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Stray voltage ?? #40170 07/13/04 09:46 PM
Joined: Jan 2004
Posts: 171
M
Merlin Offline OP
Member
I have got an interesting situation. I wonder if anyone has an answer. I have a 12 conductor, 10 AWG cable that runs approx. 750 foot. It carries 480 volt on two legs to a transformer. Ground on one conductor. Six conductors are 2 seperate three wire control circuits ( 480 V control ). These six have no connection to the to legs of 480 other than incased in the same cable. The other three cond. are spares.

My problem is when I energize the two 480 conductors, I am getting stray voltage back on the six control wires. Why ??? Is it possible they are "bleeding" over in the cable. If so, what can I do ?

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Re: Stray voltage ?? #40171 07/14/04 04:02 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Not much you can do about it short of using separate cables.

How are you finding the voltage?

Is it causing problems on the control circuits or are you just measuring it with a digital meter?


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Re: Stray voltage ?? #40172 07/14/04 04:35 AM
Joined: Jan 2004
Posts: 171
M
Merlin Offline OP
Member
Just measuring so far about 90 volts. I have not energized the hole system yet. Thanks iwire.

Re: Stray voltage ?? #40173 07/14/04 04:42 AM
Joined: Jan 2003
Posts: 4,391
I
iwire Offline
Moderator
Try measuring it with an older analog meter.

Or Connect a set of wiggy testers to the same set of wires as the digital meter is connected to.

You will probably see the 90 V disappear, the digital does not load the circuit enough to drag the stray voltage down.

Here is a NEMA statement on this problem.

You can get the original from nema.org use the search feature for "Bulletin 88"

Quote
Code
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
Massachusetts
Re: Stray voltage ?? #40174 07/14/04 02:27 PM
Joined: Oct 2003
Posts: 156
D
dereckbc Offline
Member
It is a ghost voltage from the high input impedance of your digital meter. You can prove it to yourself by just holding the leads of your meter apart, or just connect one lead to a ground and leave the other open. The erradic reading is a ghost voltage. Now short your leads together and you will read 0.

[This message has been edited by dereckbc (edited 07-14-2004).]

Re: Stray voltage ?? #40175 07/14/04 03:36 PM
Joined: May 2004
Posts: 73
R
Ralpha494 Offline
Member
I ran into something like this remodelling an office area. A 150 foot long 3/4 inch pipe with a brown, orange, and yellow #12 and a #10 gray going directly to a 480/277 panel. The orange wire was the one that was supposed to be feeding the area as per the print. I locked it off, went back to test it off with my little glow tester and found that the tester said it was hot. I went and got the Fluke, it said 89 volts. I thought it must be back feeding thru one of the other circuits and/or have the wrong neutral in some other box or something. I then proceeded take the wire off the breaker. 89 volts still. So now I have both ends of that wire disconnected and 89 volts on it. I remembered about induction and touched the wire to the box, got a little spark, retested with the voltmeter and got 15 volts and climbing. It took a few minutes and it was back up to 89. I ended up jumpering it to ground in the panel until after I was done splicing it in the box. I'm sure the 89 volts would have "surprised" me off of the ladder. I've been checking everything since I took OSHA 10 and NFPA 70E and realized how lucky I've been.

Re: Stray voltage ?? #40176 07/14/04 07:19 PM
Joined: Dec 2002
Posts: 308
E
Edward Offline
Member
As others have explained it is phantom voltage or ghost.

If it has a white sheet over it then it is probably ghost voltage. [Linked Image]


Edward


Thanks
Edward
Re: Stray voltage ?? #40177 07/14/04 08:53 PM
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
Very entertaining when it's existing wiring and close to another voltage, like 24 or 120.

Re: Stray voltage ?? #40178 07/15/04 12:13 AM
Joined: Apr 2002
Posts: 2,527
B
Bjarney Offline
Moderator
Maybe I'm misunderstanding the concern, but couldn’t both ends of the spares be temporally be insulated? Then, don't worry about them until they're needed. If they're not being used at this point, and you have no need to contact them, using a high-impedance meter for testing shouldn’t be necessary.

If you must work them while other conductors in a common cable are hot, then temporarily ground the spares at one end. Just don’t forget later that they are grounded.

Re: Stray voltage ?? #40179 07/15/04 04:37 AM
Joined: Jan 2004
Posts: 171
M
Merlin Offline OP
Member
Thanks for all the information gentlemen. Yes Bjarney, I have the spares insulated. But my concern was the "ghost" voltage interfering with the control circuit in this cable. I was told to try to install a petentiometer on these control wires and try to "bleed" it off. We'll see what happens. Thanks again for info. Kevin

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