Multiwire circuit-stray voltage

[kvl]

Fellow worker just called me and asked for some guidance on what he is reading on his meter. He has run a multiwire branch circiut for some outlets and in checking his work, has found that L1 to neutral reads 120vac, he then checks L1 to ground and has 170vac. I told him to check the ground and neutral connections at the panel and at every box, and he assures me over the phone they are proper. It would seem that he has a ground wire somewhere making slight contact with L2 giving him the 170vac reading, however, wouldn't this read a 170vac upon checking L1 to neutral? Late night and probably not thinking right. Any suggestions? Thanks in advance.

 

[solaeros]

is he using a digital meter?
I would suggest backing these readings up with a solenoid type meter such as a "wiggy". I don't know how many time I've been misled relying on my digital alone.
Dana

 

[robbietan]

could be a floating ground. make sure that there is a ground on the transformer upstream to it and that the neutral is connected there and there only.

 

[poweringtech]

This may be a case of tapping an incorrect hot line to the center tap of a delta connected transformer. Make sure the the hot conductor is the leg where the center tap is located.

 

[George Stolz]

What is the voltage of the system?

 

[stray spark]

strange voltage

strange voltage

I am going to assume that he ran these circuits from an existing panel. First check the voltage on the breaker that he tied the circuit to. There is no such thing as slight contact to ground (i.e. dead short). My guess is you will find the problem long before your first box unless you have a multitude of problems. Just my 2 cents. Let us know.

 

[iwire]

If it is not the neutral then.....

is he using a digital meter?
I would suggest backing these readings up with a solenoid type meter such as a "wiggy". I don't know how many time I've been misled relying on my digital alone.
Dana

....I would bet solaros is correct.

Check this from 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.

 

[ptonsparky]

I hate to admit this but on occasion the bifocals don't quite work right & I have chased problems when the auto ranging meter actually was displaying millivolts.

 

[plate]

Fluke makes a small adapter that plugs into your high impedance meter to provide a lower impedance measurement. I purchased one last year and it works well. TL225

TL225 SureGrip? Stray Voltage Adapter Test Lead Kit

http://us.fluke.com/usen/products/A...FlukeUnitedStates&Category=LEAD(FlukeProducts)

 

[tallgirl]

Fluke makes a small adapter that plugs into your high impedance meter to provide a lower impedance measurement. I purchased one last year and it works well. TL225

TL225 SureGrip? Stray Voltage Adapter Test Lead Kit

http://us.fluke.com/usen/products/A...FlukeUnitedStates&Category=LEAD(FlukeProducts)

What does the "It can be used with all modern meters with standard input spacing. " comment mean? And if it means something bad, is something like that available for a Sperry DigiSnap?

One of those would allow me to leave my wiggy at home (and given how messed up it is, that would be a good thing!)

 

[plate]

Tallgirl, The input spacing is 0.75" center-to-center. I looked at the Sperry DigiSnap on line and it "looks" like it would fit.

I am with you, the less tools to carry, the better.

 

[iwire]

One of those would allow me to leave my wiggy at home (and given how messed up it is, that would be a good thing!)

Julie there is a reason after all this time wiggys are still made.

They are the right tool to carry with you for typical line voltage trouble shooting.

If you get a set with a basic continuity tester your good to go.

Before someone posts about how inaccurate they are or how you can not measure resistance with it....I know and it does not matter for 99% of the typical line voltage problems.

Think about it, your looking for bolted shorts or opens 99% of the time.

If the customer is describing symptoms like voltage drop than you go grab the DMM and a load to place on the circuit under test.

I also keep a Fluke T5-1000, Fluke 87, Fluke 36, a 400 amp AC/DC current probe for use with the 87, a Simpson 260 analog multimeter. I will also check on the adapter 'plate' pointed out.

That said I see more second guessing being done with DMMs than wiggys.

To each their own.:smile:

 

[tallgirl]

Julie there is a reason after all this time wiggys are still made.

They are the right tool to carry with you for typical line voltage trouble shooting.

Well, whenever I get (a) a truck (b) new tool box (c) bigger purse I'll be able to schlep around more gear. But I ain't got no truck, my tool box is filled with tools for fixing cars, and my purse is full of iPod, Palm Pilot and girl-stuff like lipstick

Seriously tho, I do love my wiggy and I think it's a great gadget. But I have to limit how much stuff I take with me.

Think about it, your looking for bolted shorts or opens 99% of the time.

Right now I'm looking for $40 petty cash to buy a cable so I can fab a test fixture to inject errors into a 3Gb/s cable assembly. I don't think my wiggy or a DMM is going to help there

We've considered bake sales in the past. Or standing on street corners with cardboard signs.

 

[tallgirl]

Tallgirl, The input spacing is 0.75" center-to-center. I looked at the Sperry DigiSnap on line and it "looks" like it would fit.

I am with you, the less tools to carry, the better.

Thanks -- I'll get out some pieces-parts and see what the actual measurement is on my DigiSnap. Now that it's working again I can start carrying it around again ...

 

[jinglis]

What was the reading neutral to ground at the receptacle. If it was 50 volts we have a problem. Start with the checking the voltages at the panel. If all is OK there then check the connections downstream on the new installation.

 

[kvl]

If it is not the neutral then.....



....I would bet solaros is correct.

Check this from NEMA

Sorry for the delayed response on what was found. He borrowed a 'wiggy 'and everthing checked out. Thanks IWire for the posted quote on the phantom voltages, good justification for having the right tools in play to minimize time on the job. Thanks everyone for your replies.