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#99290 - 08/04/06 11:29 PM strange voltage reading  
watersparkfalls  Offline
Member
Joined: Mar 2005
Posts: 216
Washington...Not DC
alright folks this one really has me stumpted. i have a 14-3 home run which wasnt holding(red leg tripping)which upon trouble shooting discovered it was the hall three way traveler and ground dead short, nail likely suspect as a soffit was added after the fact... but while trouble shooting this dead short i discovered that the home run had/has an anomaly.
with all the wires disconnected at the box i did a volt and continuity check to the three wire home run, i got a reading of 36 volts to ground from the red leg while it was shut off at the panel and the black circuit was turned on which read 120 volts to ground and vice versa whith the red circuit on and the black circuit off.
between the two circuits with one turned off i got a reading of 72 volts conductor to conductor. i dont understand whats happening with this multi-wire home run.
and more confusing is how there is a drop of the same phase(impossible from everything i was taught)there isnt any continuity between any of the conductors(except the white and bare landed on ground bar) red to black, red to white, red to bare. black to red, black to white or black to bare. so how can i see a drop with one circuit on across red and black? it is totally disconnected and shows no ohms yet i have double voltage leg to leg and half that leg to ground.

experience is the best teacher and i am hoping you guys and all your experience can explain this one to me.

thanks,h2o


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#99291 - 08/05/06 12:19 AM Re: strange voltage reading  
mxslick  Offline
Member
Joined: Oct 2004
Posts: 803
Atomic City, ID USA
h2o:

Are you using a digital voltmeter? If you are what you're seeing is capacative coupling in the wire which will give a reading even on the "dead" leg. Same thing happens with a DVM if you try to meter a circuit breaker which is off and has no load connected. (Very small leakage across the breaker's internals.)

Try the same test with a wiggy, you will most likely see no reaction at all on the wiggy.

ANd to prove it, also tie the leads of your digital meter and thew wiggy togeter so they're both testing at the same time, I'll bet you'll see nothing on the DVM.


Stupid should be painful.

#99292 - 08/05/06 08:53 AM Re: strange voltage reading  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Is it possible you are reading through the loads? Or that this ground or grounded conductor is loose or cut?


Mark Heller
"Well - I oughta....." -Jackie Gleason

#99293 - 08/13/06 01:31 AM Re: strange voltage reading  
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
Water, if you want real voltage readings, try a night-light bulb in parallel with your test leads. That'll minimize induced voltages.


Larry Fine
Fine Electric Co.
fineelectricco.com

#99294 - 09/19/06 09:27 PM Re: strange voltage reading  
tajoch  Offline
Member
Joined: Feb 2001
Posts: 54
Quinlan texas, usa
Speakin of DVM VS Wiggies:
One plant I worked in, they did an upgrade on one of the (in - plant) Substations, and part of the up grade was that, they ran new feeders from the substation to all the bus ducts installed in the (overheads)/ceilings of the department. Well right after this upgrade, the people in the department started complaining that some of the lights, and the emergency/exit lights were no longer working. So Z who was the electrician(wannabe) for that department, was tasked w/ the job of fixing/repairing all these lights, (and this was a large portion of the facility). He went out and checked voltage w/ his DVM(fluke) at each unit, (read 120vac to ground/nuetral) tried changing the lamps.
decided that the fixtures were bad. so he changed out the bad light fixtures and the bad exit/emergency lights. ( he didn't bother to tell any of us older guyz exactly what he found or anything else.....(lol)
But (for good or bad: we had a warranty policy at that plant, if you worked on it, you worked on it til it was fixed)
So anyway he replaced all the affected fixtures, and then a week or so later he was again tasked w/ troubleshooting them again, cause they were all out again, (now keep in mind that Z wasn't the brightest lamp in the box....lol)
So he tests them again, and decides that once again all the fixtures are burned out, so he orders and installs new units..........
and another week goes by, and he again gets tasked w/ troubleshooting the exact same fixtures for not being on.....
SO once again he orders replacements....
at thsi point the supervisor is begining to wonder WTF is going on.... so he mentions to T and myself what has been going on, and what Z has done....
I could see why he was upset, each time Z replaced 20 HID metal halide fixtures, and the same number of exit/emergency fixtures (x3). Well we went and looked and w/ a DVM you had 110VAC to ground and (with all the fixtures on the circuit) about a 100vac to neutral. T was an older guy, He sent me for his Wiggie, which gave us a different reading, we had 110VAc to ground, and 0vac to neutral.
After a little bit of investigation we found that when they did the upgrade, that the contractor went to the engineer, and asked if they needed to pull a neutral for this one busduct as it was there but wasn't on the prints, and wasn't on any of the other busducts, so the engineer told them no it wasn't needed.
So with out a real Neutral Z was hooking these 120Vac fixtures up to 23VAC, no wonder that they fried.
I've kept a wiggie in my tool bag ever since.....


#99295 - 09/20/06 11:24 PM Re: strange voltage reading  
coolshoes  Offline
Junior Member
Joined: Jun 2006
Posts: 3
Hi guys.

I am a consistent follower of the forum, but haven't posted anything yet. I have seen this happen a few times. I would guess that your 14/3 is still connected to a three way switch somewhere (is there a box buried in the wall?) I have read 30+ volts coming off of the "dead" traveller in a live three/four way lighting circuit many times. By the way, can anyone tell me what causes this?


#99296 - 09/22/06 02:30 AM Re: strange voltage reading  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
Induction..... The open conductor run parralel to the phase or control wire. Reading with a DMM will show a potential - although not a usable potential.

But if it were a usable potential in voltages that low - It might not register on a wiggy. Or a voltage like ~90 would look like 120....


Mark Heller
"Well - I oughta....." -Jackie Gleason


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