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?

  • What are the ratings of the gcfi & breaker? Having these in series may not be a good idea...
    – Solar Mike
    Commented Jun 30, 2019 at 15:13
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    You'll probably be migrated to DIY as this isn't a design question. Are you sure your pool lights are on the GFCI? If you switch it off do the lights turn off? You might add your location to your user profile so we know what regulations apply in your area.
    – Transistor
    Commented Jun 30, 2019 at 15:14
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    Is the GFCI protection provided through a receptacle or deadfront-type GFCI, or through a GFCI breaker? (i.e. does the breaker itself have a TEST button on it, or is there something else in the circuit with Reset and Test buttons on it?) Commented Jun 30, 2019 at 15:25
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    I realize when you say "GFCI", you know exactly what you mean. However, I don't know. GFCIs come in many combo packages: GFCI+breaker, GFCI+receptacle, GFCI+switch+recep, GFCI standalone, etc. Commented Jun 30, 2019 at 15:27
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    A GFCI below the waterline would not meet code in an in ground pool it needs to be above the waterline and 5’ away from the edge of the pool Unless these are actually low voltage lights. I don’t have a code book handy but believe the low voltage contact level for wet locations is 15v av and 30v dc as I put in my answer below. If the lamps are low voltage and a listed transformer is used GFCI may not be required.
    – Ed Beal
    Commented Jun 30, 2019 at 16:09

3 Answers 3


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.

enter image description here

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.

  • a receptacle not likely to be splashed with water,
  • inline in GFCI-only (deadfront) devices,
  • part of a GFCI+receptacle+switch combo device
  • a GFCI breaker to replace the regular breaker.

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.

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    I'm surprised that a 12V device has a connector on it that is commonly used for 120V. Lots of people just put things together that "fit", and...
    – AaronD
    Commented Jul 1, 2019 at 1:23
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    @AaronD It's a legacy from RV's which still use it, and from the days before modern sockets. You will also find that Edison socket used for 24VDC, 37VDC and 75VDC. They harken back to a time when you were expected to know what you're doing. Heck you even find NEMA 1-15 sockets with 75VDC in locomotives; some laptop and cellphone chargers even work on them. 120V incandescents will light dimly. Commented Jul 1, 2019 at 1:27
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    My parents' old RV had 12V incandescent bulbs, but they used a different socket. I guess you could have gotten one in contact with 120V, but you would have had to try. They've sold it now and bought a new one, which has hardwired LED's. Changing one of those is basically a rewiring job.
    – AaronD
    Commented Jul 1, 2019 at 2:09
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    And I know about being expected to know what you're doing with identical connectors! A 1/4" (6.3mm) "phone plug" on stage, for example, can be used for anything from a raw guitar to a 100+ volt, 8-ohm PA! And most people don't realize the shock hazard from their trusty old system.
    – AaronD
    Commented Jul 1, 2019 at 2:14
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    @AaronD GU5.3 bulbs come in 220V and 12V. I blew several because in my house they used the same fixtures, sometimes with a driver inside and sometimes without.
    – AndreKR
    Commented Jul 1, 2019 at 7:25

It sounds like your existing GFCI is not working, pool water (if clean is not really a good conductor) but should have tripped a GFCI prior to tripping a breaker (unless the breaker is a GFCI). The type of pool can make a difference also an in ground concrete/ gunnite or similar pool has a bonding grid that should have tripped the GFCI. A fiberglass in ground pool or a poly lined may not trip a GFCI. The GFCI should not be embedded in the fixture you should be able to push the test button at the breaker panel (monthly) to test if it is working. If the GFCI is an outlet type feeding the light fixtures it should not be within 5’ of the pool but it should also be able to be tested without removing the fixture. The type of fixture can also make a difference if the shell of the fixture is plastic with a glass bezel it is insulating material so even though there was a leak and clean water is not a good conductor the contacts may have been in the water conducting from hot to neutral and an air space so the dripping water did not or could not create a leakage path to trip the GFCI , so with this there are several cases that lighting won’t trip a GFCI but may trip a breaker. I have always used low voltage lights but still provide the transformer with GFCI protection, you should be able to test the GFCI device and this should be done monthly.

If the GFCI is faulty how would I know if the test button is functioning correctly. The test button is not part of the fault sensing circuitry it is an external portion that creates an imbalance in the hot and neutral line and when this imbalance reaches 5ma it should cause the fault or open the circuit. if not the GFCI has failed and needs to be replaced. You should be able to test without removing the fixtures, if you need to remove the fixtures to test the GFCI the breaker needs to be updated to GFCI unless the circuit voltage for the lights is below the low voltage contact level of 15v ac or 30dc (I believe these are the correct values but I don’t have a code book handy).

  • Thank you. I was confused about many things and this answer (together with Harper's) left me with a much clearer picture. I wish I could accept both answers.
    – WillO
    Commented Jul 4, 2019 at 3:02

It is also possible that you have a low voltage pool light, in which case the topology would look like line->gfci->transformer->light. The transformer's secondary windings, which produce the voltage going to the pool light, have no reference to ground, and therefore there would be no current leakage through the water into ground, and therefore would not energize the pool water or trip the GFCI. However, the load drawn by the short circuit in the pool light could be enough to trip a breaker

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Commented Jul 1, 2019 at 19:37
  • No, it's certainly not low voltage, because the old bulb was incadescent (though I replaced it with an LED).
    – WillO
    Commented Jul 3, 2019 at 19:27
  • I have low voltage Edison lamps, these screw in the same as a normal lamp (mine are for lighting in a generator room) these are still available today if the generator is not running the system changes over to the battery lights I think they are 24v but will have to look next week, so be cautious there are many different voltage lamps that use a medium Edison base that we all know so well.
    – Ed Beal
    Commented Jul 4, 2019 at 15:07

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