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Just want to make sure my outlet's ground is working. How to do that?

UPDATE: My main aim is to test resistance of anti-static wrist strap, but since I know it works by testing it with multimeter, I now want to be sure wall socket's ground works as well. This is why I asked this question.

Maybe I can test wall socket's ground with a multimeter?

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3 Answers 3

I use a digital voltmeter and measure three ways.

  • line to neutral shows 120VAC (in Canada)
  • line to earth shows 120VAC since neutral is earthed some distance away
  • neutral to earth shows a very low voltage (not always zero, but very small)

If the earth is open, you won't see the line-to-earth voltage on the meter.

This is essentially replicating the functionality of the circuit tester gadget described elsewhere in the thread.

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What does "earth is open" mean? –  Boris_yo Sep 11 '11 at 15:34
    
It means that the earth connection is absent from the receptacle. Open-circuit. –  Adam Lawrence Sep 13 '11 at 21:07

I don't know how things are wired in Israel, but here in the US the neutral is tied to ground at the breaker panel. For most cases (exception below), that means neutral and ground are equivalent at the outlet, except that neutral is intended to carry the return current whereas ground is intended for a safety return path when something goes wrong. The ground might be tied to a metal chassis, for example. Normally that is a open connection, but if something inside shorted to the chassis, the current would be carried by the ground lead instead of a person touching the chassis.

Therefore, you can test if the ground is working by very carefully connecting a small test load between the hot lead and ground. A small lightbulb is ideal for this. The lightbulb should light just as if it were connected between hot and neutral.

Now for the exception. Since there isn't ever supposed to be substantial current on the ground line (think of the chassis, just little capacitive coupling and some small leakage perhaps), current on the ground line indicates something went wrong. This can be exploited for additional safety by shutting off the hot line when ground current is detected. This is called "ground fault detection", and you can get "ground fault" breakers that have this built in. Around here at least, you can also get outlets that have a ground fault interruptor built in. These are often found in bathrooms and other places where the human user is more likely than usual to be connected to ground.

The lightbulb test described above will therefore not only test the ground lead, but by putting current on the ground line it will also test any ground fault interruptor on the line. If the bulb lights normally, then you've got a working normal outlet with a good ground line. If the bulb lights for a fraction of a second then goes out, you have a working ground fault breaker somewhere. In that case, you will have to reset that breaker to get power back at the outlet. If the bulb doesn't light at all, then you have a broken ground lead, which should be taken care of promptly.

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1  
A brief note on Olin's great answer: you're more likely to be connected to ground in the bathroom because the plumbing acts as ground. Even though it's not connected to the electrical system, it's a lot of metal and may be deliberately grounded to earth somewhere in the basement or back yard, in case of lightning strike or wire shorting to it. Same in the kitchen. Other rooms in the house don't have such ready ground, or water to accidentally connect you with it. –  Matt B. Sep 11 '11 at 23:55
    
You should always assume that plumbing might be grounded for safety reasons, but more and more plumbing is being done with PEX pipe so it is no longer safe to assume that a plumbing fixture is always at ground. –  Joe Hass Dec 11 '13 at 19:35
    
Regarding the safety of the bulb test: what if all the plug grounds in the house are connected together but are not connected to the Neutral inside the breaker panel or inside the mains company meter? The test will essentially transfer the phase to the ground wire through the bulb so anyone touching a grounded metal device in the house may get a shock. –  alexan_e Dec 11 '13 at 21:59
    
@elexan: No, because "grounded" specifically means connected to ground. Also, it takes touching two things with sufficient potential between them to get a shock. Your whole question therefore makes no sense. –  Olin Lathrop Dec 11 '13 at 22:09
    
It is possible to have all the plug ground wires connected together with the path to the GROUND missing for any reason (which means that all the ground wires of the plugs will be floating), that is the case for which I find it dangerous to raise the plug ground to the PHASE (hot) potential through the bulb resistance. Touching just the PHASE (hot) while having insufficient insulation with the ground (the ground we step on) is enough to cause a shock. I don't say this to doubt your knowledge in electronics, I'm just having safety concerns and I think readers should be warned. –  alexan_e Dec 12 '13 at 0:36

With a socket testing plug.

In the UK there are many, and they look like this:

enter image description here

I don't know which country you are in, and what socket type you have, but check your local hardware store - they may well stock your local equivalent.

The one pictured costs about GBP 16 / EUR 19 / USD 26.

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