed the fixture and flipped the breaker back on, the lights worked, even though I never reset the GFCI. Is that evidence that the GFCI is not working?
Question 3: In pondering Questions 1 and 2, I was led to wonder how I would ever know if my GFCI is not working. Removing the underwater fixtures once a month to hit the test button is not terribly practical. Is there another way to test this?
Best Answer
An underwater GFCI doesn't matter
That is to say, it doesn't perform any useful function underwater. It does nothing to prevent the water from being electrified, which is its one job. Here's how a GFCI is laid out.
As you can see, if water can get to the "Line" side of the device, then it electrifies the water. And the GFCI cannot do a thing about it.
So this is a lost cause. I really don't care whether your friend thought he saw a GFCI down there; if it exists it is useless.
Besides, having a "test" button in such an inaccessible location is a blatant code violation. It simply makes no sense, unless it's a holdout from the early days of GFCI requirements - maybe it's something the light manufacturer put in there to CYA. Regardless: treat it as if it doesn't exist.
GFCI and overcurrent are different things
A breaker trips on overcurrent, when the total current flow exceeds the breaker limit by a wide enough margin or a long enough time. (breaker trip curves are pretty generous).
A GFCI actively looks for current differential on the two conductors. When it detects a small amount (8ma) of unequal current, that means current is seeking a third path, and it trips.
While they both relate to current, that is the end of the similarities.
That said, if the breaker is a combo GFCI+breaker device, it may seem like an overcurrent trip. You have to pay close attention to its indications to see whether you are dealing with a GFCI or overcurrent trip. If it is a GFCI breaker, it will have a TEST button.
Otherwise if the breaker tripped, it's because either a massive amount of current flowed, or the circuit was mildly overloaded for awhile. This could simply be from too many appliances plugged into the circuit.
GFCI protection is required for pool circuits
As said, the GFCI units underwater don't count. Every circuit within 6 feet of the water (length of a common appliance cord) needs GFCI protection. Fortunately any GFCI device can confer GFCI protection to devices down-circuit of it. At extremes, a GFCI+breaker protects the whole circuit.
So the right way to protect the circuit with the pool lights is find a point along that circuit before it nears the pool, and fit an appropriate GFCI device there. E.G.
Alternately, make the pool lights low-voltage DC
12 volt DC power is incapable of shocking swimmers. Old incandescents draw too much current for that to work with existing wiring, but LED is so efficient that it works fine at 12V on existing wiring. So why have special devices which require constant testing to protect swimmers from shock, when you can moot the entire point by switching to 12 volts DC?
I recommend fitting a 12VDC power supply at least 6' from the pool and running all the pool lighting on that, using LED. This also means you never need to change another bulb. It's even possible to do it using existing fixtures, since they make Edison base LED replacements that are 12 volts DC.