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#56575 - 09/25/05 11:57 PM Stray Voltage  
skingusmc  Offline
Member
Joined: May 2004
Posts: 116
Simi Valley, CA USA
Here is my situation -

There are 2 separat hot leads (yes, we have ground and neutral also) running from a sub-panel, through PVC conduit for about 25 feet (about 20 feet is underground)to a junction box. Here they go their separate ways, with each circuit supplying different pairs of outlets (receptacles).

We will call these circuit A and circuit B.

Circuit A is connected to the outlets and the CB in the sub-panel.

Circuit B is not connected at the sub-panel nor is it connect to any outlets.

With circuit A energized, I get a reading of 20VAC - 22VAC on the circuit B hot wire. The votage is readable from hot to neutral and from hot to ground. There is no voltage reading from neutral to ground.

Where is the votage on circuit B coming from and why?

I have continuity checked each hot wire, and they are not connected.

I have ruled out a short to ground or neutral in circuit A, as that would trip the CB.

I thought that perhaps it could be "induced" voltage, but then why on just the other hot wire (as electricity does not know what color the wires are) and why so much voltage.

The underground run does pass under a 1/2 HP , 240VAC pump motor, but it is approx. 24" underground and beneath a 6" concrete reenforced (wire mesh) slab. I don't think this has anything to do with it, but that is the layout.

All help is appreciated, and instant knolwedge in pill form would be even more appreciated (I hate shots).

Steve


Thanks
Steve

Work Gear for Electricians and the Trades

#56576 - 09/26/05 12:11 AM Re: Stray Voltage  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,057
Estero,Fl,usa
If you are measuring this with a digital meter it is probably normal. Just about any floating wire will show something on a digital. You don't see it on the ground and neutral because they are terminated in the panel.


Greg Fretwell

#56577 - 09/26/05 12:14 AM Re: Stray Voltage  
livetoride  Offline
Member
Joined: Jan 2005
Posts: 109
san diego ca usa
I think you have induced voltage and if you disconnected the white wire you would read the same voltage to ground on it as the hot wire. Just my best guess. Rod


#56578 - 09/26/05 12:31 AM Re: Stray Voltage  
skingusmc  Offline
Member
Joined: May 2004
Posts: 116
Simi Valley, CA USA
gfretwell -

Yes, I used a DMM, and have usually seen a few (less than 5) volts, I just thought that 20 - 22 volts was a bit much.

Livetoride -

I thought of induced voltage as well, but then thought that 20 - 22 volts was a bit much to be induced.

Perhaps this is fine though.

I am going to use a different DMM tomorrow and see if I get they same readings, as I guess my DMM could be whacky.

I noticed that neither of you offered an instant knowledge pill...darn!! (grin)(I would even have paid shipping)


Thanks
Steve

#56579 - 09/26/05 12:43 AM Re: Stray Voltage  
gfretwell  Online Content


Member
Joined: Jul 2004
Posts: 9,057
Estero,Fl,usa
DMMs are all "wacky" that way. The input impedance is so high that any "antenna" will show lots of volts. The fact that you have it in a pipe with a current carrying conductor will make a very effective "one turn" transformer for a few micro amps. Hang your wiggy on it and you will see the volts go away.


I will trade you a knowlege pill for a spelling pill [Linked Image]

[This message has been edited by gfretwell (edited 09-26-2005).]


Greg Fretwell

#56580 - 09/26/05 02:24 AM Re: Stray Voltage  
AllClear  Offline
Member
Joined: Aug 2005
Posts: 44
Belmond Iowa US
Use a Wiggy! It Doesn't Lie.

I love my Fluke 87 and 189 DMM's but I Still use a wiggy alot.
I received a MILD jolt ot of "dead" wires that we had just pulled in, because they were in a raceway with alot of motor runs, (3phase 480 volt)Some that were controlled by VFD's. It Was Strange to me because i did get out my DMM and the voltage would fluxuate between 30 to 90 volts about every 30 seconds. I've never figured out why though.
Someday I hope to understand why the voltage was so high. Probably harmonics, But I hate to use harmonics as an answer to everything.

Justin


It's Not The Fall That Kills You...
It's That Sudden Stop At The End

#56581 - 09/26/05 03:50 AM Re: Stray Voltage  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
skingusmc it is called phantom voltage and it is nothing to worry about. If I use a DMM on a dead conductor in a raceway with 277 volt conductors the dead one may show 90 volts.

Justin I can not agree more with your post. [Linked Image]

I also have a Fluke 87 and I think it is a great meter. However for basic line voltage troubleshooting I leave it in the truck.

A Wiggy with continuity tester is a better choice and as you said they do not lie. [Linked Image]

Check out this NEMA bulletin

Quote

NEMA

ENGINEERING DEPARTMENT

BULLETIN 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.


" NEMA recommends the use of a Listed low impedance multimeter...."

A wiggy works. [Linked Image]


[This message has been edited by iwire (edited 09-26-2005).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#56582 - 09/26/05 07:50 AM Re: Stray Voltage  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
The readings you obtained aren't at all high considering that the input impedance of the typical DMM is around 10 meg.

You can work out the voltage reading with some basic formulas. Assume, for example, a capacitance of 50pF between the hot wire and the unattached conductor.


Xc = 1 / (2 * pi * 60Hz * 50pF) = 53 meg. approx.

Z = SQRT (Xc^2 + R^2) = SQRT (53^2 + 10^2) = 54 meg.

I = E / Z = 120V / 54 megohms = 2.2 microamps.

Thus voltage across the meter = 2.2 * 10 meg = 22 volts.

The 50 picofarad figure I started from there is easily obtainable with a relatively short run of cable. Capacitance values much higher than that are common on longer runs.

Over here in the U.K. where we're normally dealing with circuits at 240V to ground, it's quite common to put a DMM on a disconnected conductor and read 100V or more. If you place a load on that wire though (e.g. a 60W bulb to ground), the voltage will disappear.


#56583 - 09/26/05 08:19 AM Re: Stray Voltage  
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
A simple test: place a regular light bulb on the dead hot-wire and the neutral, and then see if your meterreads anything.

This is one reason I prefer a solenoid-type of voltage meter (Is that a "wiggie"?); it places a decent load on the circuit.


Larry Fine
Fine Electric Co.
fineelectricco.com

#56584 - 09/26/05 12:55 PM Re: Stray Voltage  
skingusmc  Offline
Member
Joined: May 2004
Posts: 116
Simi Valley, CA USA
All -

Once again, thank for all the replies.

gfretwell - I hate it when that happens! [Linked Image] Guess I need both types of pills!

OK, which "wiggy" would you all recommend?

Steve


Thanks
Steve

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