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I am swapping out an existing floodlight for this:

https://www.amazon.com/gp/product/B07873SR5S/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1

The existing light works fine. The new light works as long as it is NOT screwed into the electrical box. If it is touching the electrical box when the light comes on, the circuit breaker will trip (accompanied by a complementary popping noise and some sparks).

Some background information: My house was built in the 50s. I'm not aware of any outlets that are truly grounded. The power to the floodlight is controlled by a switch in my kitchen. Conduit runs from the wall just outside the switch to a carport fluorescent light and then out to the floodlight.

At the electrical box for the floodlight, there is a red wire (hot) and a white wire (neutral). The floodlight has three wires: black (hot), white (neutral), green (ground). I connect the two hot wires (red to black), the two neutral wires (white to white), and leave the green ground wire unconnected and capped. Again, it works perfectly when the wires are connected but the light is not screwed into the electrical box.

Any testing I have done with an analog voltmeter seems to be correct. When the power is on: red(hot) to white (neutral) reads ~110V. Red(hot) to the electrical box reads ~110V. White(neutral) to the electrical box doesn't register a reading. With the power off, there is no reading.

I originally assumed something was wrong upstream. I have verified everything is I would expect behind the kitchen switch (same testing procedure as above). I haven't pulled the fluorescent light fixture yet to check its electrical box, but the wiring at the fixture looks okay (nothing exposed).

I'm left wondering if something is wrong inside the new light, lol. That seems unlikely, but I guess it is possible. Hopefully, I've explained this well enough. I'm sure it's obvious I am no expert with electricity.

  • Popping/sparks would usually indicate a short somewhere. Is the electrical box metal? – Chris M. Jun 11 '18 at 16:53
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The green/ground should be connected. My understanding (others more expert will correct me if I'm wrong) is that if the box for the floodlight is metal and has metal conduit going back to the switch, etc. then the box should be grounded and that the normal thing to do without a ground wire coming in to the box is to connect the floodlight ground wire to the metal box. If the box is NOT grounded that wouldn't hurt anything (but would NOT be providing the expected ground, but no worse than a capped ground wire, which does no good at all and is most likely not considered a complete/proper installation). But if the box is somehow hot then (a) you have a dangerous situation and (b) every time you touch the case of the floodlight (which should be connected to the floodlight ground wire internally) to the metal box you will trip the circuit. So I see two likely scenarios:

1 - Floodlight is good. Box is bad (shorted to hot?). Need to diagnosis/fix box problem, which may be a bit complicated. Check for voltage between neutral and metal box - should be 0 V, if I understand the nature of neutral/ground bond correctly. If it is 120 V (or anything more than a few V) then I think either your metal box is hot (VERY bad) or hot/neutral reversed somewhere (also VERY bad) - figuring out which one would require a bit of detective work - but definitely need to fix before installing the light. I just noticed you DID test neutral/box = 0 V and hot/box = 110 V, so that would indicate the box is OK (though unknown whether it is a proper ground or not) which leads to...

2 - Floodlight is bad. With the floodlight disconnected from the circuit, use a tester to check for continuity (or low resistance) between (a) ground wire and metal case - (good), (b) hot wire and metal case (VERY bad), (c) neutral wire and metal case (bad, but if I understand things correctly, that shouldn't cause a sparks-flying-breaker-trip). If (b) or (c) then I think you have a defective floodlight.

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    I should have taken the floodlight apart sooner. You know what they say about assumptions... The voltmeter showed no resistance between hot and the light frame. It turns out that when the metal cover was screwed onto the flood light, one of the hot wires was caught against the frame and created a short circuit. After I fixed this wire, everything works as expected. I spent a ton of time troubleshooting my side of things because I assumed the problem must be my old house. Anyway, experience gained :) – Matt Jun 11 '18 at 18:12
  • Glad you got it fixed. Not as crucial as in some situations (assuming the floodlight is out of reach as most are), but you should connect the ground wire to the metal box instead of capping it. Since the box shorting to hot was a problem but case to hot was not, that would indicate that the metal box is grounded (as it should be) and therefore connected to neutral at the main panel and therefore tripped the breaker. So since it is grounded, connect the ground wire. – manassehkatz-Reinstate Monica Jun 11 '18 at 18:20
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    Agreed, and once I shielded the wire inside the light I went back and connected the ground wire. Thanks. – Matt Jun 12 '18 at 16:51
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First, if the conduit is metal, and not damaged or badly corroded, then that is a proper ground path. I have a factory full of the stuff, miles of wire and not a ground wire to be seen. This shocks newcomers, but the pro electricians who work on the 480 don't bat an eye. This is normal.

The problem is a) you didn't believe you were grounded so b) you didn't bother to hook up ground first. You are effectively connecting ground when you bolt up the fixture to the junction box, which is why it only fails then.

The trouble is, for non-philosophers, hooking up ground last triggers the logical fallacy of cum hoc, ergo propter hoc: since Y follows X, therefore Y is caused by X. Since attaching ground failed it, "ground must be bad".

If you had hooked up ground first, this would be quite a different tale (and a simpler one by far). Since attaching hot fails it, therefore the lamp must be defective. Still a logical fallacy, but the correct answer.

  • Hopefully This shocks newcomers is only a figure of speech... – manassehkatz-Reinstate Monica Jun 11 '18 at 20:38
  • Would it be more correct to say that conduit can be used as a proper ground path? I thought a "proper ground" must be connected to...well, the ground (earth). A piece of conduit lying on a rubber mat wouldn't be considered grounded, would it? I wasn't aware of a ground path, but I guess the voltmeter test between hot and the box reveals that is grounded (reads 110V)? If not, it wouldn't read anything? – Matt Jun 12 '18 at 16:45
  • Last thing, when did anyone say "ground must be bad"? The ground and box connections were points of failure. I also don't understand about connecting "ground first". The power is off anytime connections are made. The order would seem to be irrelevant. It is a binary decision to connect ground or not. Equally confusing is, "since attaching hot fails it." There is no electricty without hot attached. As far as I know, every electrical device will fail if hot isn't attached. Sorry, didn't mean to be flippant. Maybe I'm misreading. – Matt Jun 12 '18 at 16:50
  • @Matt "conduit can be used as a proper ground path" - That is correct. Basically you will normally find either metal box/metal conduit that goes all the way back to ground at the breaker (by way of metal panel box) or an actual ground wire (green or bare). The hot<->box measurement 110V was confirmation of that. The actual ground (e.g., grounding rod in the dirt, or cold water pipe, etc.) is one connection somewhere in the "main panel, service disconnect, meter" area for the entire building. – manassehkatz-Reinstate Monica Jun 12 '18 at 17:25
  • @Matt As far as "ground first", I think what Harper meant was that if you connected ground & hot at the same time, before turning on the breaker (and before moving the light fixture into the box) then you would have found the problem sooner. Since ground wire wasn't connected to anything, light fixture case was "hot" but not obviously so - actually quite a dangerous situation if you had touched that case while the light was on and grounded yourself. – manassehkatz-Reinstate Monica Jun 12 '18 at 17:29

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