Disconnected LOAD cable is somehow grounded?

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  #1  
Old 05-03-11, 10:45 PM
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Disconnected LOAD cable is somehow grounded?

I have a circuit spanning too many lights and receptacles. My goal is to run a new circuit from the breaker box and intercept this existing circuit, cutting it in half. I did this intercept at a ceiling light box. For protection, at the first receptacle downstream (of the newly split circuit), I added a GFCI receptacle. Inside this metal receptacle box were two cables. While attempting to determine the line/load cable, I found something interesting...I disconnected all the wires inside the metal receptacle box, however I left the safety ground intact (so all boxes on the circuit were safety grounded). When testing the line hot to the line neutral, my voltmeter showed '120'. However, when testing the line hot to the LOAD neutral and LOAD hot respectively, both also showed '120'. Testing the LOAD hot to the line neutral and LOAD neutral both showed 0 voltage.

After hooking up the GFCI receptacle, my polarity tester showed the hookup as being correct, the GFCI receptacle tripped when tested and my new AFCI breaker also tripped when tested (by pressing the AFCI trip button directly on the breaker). When powered, the new circuit acted completely normal.

Why would a voltmeter get a correct voltage reading when connected to a LOAD cable that's disconnected from the circuit? I was under the impression that a voltmeter needed to be connected to a properly grounded source in order to get a voltage reading...am I wrong? How could a LOAD cable be grounded while both the LOAD neutral and LOAD hot lines were disconnected? What am I missing here?
 
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Old 05-04-11, 09:25 AM
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I should add, I am using a Craftsman Multimeter and setting it to "600V AC".
 
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Old 05-04-11, 10:14 AM
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Voltage measurements to isolated cable segments are unreliable because they are subject to a phantom voltage effect so they can be disregarded. When the wires are all disconnected at the junction box, the load side wires are no longer connected to the electrical system, so they are in a sense floating in space. It would sort of be like measuring voltage with one probe in a receptacle and the other touching your hammer; the hammer isn't connected to anything so the measurement is not meaningful. It sounds like your circuit is wired normally given that the AFCI and GFCI devices work as expected (both would trip if the load neutral was actually grounded).
 
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Old 05-04-11, 11:05 AM
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Is the multimeter accurately reading the phantom voltage (not sure if this question even makes sense)? Because the reading is '120' and pretty consistent, I would assume it to either fluctuate or be lower. The floating circuit is not near any live circuits. There are appliances plugged into the floating circuit (computer, TV, gaming systems, DVD, etc), could these be contributing to the phantom voltage reading?
 
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Old 05-04-11, 12:54 PM
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Originally Posted by k19_1234 View Post
Is the multimeter accurately reading the phantom voltage (not sure if this question even makes sense)?
Yes, the effect is because the meter is too sensitive. The voltage (difference in potential) is actually there, but the energy that could be delivered is minuscule. From the point of view of a physicist you could say the measurement is correct, but as a power system diagnostic tool you could say it is not useful. Similar to how a static shock generated by rubbing your socks on the carpet could be measured at 50,000V by a sensitive instrument, but the actual energy dissipated by the shock is not harmful or useful.

could these be contributing to the phantom voltage reading?
Yes modern meters have electronics in them so sensitive as to pick up very weak phantom effects. The way to mitigate the effect is to put a resistor (such as an incandescent table lamp) in parallel with your meter probes, but it's not the safest thing to do in your specific situation where you have live wires hanging out of the box during testing.
 
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Old 05-04-11, 01:11 PM
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Using a solenoid tester or an analog multimeter rather then a digital multimeter will also help.
 
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