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05-27-2010, 11:48 AM   #1
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## Electric thread Hi-jack

Quote:
 Originally Posted by AllanJ Yes the hot to neutral should be around 118.5 volts, except that, as you said, it is not working. With one batch of outlets dead and another batch live on the same circuit, it is possible that the bad connection is at one of the live outlets, specifically where the wires continue on to the dead group. I am guessing that the neutral is broken somewhere and you are measuring phantom voltage of 97 volts. Phantom voltage is a phenomenon that is very lengthy to explain and it is more likely to be measured with modern digital voltmeters. Phantom voltage is measured in a dead line that runs within a cable juxtaposed with another line including a ground wire that could have completed the circuit. Example: Hot wire is intact supplying 120 volts. Ground wire is intact. Voltmeter probes touched to hot and neutral. Neutral is dead but miniscule amount of current can leak between it and the ground wire, through insulation and all. A circuit is indeed complete although only a few microamperes can make the jump from neutral to ground, but enough that the meter's internal circuitry rates it as 97 volts. Next, measure neutral to ground. Hot wire in the same cable leaks a miniscule amount of current into the dead neutral running alongside, the voltmeter picks it up and through the voltmeter circuitry to ground, the current happens to cause a readout of 1.5 volts on the meter. Different brands of meters will read out different numbers. A test that electricians often do to avoid confusion with phantom voltage is to use a small incandescent light, say 25 watts, with the terminals or leads acrross the supposedly 120 volt circuit. When phantom voltage is involved, it will not light up the bulb and will be seen to drop to zero when both the light and the voltmeter are put across the circuit.
Can you give us some documentation about how current leaks through insulation? I'd love to know about it. I'd also like to know about currents between neutral and ground in a properly installed circuit.

05-27-2010, 03:52 PM   #2
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Quote:
 Originally Posted by AllanJ Phantom currents "leaking through insulation" are due to capacitance. When AC voltage is present (relative to something such as ground) on one metal object and another nearby metal object is bonded to the same something (ground) then a flow of current is induced. In the situation, the two metal objects are lengths of wire in the same Romex cable. The material (plastic, air, glass, waxed paper etc.) separating the two metal object influences how much current will flow, as well as other things such as voltage, AC frequency, distance between the metal objects, surface area of the metal objects, etc. Probably not. If something inside the switch or receptacle got deformed during your efforts to yank the wire out, it's not safe to use any more. Also, the loose piece of wire could migrate within the switch or receptacle and cause a short circuit.
That's called induction. It has nothing to do with currents "leaking" through insulation.

05-27-2010, 03:56 PM   #3
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Quote:
 Originally Posted by brric Can you give us some documentation about how current leaks through insulation? I'd love to know about it. I'd also like to know about currents between neutral and ground in a properly installed circuit.
It's a capacitance effect. When AC voltage is present (relative to something such as ground) on one metal object and another nearby metal object is bonded to the same something (ground) then a flow of current is induced. In the situation, the two metal objects are lengths of wire in the same Romex cable.

Say you have the meter probes on the hot wire and the (dead) neutral wire. On the positive half cycle of the AC, current flows from hot through the meter to neutral and in general doesn't go any further but enough goes through the meter to build up a positive charge on the dead neutral wire. At the same time a negative charge builds up on the ground wire because that is bonded to neutral back at the panel.

On the negative half cycle the positive charge builds up on the ground wire and a negative charge coming through the meter builds up on the dead neutral.

The changeover from positive charge to negative charge and back again is synonymous with an AC current flow, although small, probably on the order of microamperes.

The material (plastic, air, glass, waxed paper etc.) separating the two metal object influences how much current will flow, as do other things (voltage, AC frequency, distance between the metal objects, surface area of the metal objects, etc.)

'
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Last edited by AllanJ; 05-27-2010 at 08:38 PM.

 05-27-2010, 06:39 PM #4 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 This is how much capacitance you should see http://en.wikipedia.org/wiki/Capacitance see Pair of parallel wires The current seems to flow through the dielectric/insulator, but it doesn't. The charge builds up and bleeds down on each conductor over each cycle. For 10' of Romex connected to 120vac @ 60 Hz you should see about 60 uA of current due to the capacitive reactance. Or, just use a C-meter. Last edited by Yoyizit; 05-27-2010 at 06:48 PM.
05-27-2010, 06:59 PM   #5
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Quote:
 Originally Posted by Yoyizit This is how much capacitance you should see http://en.wikipedia.org/wiki/Capacitance see Pair of parallel wires The current seems to flow through the dielectric/insulator, but it doesn't. The charge builds up and bleeds down on each conductor over each cycle. For 10' of Romex connected to 120vac @ 60 Hz you should see about 60 uA of current due to the capacitive reactance. Or, just use a C-meter.
\
Capacitive reactance is an impedance to current flow not a producer of current.

05-27-2010, 07:15 PM   #6
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Quote:
 Originally Posted by brric \ Capacitive reactance is an impedance to current flow not a producer of current.
A voltage/current source, and a circuit, was implied.

What makes phantom voltages weird is that, usually you are measuring the output of a voltage source with a voltmeter.
In the phantom voltage case you are measuring the voltage output of a current source, that source being 120vac in series with a few hundred K of capacitive reactance. In this case, a cheapie 1000 ohms/volt meter which input impedance changes with each range is telling you more truth than a constant high input impedance DVM.

Last edited by Yoyizit; 05-27-2010 at 07:19 PM.

05-27-2010, 07:18 PM   #7
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Quote:
 Originally Posted by Yoyizit A voltage/current source, and a circuit, was implied.
So what? Capacitive reactance is expressed in ohms not uAmps.

05-27-2010, 08:53 PM   #8
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Quote:
 ... currents between neutral and ground ...
In a properly installed and properly functioning system the neutrals are bonded to the grounds onlyh at the main panel.

If there is no fault (unwanted contact between wires and/or other metal objects) then there is no load current returning to the panel via the ground but positive/negative charges can still build up on the ground wire running with (juxtaposed with) the neutral and also the hot wire. Any net positive/negative imbalance between neutral and ground is drained off, or cancelled out if you insist, at the panel neutral/ground bus bar. The act of draining this off is a minute current flowing down the ground wire to the panel.

Voltage across any two points in the electrical system equals current times resistance between those points. It is possible that for a circuit with several amperes of load, there is a difference of a few volts between the neutral at the load location and ground (due to resistance in the neutral and therefore voltage drop). But any phantom current induced in the ground is so small that there is almost zero voltage drop in the (probably similarly sized) ground wire from the same location we measured the voltage drop in the neutral. Then measuring the voltage between neutral and ground out at that location reveals just the few volts of voltage drop in the neutral.
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Last edited by AllanJ; 05-27-2010 at 09:14 PM.

05-27-2010, 09:17 PM   #9
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Quote:
 Originally Posted by brric So what? Capacitive reactance is expressed in ohms not uAmps.
Ay, yi, yi. . .

05-27-2010, 09:22 PM   #10
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Quote:
 Originally Posted by AllanJ In a properly installed and properly functioning system the neutrals are bonded to the grounds onlyh at the main panel. If there is no fault (unwanted contact between wires and/or other metal objects) then there is no load current returning to the panel via the ground but positive/negative charges can still build up on the ground wire running with (juxtaposed with) the neutral and also the hot wire. Any net positive/negative imbalance between neutral and ground is drained off, or cancelled out if you insist, at the panel neutral/ground bus bar. The act of draining this off is a minute current flowing down the ground wire to the panel. Voltage across any two points in the electrical system equals current times resistance between those points. It is possible that for a circuit with several amperes of load, there is a difference of a few volts between the neutral at the load location and ground (due to resistance in the neutral and therefore voltage drop). But any phantom current induced in the ground is so small that there is almost zero voltage drop in the (probably similarly sized) ground wire from the same location we measured the voltage drop in the neutral. Then measuring the voltage between neutral and ground out at that location reveals just the few volts of voltage drop in the neutral.
That is ridiculous. You cannot have cureent flow without a difference in potential. Neutrals and equipment grounds are at the same potential. There is no current between neutrals and grounds in a normal, correctly installed, fault free circuit.

 05-27-2010, 09:27 PM #11 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 Here. Take two of these, http://www.youtube.com/watch?v=uU41S...eature=related go to bed and call me in the morning.
 05-28-2010, 11:59 AM #12 Member   Join Date: Jul 2008 Location: NW of D.C. Posts: 5,990 Rewards Points: 2,000 Yo, brric! Did you survive the night with those two women I sent you? If so, you're a better man than I BTW, in general, this statement You cannot have cureent flow without a difference in potential is false. Last edited by Yoyizit; 05-28-2010 at 12:16 PM.
05-28-2010, 12:28 PM   #13

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Quote:
 Originally Posted by Yoyizit BTW, in general, this statement You cannot have cureent flow without a difference in potential is false.
Care to explain how two items at the same potential would have a flow between them?
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05-28-2010, 12:45 PM   #14
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Quote:
 Originally Posted by Jim Port Care to explain how two items at the same potential would have a flow between them?
Thank you, Jim.

05-28-2010, 03:28 PM   #15
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You hook up a current source to a short circuit.
By definition, the setpoint current flows; 1A, 0.1A, whatever.
By definition, there is zero voltage drop across a short circuit.

And this is without using superconductors.

Congrats on surviving those two women. Next time I'll send you four.

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