[andyfb]

Hi all,

I noticed when flipping off some 125V / 20A breakers in our warehouse that the AC voltage on my multimeter read 15V, with the breaker in the OFF position. I tried another breaker and it showed 1.8V, and another showed 0.7V when off.

I would have expected <100 mV as a guess for what the OFF voltage should measure on the screw terminal, but I am not from an electrical background so maybe that is somewhat standard...

So my question is whether these are expected voltages or whether my breakers may be bad. They are definitely very old.

 

[Know A Little]

You are most likely reading what is commonly referred to as ghost or phantom voltages, it is a byproduct of the type of meter you are using.

BUT You need to verify this before putzing around. A voltage tester with a low impedance input will resolve the problem. There are attachments you can by for your meter to minimize this problem.

http://support.fluke.com/find-sales/Download/Asset/2105317_A_w.pdf


http://www.amazon.com/Fluke-SV225-Voltage-Eliminator-Adapter/dp/B005H0NKDC

 

[andyfb]

Thanks very much !! This definitely sounds like the next step to check into...

 

[Msradell]

Actually just a cheap analog meter is better in this case. They are not as susceptible to picking up phantom voltage because they have a much lower input impedance compared to cheap and most middle of the line digital meters.

 

[gregzoll]

My question is why you are playing around in that panel and why do you have the meter with you. You are dealing with not only a dangerous situation, but also that you are dealing with 3 phase power. If your meter is cheap, you are going to read whacked out readings when the breaker is off. Just means that there is nothing wrong with the breaker, just that you have a cheap uncalibrated meter.

 

[Know A Little]

My question is why you are playing around in that panel and why do you have the meter with you. You are dealing with not only a dangerous situation, but also that you are dealing with 3 phase power. If your meter is cheap, you are going to read whacked out readings when the breaker is off. Just means that there is nothing wrong with the breaker, just that you have a cheap uncalibrated meter.

Three phase or single phase both can be dangerous, but based on your assumption he was where he should not by, by OSHA 1910, NO DIY'er should in any panel that is energized.


That is just not true some of the older higher end DMM's had this issue.

And calibrated or not the meters could read ghost/phantom voltage..

 

[WyrTwister]

Induction ?

Back feed ?

A trickle of voltage leaking through the circuit breaker ?

A poorly grounded neutral ?

Take your pick . Or come up with another answer .

I never much worried about it as long as it was around 24 VAC or less . Other than to check the neutral or neutrals .

God bless
Wyr

 

[andyfb]

Hi all, I don't have 3 phase power, just a bunch of single phase in a warehouse space. When I mentioned checking three breakers, it was not three legs of a 3 phase system, just an example of a few others that I checked. Little difference though.

I'm replacing some duplex wall outlets for quads because we keep running short on outlets in our warehouse. We're just powering small table-top devices like a battery charger, small dremel, etc... no issue with overloading amperage. Just needed more outlets to access and I of course wanted to see if the lines were truly dead when I flipped the breakers before handling the outlets.

My multimeter is indeed a cheap $20 multimeter that's been pretty accurate for other uses thus far. The phantom voltage idea sounds reasonable to a volt or so, and we do indeed have multiple lines running in the raceways. But I think the ~15V I measured on the line seems quite high for being a capacitive effect.

After coming back to it later, depowering the whole panel, and popping out the questionable breaker and putting it back in and repowering the panel, it then measured the same ~1V as the others when OFF. It seems to be more normal behavior now, although the cause of the higher measurements earlier are still a curiosity.

 

[mm11]

So, you're not an electrician, yet you're doing commercial electrical work?

 

[Know A Little]

So, you're not an electrician, yet you're doing commercial electrical work?

Depends on his location.