Why does my new floodlight trip the circuit breaker?

Question

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.

Answer 1

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.

Answer 2

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 surprises newcomers, but the pro electricians who work on the 480 don't bat an eye. This is normal.

The problem is a) you did not think ground was available, so b) you didn't hook it up. However, ground was present after all, and it snuck up on you, when you bolted up the fixture to the junction box.

The root problem was that there was a short between hot and ground. This is pretty bad, but it did not become apparent until ground was connected.

Enter the logical fallacy of cum hoc, ergo propter hoc: since Y follows X, therefore Y is caused by X. Since attaching ground made it fail, it seems apparent ground must be the root of the problem!

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