testing for ground/ground faults

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Old 05-15-04, 11:44 PM
doingitmyself
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testing for ground/ground faults

could someone explain how to go about checking (with a multimeter) for current leakage to the metal frame of, say a washer or dryer. If, for instance, a hot wire or a neutral came loose and contacted the metal case, would there be a way to test for voltage on that frame? I know to touch the red lead of the meter to the cabinet, but where would I find a ground reference? Also, how to verify, with a multimeter, that the EGC is actually attached to the frame.

This is just for information only - not an actual problem. I was just wondering.

Thanks
 
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Old 05-16-04, 12:58 AM
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Grounded equipment has two possible current paths. Current coming into the machine from the hot lead can either exit through the neutral (desireable) lead or can exit through the grounding lead. That current exiting via the ground lead is, by definition, leakage current. You could hookup your multimeter as an AC ammeter and put it in series with the gound lead and measure any leakage current. Break the green wire and insert your ammeter in series with the wire. Watch out, if you have an old analog meter and you do have significant leakage it could damage the meter.

What you really need for a test like this, though, is a megger. That will give you an excellent test of the insulation quality of your equipment quickly and easily.
 
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Old 05-16-04, 05:07 AM
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There are different levels of test that you can perform, and different tools available. Some require _disconnecting_ wires either from the load or from each other.

If you want to test the quality of the equipment ground connection or check for leakage from to the frame of a piece of equipment, I would advise _against_ doing any tests that require opening connections. After you break the connection, you need to remake it, and this new splice is not tested

You can check for an energized frame by touching one volt meter probe to the frame, and touching the other probe to any convenient grounded metal. If necessary, use some sort of log clip on lead to get to grounded metal. Metal water pipes are reasonable for this.

You can also check the continuity of your equipment grounding conductor and ground bonding by measuring the _resistance_ (ohms) of between exposed metal on the frame and your convenient grounded metal. The resistance should be the _minimum_ reading that your multi-meter will give.

There are quite expensive circuit testers (several hundreds of dollars) which give an accurate enough resistance measure to detect 'bootleg' grounds. These are specialized pieces of equipment, not your standard multimeter.

For detecting 'ground leakage' (where current is getting into the equipment ground conductor, but not enough to cause significant voltage on the frame or trip a breaker), a 'clamp on' amp meter is hard to beat. This measures the current flowing in a wire by the magnetic field produced on the outside of the wire. Simply feed the _unbroken_ wire through the clamp, and make a measurement. You can also detect leakage by measuring all the circuit conductors (both hot and return) at the same time; the _net_ current should be zero, because the current on the hot and the current on the return should balance and be in opposite directions. Any net current flow means that current is going someplace else.

Meggers can be quite useful, but require significant training in their use. Meggers operate at high voltage (500V, 1000V, 2000V and above are common), and so the meter itself can give you quite a shock (it is current limited, so the high voltage won't kill you...but the voltage can jump in ways that you don't expect. In addition, you need to disconnect any equipment which would be damaged by the high voltage!

-Jon
 
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Old 05-16-04, 11:28 AM
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You could perform such tests using GFI protective devices. For a 120-volt appliance,succh as a washer. simply use a GFI receptacle.

Testing 220-volt appliances would require 2-pole GFI circuit-breakers which are expensive.

Good Luck & Enjoy the Experience.!!!!!!!!!!!
 
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Old 05-17-04, 02:36 AM
doingitmyself
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ground testing

Originally Posted by winnie
You can check for an energized frame by touching one volt meter probe to the frame, and touching the other probe to any convenient grounded metal.
Black lead to the grounding slot on a receptacle and the red lead to the frame, for instance?

Originally Posted by winnie
You can also detect leakage by measuring all the circuit conductors (both hot and return) at the same time; the _net_ current should be zero, because the current on the hot and the current on the return should balance and be in opposite directions. Any net current flow means that current is going someplace else.
do you mean to clamp both hot and neutral/grounded return together? I have a clamp on ammeter, but I have always checked current in each conductor separately - wasn't aware that one could safely place both hot and return together. This, then, would be a good way of testing if the load was balanced? For instance, I could place the clamp around both the hot and neutral wires of the incoming 3-wire SE cables together inside and determine the difference, in amps, for both sides of the panel?

I appreciate your responses and the time you all take to help us doitourselfers. Thanks so much.

-doingitmyself
 
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Old 05-17-04, 10:44 AM
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I often find it hard to be sure that a volt meter probe is touching the metal in the ground hole, rather than the plastic walls...and you also need to be certain of the receptacle ground...but if you are certain of the receptacle ground, then measuring between the frame and the ground pin is exactly what I am describing.

If the wires are fully insulated and flexible enough to both fit in the clamp, then it is safe to run the clamp around multiple wires in order to measure the _net_ current flowing. If you are willing to spend enough money, you can buy 'clamp on ammeters' where the 'clamp' is actually a flexible cable that can be threaded around various conductors.

Note however that current _must_ go somewhere, meaning that you can often find a single conductor which carries the net current that you are looking for. If you want to measure how well your hots are balanced, then measure the current on the _neutral_, which carries the imbalance. If you put the clamp around the entire SE cable, the reading _should_ be zero, because any current coming in on the hots will be balanced either by the other hot or by the neutral. If (with the clamp around the entire SE cable) you measure some current flow, this indicates some current leaking through your grounding electrode system.

-Jon
 
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