Can blown bulb trip GFI?

I have an underwater pool light. Whenever I try to turn it on, the GFI in the circuit trips. I replaced the GFI - same problem. Could a blown bulb be tripping the GFI?
I ask because it's a pain to change the bulb - the screw head on the face plate is damaged, and I would need to drain the pool to be able to use a "screw-out" head to back-out the screw and change the bulb.
Ideas?
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A simply blown (open) bulb will not trip the GFCI. It is more likely that you have a short between one of the lamp terminals and the faceplate or housing (and thus the grounded pool water). It may be as simple as a damaged rubber gasket or plastic spacer. Is there any water inside the housing?
If the Breaker is GFCI it can be impossible to know if the trip is a ground fault or a real short. If you have a breaker and a seperate GFCI outlet, you can tell the difference.
Simplest short term solution is to cut and tape the wire to that bulb until it is more convenient to fix the lamp housing. To troubleshoot, you need an Ohmmeter and with it you can measure the resistance between each of the wires to the bulb and a grounded point on the pool (like the ladder). The resistance should be open, any resistance at all indicates a ground fault and the breaker is doing its job saving your life.
Furthermore, with the bulb removed or dead, the resistance between the wires should also be infinate (open) but if the bulb is not dead, there will be a somewhat low resistance almost a short between the wires.
Hopefully you can gain access to the wires to the pool light somewhere outside the pool of course, there may be an access cover on the deck or a panel on a wall.
If your lamps are low voltage and powered by a transformer then the answer would be a bit different as the ground fault would be near the transformer.

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Thanks for the info - Pipedown, you said:
"If the Breaker is GFCI it can be impossible to know if the trip is a ground fault or a real short. If you have a breaker and a seperate GFCI outlet, you can tell the difference."
I have a non-GFI-breaker that feeds three boxes in a row: a light switch in a box, a GFI outlet plug and the third is a plain duplex convenience receptacle. I replaced the light switch and GFI and used a GFI tester to verify that it was correctly wired (it's got three leds that show correct wiring and a "tripper" that works as it should).
The easiest place to gain access to the light wires seem like it would be at the light switch - there are two blue wires there, but at least one of them goes to the GFI, so if I remove the switch and test continuity on the wires, won't I be testing *through* the GFI? Will that be accurate? Do you think those two blue wires go directly to the bulb?
No, not a low-volt system.
RBM: I see a black plastic square box right next to the electrical boxes. Maybe that's where the light wires come up from the metal niche. I'll check it out tomorrow AM! (Too hot in Phoenix to check anything at 4PM - it's 109 right now!
I see no water in the light fixture itself - and just FYI, the bulbs are supposed to have a life of 1000 hrs or so....this puppy was still pretty new when it stopped working!
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Most likely water has entered the sealed housing. There is a pipe which goes from a metal niche that that light is installed in, to an above ground splice box called a deck box. It's probably not far from the fixture and possibly concealed under a diving board or in a bush. It has a plastic cover held on by four screws. Open it up and disconnect the wires going to the fixture and cap the feed wires. Then you should be able to reset the gfci

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RBM wrote:

You already have received the correct answer to your question.
Now, displaying my near total ignorance of swimming pool technology I have to ask, can you properly change the bulb *without* lowering the water level even if the screw head isn't damaged?
Jeff
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Jeffry Wisnia

(W1BSV + Brass Rat \'57 EE)
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If you have a wet niche fixture,,,,,,,,,,,,,,,,,,,Yes. Dry niche fixture ,,,,,,,no. Most pool fixtures are wet niche.
Changing the bulb will not fix the GFCI issue. If the fixture seal is intact which I will bet it is. Your looking at an new cord for the fixture. If it is old they can be a bear to get out and replace. At least the last one I tried was a real pain.
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Jeff, on standard wet niche fixtures there is one screw in the face plate which when you remove, you have a hermetically sealed unit that pulls out and if properly installed has enough rubber cord attached to lift the whole thing onto the pool deck where you can attempt to make repairs Roy

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RBM wrote:

<snipped>
Thanks, I'm glad I asked. Now I understand why he feels he needs a dry access to get that mangled screw out.
Jeff
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RBM & Jeff: You raise an interesting physics problem - and that is how to be in the pool and exert a force against a screw that is underwater. The more you *push* against the screw with a screwdriver, the more you force your body away from the screw --- so it's almost impossible to get much torque on the screw head (at least as far as I can figure). I've ruled out taking my new battery drill under water. Draining is the only solution I can think of unless you guys have a good trick!
Chuck KG7JD - and lots of pals from tech in Beantown!
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ron snipped-for-privacy@hotmail.com wrote:

A pneumatic drill is a possibility (rental?)
Wet niche lamp housings are water cooled - don't run them when removed on the surface (or water below light).
Bud--
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On Fri, 01 Jul 2005 16:05:28 -0400, Jeff Wisnia

I was thinking the same thing. How in the heck can you change the bulb underwater withour filling the socket and fixture with water? Dont seem possible to me.
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The entire socket assembly is on a long cord that allows you to life it above the water surface, open it, and replace the bulb. I just can't figure out how to remove the fixture without draining the water because of the bad screw head.
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On 2 Jul 2005 12:32:54 -0700, "ron snipped-for-privacy@hotmail.com"

Be sure you get a new gasket. If this thing has been there a while the old one will have taken a "set" and will leak.
As for getting it out maybe you could tie a rope to something solid on the deck and around your waist. Put a strain on it by pushing off the wall with your feet and then lean on the screw with something. These are just threaded in sheet metal so it shouldn't be that tight but if you can't touch bottom it is hard to get much pressure on it. Once you get it out replace it with a hex head.
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On Sun, 03 Jul 2005 00:13:07 -0400, snipped-for-privacy@aol.com wrote:

See if you can get a vice grips around the head. That works sometimes. At least I know now how the bulb replacement and socket works. I know nothing about pools. I could not imagine how you would change the bulb underwater.
Mark
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If you have an air compressor, you can use an air grinder to grind the head off the screw. Once the bezel is removed, you can grab onto the remaining shaft with vice grips to remove it the rest of the way. Visibility may be poor using an air tool underwater but there should be no safety issue as with electric power tools.

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