Pool Light Keeps Burning Out

Question

I’m troubleshooting an in-ground pool light that burns out within 5 seconds of turning it on. The old light burned out like this the first time I turned it on this year. The old fixture was shot, so I replaced it with a new one and the bulb burned out within five seconds. Water is not leaking into new fixture.

I disconnected the feed wire from the fixture wire and read 125V at the feed wire, about as expected. I then isolated the hot and neutral wires by disconnecting them at the breaker box (about 66 feet away, cable running underground) but the ground wire is still connected. A continuity check between the disconnected hot leg and neutral or ground wires shows minute continuity (about 5,000 ohms resistance). Is that normal? If not, would a small leak to ground like that cause a light bulb to burn out?

The light bulb is 500 watts on a 20 amp circuit with a GFCI circuit breaker. The breaker never trips and the GFCI test function operates normally.

I’ve checked and ensured no loose connections at all wire connection points (including switches) from the breaker to the fixture and I find no evidence of arcing or overheating at those points. The circuit is wired with two three-way switches. Nothing else is on the circuit but the 500 watt bulb.

If nothing else I’ve reported here would burn out bulbs without tripping the breaker, then what else could the problem be? Thanks, Rich

Answer 1

My first comment is old lamps if correct may have leaked and allowed atmosphere in and the filament burned up, lamps over 300w are quite rare and most have been replaced by HID over the last 40 years.

More realistic HID ( metal halide or sodium vapor ok with a ballast) Are you sure you don’t have a ballast in there somewhere? Open circuit voltage for a sodium s55 could be there but when the starter strikes in a few seconds it’s high voltage pulse would blow a incandescent filament. I think this may be what is happening if not old stock.

the original lamp could have been a 500w equivalent sodium at 150 actual watts because hid’s usually are 400w the next size is 1000w that is actual watts but equivalent is Close to 150w so now you know why most changed years back

a 500w lamp will have a mogul base and only produce 10k lumens . A 150w high pressure sodium may have been classed as a 500w equivalent the open voltage would be in the range you measured Or close 110+ I did not discuss metal halide because their open circuit voltage is 2x what you measured and HPS has a much higher strike value.

So we have old stock incandescent mogul base or a possible HID lamp that has a ballast and starter , I can caution you if your meter is not rated do not try and test the strike voltage most meters are only good to 1kv for the test leads up to 5kv but some HPS have strike voltages from 12-20kv , I learned this with a fluke 87 many years ago. For well under 100$ you can get a LED flood almost any brand will put out 500w equivalent and consume 75w with higher end ones only drawing ~60w And they will last 50000 hours compared to 2000 of an incandescent.