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Author Topic: Interesting outlet at show last night  (Read 7338 times)

Aaron Maurer

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Interesting outlet at show last night
« on: March 20, 2016, 11:39:42 am »

As I have learned from all the great folks on this site I always test each outlet with my non contact voltage detector. Found an outlet with both hot and neutral energized and ground was fine. To be honest I wasn't going to use it anyway as the venue plugged something into it which I think were Christmas lights. All other outlets were in good shape and the night went without issue. The one thing I learned to be most valuable was to test the microphones and all the chassis on the instruments. Had I not tested the outlet and potentially plugged into it I suppose conditions would/could have changed? 

So the question is what may have caused this?  Is this the floating neutral syndrome?  Why none of the other outlets in the room were effected and is it common to see this in one singular outlet? 
« Last Edit: March 20, 2016, 11:42:26 am by Aaron Maurer »
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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #1 on: March 20, 2016, 11:46:13 am »

Found an outlet with both hot and neutral energized and ground was fine.

One thing I can think of is that someone wired a standard 120-volt "Edison" receptacle with 240-volts. I've seen this a number of times at churches and gas stations as "special" outlets for the floor buffer or air compressor. Yes, that would probably cause your sound system to burn up anything that didn't have an auto-switching power supply. Always best to test first.
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John Roberts {JR}

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Re: Interesting outlet at show last night
« Reply #2 on: March 20, 2016, 12:11:11 pm »

As I have learned from all the great folks on this site I always test each outlet with my non contact voltage detector. Found an outlet with both hot and neutral energized and ground was fine. To be honest I wasn't going to use it anyway as the venue plugged something into it which I think were Christmas lights. All other outlets were in good shape and the night went without issue. The one thing I learned to be most valuable was to test the microphones and all the chassis on the instruments. Had I not tested the outlet and potentially plugged into it I suppose conditions would/could have changed? 

So the question is what may have caused this?  Is this the floating neutral syndrome?  Why none of the other outlets in the room were effected and is it common to see this in one singular outlet?
The floating neutral will only be energized if there is something plugged into that outlet, since the current path is through whatever is plugged in. You mention something was plugged in so perhaps an open neutral connection at that outlet.

JR
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Corey Scogin

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Re: Interesting outlet at show last night
« Reply #3 on: March 20, 2016, 02:46:28 pm »

I wouldn't think that a receptacle and the wires in a receptacle box are isolated enough to guarantee that an NCVT would not light up in the neutral slot. I would imagine that often, even being that close to the hot may cause some NCVTs to alarm.
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Jonathan Johnson

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Re: Interesting outlet at show last night
« Reply #4 on: March 20, 2016, 06:45:31 pm »

I wouldn't think that a receptacle and the wires in a receptacle box are isolated enough to guarantee that an NCVT would not light up in the neutral slot. I would imagine that often, even being that close to the hot may cause some NCVTs to alarm.

If the run from the panel is particularly long, and the NCVT is particularly sensitive, there could be capacitive charging of the neutral wire detected by the (extremely high impedance) NCVT.

If in doubt, check with a voltmeter.
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John Roberts {JR}

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Re: Interesting outlet at show last night
« Reply #5 on: March 20, 2016, 06:51:11 pm »

I wouldn't think that a receptacle and the wires in a receptacle box are isolated enough to guarantee that an NCVT would not light up in the neutral slot. I would imagine that often, even being that close to the hot may cause some NCVTs to alarm.
My (very cheap) NCVT can't even be close to the outlet without going off...

Since it doesn't have an on/off switch I can't even leave it on my work bench... I was going to remove the battery but now I can't remember where I put it so it wouldn't keep going off.  :o

JR
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John Roberts {JR}

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Re: Interesting outlet at show last night
« Reply #6 on: March 20, 2016, 06:59:41 pm »

If the run from the panel is particularly long, and the NCVT is particularly sensitive, there could be capacitive charging of the neutral wire detected by the (extremely high impedance) NCVT.

If in doubt, check with a voltmeter.
Not sure that I follow? The neutral should be a low impedance bonded to ground at the panel (I believe). Any capacitive coupling into neutral should be easily absorbed and dissipated.

JR
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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #7 on: March 20, 2016, 09:30:11 pm »

If the run from the panel is particularly long, and the NCVT is particularly sensitive, there could be capacitive charging of the neutral wire detected by the (extremely high impedance) NCVT.

If in doubt, check with a voltmeter.

I don't think that's what causes false positives. If the NCVT is too sensitive, and/or the physical construction of the outlet creates a large hot spot, then it can trigger most anywhere near the receptacle. For instance, a NCVT will often false trigger near the ground slot of a GFCI simply because there's a lot of electrial wiring near the center of the outlet.

To know for sure, there's no substitute for a meter on the outlet. Just remember that without an external ground reference you won't be able to find a RPBG with a meter. But the beauty of the NCVT is that it's really good at finding RPBG mis-wiring. So I generally use both for outlet checking.
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Stephen Swaffer

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Re: Interesting outlet at show last night
« Reply #8 on: March 20, 2016, 10:02:07 pm »

I don't think that's what causes false positives. If the NCVT is too sensitive, and/or the physical construction of the outlet creates a large hot spot, then it can trigger most anywhere near the receptacle. For instance, a NCVT will often false trigger near the ground slot of a GFCI simply because there's a lot of electrial wiring near the center of the outlet.

To know for sure, there's no substitute for a meter on the outlet. Just remember that without an external ground reference you won't be able to find a RPBG with a meter. But the beauty of the NCVT is that it's really good at finding RPBG mis-wiring. So I generally use both for outlet checking.

Actually a standard 3 light tester PLUS a NCVT is a pretty reliable quick test-obviously doesn't test the actual voltage though.  If you run into the new TR resistant receptacles a meter is tough to use-unless you have a replacement cord end handy.
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Steve Swaffer

Jonathan Johnson

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Re: Interesting outlet at show last night
« Reply #9 on: March 20, 2016, 10:28:20 pm »

If you run into the new TR resistant receptacles a meter is tough to use-unless you have a replacement cord end handy.

The trick, when metering from one slot to ground, is to first poke the probes into both slots, then move one of the probes to ground.

The TR shutters in the slots are designed so that both must be pushed out of the way simultaneously.
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