DIY: Old ceiling light, no ground -- what's the worst that can happen?
October 21, 2014 12:13 PM   Subscribe

I replaced a ceiling light in a 1950's-wired apartment. There are insulated black and neutral wires, but no ground connection (fixture has ground wire but just connected it to mounting bar). If wired backwards, what happens when I flip the switch? When I touch the fixture while power and/or switch are on?

Have read many threads, especially on DIY forums, but haven't found an answer.

My new fixture (fairly low-power single pendant light) has a ground wire, but I just wrapped it around the grounding screw in the mounting bar. I'm also very certain I connected black to black and white to white, but you never know... I tried very hard to make sure the wire caps covered all bare wire.

I've turned the circuit breaker back on and nothing happened (good), but have been too scare to flip the light switch on. If it's wired backwards, and/or if it's the worst thing ever not to ground, what will happen when I flip the switch? What will happen when I touch the fixture with the switch on? What about when switch is off? How can I test this without killing myself or getting a shock - can I insulate myself before touching it?

I'm a student - I can't afford to pay an electrician just to come in and look at it.
posted by ArgyleGargoyle to Home & Garden (10 answers total)
Best answer: Normally, the sleeve/screw-in parts of the socket are neutral, and the little thing deep inside the socket is hot. Which means that if you stick your finger in the socket you have to get all the way to the bottom before you hit hot.

If you got it backwards, you just have to touch the threaded parts to hit hot.

So as long as your ground wire isn't near anything that's hot, that's the worst that could happen. As long as the switch is wired correctly, hot is still cut when the light is turned off.

With similar questions in the past other (knowledgeable) people here have expressed more concern than me. I've gotten hit by really egregiously wired wrong circuits so many times in the past that if that's the worst goof on old wiring, I'm barely gonna raise an eyebrow. But I did just re-wire my entire house so that I now know it's done right.
posted by straw at 12:22 PM on October 21, 2014 [1 favorite]

I don't think incandescent/fluorescent light bulbs care about polarity (LEDs are a different story), so you should be fine. Most free-standing lamps only have 2 prongs and some are the older reversible plugs, I think the same holds true for in-wall wiring, even if there's A Right Way to Do It.
posted by JauntyFedora at 12:24 PM on October 21, 2014

Best answer: This does not matter for incandescent light bulbs.

Nothing will happen.

If it's wired backwards, you might get a shock when changing the lightbulb (because the socket is the hot wire per straw's answer above), so just make sure you turn the light switch off before changing the bulb.

You can test this for less than $10 bucks by going to your local big box DIY store / hardware store / electrical supply store and buying a circuit tester. Take the bulb out of the socket. Turn the switch on. Touch one end of the tester to the socket housing and the other end to any other metal portion of the light fixture (ground). If the circuit tester lights up, you're wired backwards. If not, you're good.
posted by tckma at 12:29 PM on October 21, 2014

Best answer: I don't think incandescent/fluorescent light bulbs care about polarity (LEDs are a different story)

home current is AC, which means there really isn't true polarity. The relative voltages of the two conductors go back and forth all day, and neither one is +. So when everything's working correctly, it doesn't matter which is which.

That said, one of the wires is "hot" and the other is "neutral". The two ways you can get trouble are completing a circuit between hot and neutral, or between hot and ground. So wiring things up correctly minimizes exposure to the "hot" side of things. So it hides it at the bottom of a lightbulb socket instead of in the sleeve, and off switches are on the hot side, so that when they're off, everything after the switch is neutral.

If it's wired backwards, you might get a shock when changing the lightbulb (because the socket is the hot wire per straw's answer above), so just make sure you turn the light switch off before changing the bulb.

If the switch is inside the fixture, this will not help, because the switch will be switching the wire it expects to be hot, but which you wired as neutral.

Finally: it's also useful to wire up everything consistently to keep things safer for future electricians. It's standard safety procedure to always treat everything as potentially wired incorrectly, but not everyone always remembers that.
posted by aubilenon at 12:38 PM on October 21, 2014 [4 favorites]

Best answer: When it's working, there isn't a problem to not having a ground. Fixtures didn't have grounds for decades.

But, if the wiring frays in the future, it's possible for the fixture to become electrified, and you'll get shocked if you touch it. The grounding wire is supposed to draw as much current as possible, were this to happen, so that your circuit breaker would trip.
posted by hwyengr at 12:49 PM on October 21, 2014

Best answer: So the original poster was asking what might happen if, despite best efforts, they happened to write the new fixture in backwards. As aubilenon mentioned, house wiring is AC, even though neutral should be the same as ground, from the view of the bulb both legs are the same. CFL, LED and incandescent will all work.

So assuming it's externally switched, try your best to match white to white and black to black, but if you're putting in the effort you're not that much more likely to get it worrying than anyone else (even a professional), and the universe isn't going to implode on the off chance you got it backwards.
posted by straw at 2:38 PM on October 21, 2014

And I'm trying to think of how one could safely check the outlet, with an emphasis on paranoia. One way might be to get one of those socket to plug adapters, and, using a voltmeter, measure from hot and neutral to a known ground. The presumed "hot" side should be floating relative to ground, neutral should pull that voltmeter to zero.

You could also build a cord with an external ground wire to its own plug (or a 3 prong adapter with a wire to ground running from the silly tab) so you can plug that cord into a known grounded socket, plug the 2 wire plug into such an adapter, and use a circuit tester on it.
posted by straw at 2:55 PM on October 21, 2014

Do you know for sure what's in your ceiling? In my last house, the lights all had no ground wire because there were transformers in the ceiling that they were all plugged into so they were running on 12V power. Which makes them pretty damn safe. You might be lucky And have the same thing.
posted by lollusc at 5:16 PM on October 21, 2014

Best answer: You can test polarity by using a digital voltmeter that you could borrow from a friend. You don't need a ground. You can test using just a single probe.

Set the meter for AC voltage, turn on the light switch, then carefully touch one probe to first one terminal of the light and then the other. It doesn't matter which probe you use. Let the other probe dangle free, away from your body or hold it safely in your other hand by the plastic. You don't want to touch the metal of the second probe because you would be depending on the circuitry of the meter to protect you. If there is a fault in the meter (or you accidentally set it to amps instead of volts) you could get a shock, so don't touch the metal of either probe with your body.

The neutral wire will indicate a voltage of 0 to no more than 2 volts. The hot wire will indicate anywhere from 10 volts to 120 volts, depending on the voltmeter. This works because the digital meter is very high impedance and the AC voltage couples weakly to its surroundings capacitively.

Or if you can't find a digital volt meter get one of these non-contact voltage testers or this. It beeps or lights up when you are very close to a hot wire. I would never trust one of these to determine if a circuit is live or dead, but I would trust it to distinguish between hot and neutral.

If you connected black to black and white to white, you are most likely okay. If you have any doubt, you should check it as suggested above. While the fixture will work okay with the wires swapped, it does present a shock hazard when you change a light bulb. If you happen to touch the shell part of the socket, you could get a shock. If the switch is in the light itself, then the socket will be hot even if the switch is off.
posted by JackFlash at 5:43 PM on October 21, 2014 [1 favorite]

Response by poster: Thank you all for your very helpful answers! You are fantastic!

I was too tired to drive to the hardware store yet again (far enough away to be a pain) to get a voltage tester, so I was brave, and flipped the switch, and yes, the light bulb turned on and nothing sparked. Excellent.

Now I'm hoping no wires fray, but since the previous fixture was in there for FOREVER with no ground I think I'm OK.

Whew. No more home electrical DIY for me.
posted by ArgyleGargoyle at 8:57 PM on October 21, 2014

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