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#66719 - 06/15/06 12:30 PM Residual Voltage  
TNSunny  Offline
Joined: Dec 2005
Posts: 44
I've ran into this situation a couple of times now and was wondering if any of you had any idea as to what is causing it.

Residential - After turning off a standard single pole 15A toggle switch (i.e. no pilot light), I still get about 15 Volts coming through to the outlet in the ceiling when changing a light or ceiling fan. When the circuit breaker is turned off, the voltage drops to zero. Considering that this has happened several times in several different homes, and considering that the standard switch is breaking the connection on the ungrounded wire, then I have to conclude that this stray voltage is coming through on the grounded (neutral) conductor. But why?

I have two theories. One, is that the voltage is being induced on the wire. The other is that at least two circuits are sharing a neutral and the voltage is being back-fed. What do you think??? [Linked Image]


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#66720 - 06/15/06 12:46 PM Re: Residual Voltage  
Dnkldorf  Offline
Joined: Dec 2004
Posts: 1,064
nowhere usa
loose bonding connection?

#66721 - 06/15/06 12:56 PM Re: Residual Voltage  
hbiss  Offline
Joined: Dec 2003
Posts: 886
Hawthorne, NY USA
How about a third theory, but really an expansion on your first one about voltage being induced.

I'll bet you are using a digital VOM. The input impedance is quite high, so high that it puts very little load on the circuit. Because of that you are reading the induced voltage. Put any load between the test leads (I would say your fingers but be careful here!) and the voltage goes to zero.


#66722 - 06/15/06 01:42 PM Re: Residual Voltage  
TNSunny  Offline
Joined: Dec 2005
Posts: 44

Correct - I'm using a Fluke digital VOM. So, in essence what you're saying is that a load on the circuit would bleed the induced voltage to zero?

You may be right. It's just aggravating not being able to simply use my voltage sensor to verify if the power is actually off. It lights up and bleeps with 15 volts just as if it were 120 volts. I guess it's probably just better all the way around if I take a few extra seconds to shut off the breaker. [Linked Image]

[This message has been edited by TNSunny (edited 06-15-2006).]


#66723 - 06/15/06 03:16 PM Re: Residual Voltage  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA

No. 88

October 1998

Revised February 2003

Phantom Voltages

This Bulletin is intended to address the occurrence of so-called “phantom” voltages, a phenomenon detected during the testing of electrical conductors in the field.

Due to the high impedance of measuring instruments, a voltage reading may be detected on open conductors where there is no hard electrical connection to a voltage source. Conductors that are installed in close proximity to one another, and are capacitively coupled to each other, can cause this a.c. voltage reading. Such a reading could be 2 or 3 volts, or it may be as high as the voltage on the adjacent conductors. This is what is referred to as a “phantom” voltage.

According to Underwriters Laboratories Inc., this can be a harmless reading and can be caused by the high input impedance of the measuring instrument, which places very little loading on the circuit under test. The capacitance is increased as the length of the run is increased. A 50-foot run may produce a pronounced capacitance effect whereas a one-foot sample may not produce any.

Since the “phantom” voltage is a physical phenomenon involving very small values of capacitance, it cannot energize a load or cause physiological damage to a person.

Care must be taken to be sure that the voltage reading is a phantom voltage, which is caused by improper use of high impedance multimeters, and not as a result of a cable defect or improper installation, which may result in a shock hazard.

In order to help minimize the likelihood of reaching a wrong conclusion from this phenomenon, NEMA recommends the use of a Listed low impedance multimeter in place of a high impedance multimeter or other high impedance measuring device for testing on open conductors where there is no hard electrical connection. Without a low impedance measuring device, a high voltage reading is an inconclusive indication of possible faults in the cable.

Bob Badger
Construction & Maintenance Electrician

#66724 - 06/15/06 03:33 PM Re: Residual Voltage  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
I am very surprised meters sold to electricians don't have a setting that does put a 100 k ohm resistor <or so> across the leads to eliminate this problem. That high input impedance is handy if you are probing around in CMOS logic but it really serves little function when you are looking at line voltage.

Greg Fretwell

#66725 - 06/15/06 05:22 PM Re: Residual Voltage  
feather  Offline
Joined: Feb 2006
Posts: 25
Santa Maria CA USA
I issue a Fluke 179 and a Square D Wiggy, the Wiggy is for LOTO verification.

#66726 - 06/15/06 06:45 PM Re: Residual Voltage  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
I don't don't go along with induced voltages from other conductors. I think you are getting leakage resistance or capacitive effects across the switch. You have a much greater length of parallel conductors if your switch is on and your panel breaker is off so measured voltages shoud be higher. But they're not. If you have a Fluke 179, the input impedance is spec'd at >10Mohms. If you do the math, you come up with 105 volts dropping across >70Mohms to get the 15 volts that you are reading. If the resistance is infinate, <40pF of C would cause that much leakage at 60Hz. But just take the input impedance of your meter and multiply it by 7 in this case, to determine the amount of leakage resistance you're dealing with.

#66727 - 06/15/06 07:53 PM Re: Residual Voltage  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,142
Joe, what do you figure the coupling is along a 40-50' switch leg and how would that show up against several megohms of input impedance? That math always boggles my crusty old mind but I do know a digital metyer is a random number generator on a floating line voltage conductor. I have had a fancy Fluke 6080 (8060?) for well over 2 decades but I never trusted it a lick on conductors that were not solidly connected to a source or a load.

Greg Fretwell

#66728 - 06/15/06 08:47 PM Re: Residual Voltage  
JoeTestingEngr  Offline
Joined: Nov 2005
Posts: 791
Chicago, Il.
Greg, I couldn't even guess how much coupling we might get. Experience in the field has probably been in the 100s of mV to couple volt range. But coupling would be intuitively greater with the switch on and running the whole distance back to the open beaker at the panel, At any rate, you wouldn't expect a change when he opened the feeder breaker if the switch looked infinately open. Plus, you gotta admit that a switch that looks like 70M when it's open, is a pretty good switch.

If you're talking about the 8060 with the buttons down the left side and dB capable, I love it. The only thing I like better on my 187, is being able to capture SCADA burst amplitudes by using the min/max record function. At least we can externally lower our input z on hi-z meters and it's a lot harder to go the other way.

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