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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Aaron Maurer on March 20, 2016, 11:39:42 AM
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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?
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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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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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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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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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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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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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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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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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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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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.
Yes, it's low impedance. It's VERY low impedance. But there is still capacitive coupling, and if the impedance is *just* high enough and the test equipment has VERY high -- virtually infinite -- impedance (such as an NCVT), the capacitive coupling should be measurable.
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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.
I generally take along a SureTest Analyzer which tests EVERYTHING at once including voltage, frequency, voltage drop at 15 and 20amp loads, ground impedance, etc... However, it still can't find an RPBG, so I always use a NCVT to check for a hot ground. http://www.idealindustries.ca/products/test_measurement/circuit_analyzers/suretest_circuit_analyzers.php
At $300 a SureTest Analyzer is pretty expensive for casual testing, but maybe worth it for touring where you don't want to plug a $100,000 console into an unknown receptacle. Even if you bring along your own distro, you never really know what it's plugged into unless you meter it first.
Now that I think about it, I've never plugged a SureTest Analyzer into an outlet miswired with 240-volts, so I'm going to talk to their engineers to see if it will survive that kind of over-voltage before I blow up a $300 tester.
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Yes, it's low impedance. It's VERY low impedance. But there is still capacitive coupling, and if the impedance is *just* high enough and the test equipment has VERY high -- virtually infinite -- impedance (such as an NCVT), the capacitive coupling should be measurable.
Sorry to press on this but hypothetically a neutral with <1 ohm of resistance to ground and perhaps 1000pF of capacitance between line and neutral, at 60 Hz and 120VAC will not generate much current (<50 uA). This current times 1 ohm is <50 uV.
The impedance of the NCVT is not important but it's voltage sensitivity, or how small of a voltage it will detect is. Even if you scale up both my estimates by 10x we are still talking only mV of neutral voltage which IMO is below the threshold for even the most sensitive NCVT.
OTOH if that neutral wire were completely floating we wouldn't have 1 ohm to ground, but more like a capacitive divider with perhaps 1/2 the mains voltage present to a high impedance VOM. This might be detected by a NCVT. However if something is plugged into the outlet (like the OP mentioned) with a fully floating neutral (my speculation), there could be a significant current path between line and neutral, and measurable voltage on the outlet neutral lead.
Of course this is speculation, but i don't think my hypotheticals are that far from reality.
JR
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Sorry to press on this but hypothetically a neutral with <1 ohm of resistance to ground and perhaps 1000pF of capacitance between line and neutral, at 60 Hz and 120VAC will not generate much current (<50 uA). This current times 1 ohm is <50 uV.
If the neutral was open and floating, anything you plug into it would be non-operational.
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Guys, NCVTs and 3-bulb testers are indicative, and not a complete test. Get a decent voltmeter or receptacle tester and the guesswork will be gone.
My favorite DMM for production - $150, does high and low impedance AC voltage measurements, has built-in NCVT:
http://en-us.fluke.com/products/digital-multimeters/fluke-117-digital-multimeter.html
The receptacle tester I use - ~$160, does load testing, ground impedance testing, and can survive being plugged into 240v:
http://www.extech.com/instruments/resources/datasheets/CT70data.pdf
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Guys, NCVTs and 3-bulb testers are indicative, and not a complete test. Get a decent voltmeter or receptacle tester and the guesswork will be gone.
My favorite DMM for production - $150, does high and low impedance AC voltage measurements, has built-in NCVT:
http://en-us.fluke.com/products/digital-multimeters/fluke-117-digital-multimeter.html
The receptacle tester I use - ~$160, does load testing, ground impedance testing, and can survive being plugged into 240v:
http://www.extech.com/instruments/resources/datasheets/CT70data.pdf
I have a CT70 also and use it less than I thought I might, but it's a great tool.
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I have a CT70 also and use it less than I thought I might, but it's a great tool.
I have an Extech CT70, an Amprobe INSP-3 and an Ideal Suretest on my bench, plus a vintage Woodhead GLIT meter from England that I bought back in the mid-70's. Since I'm looking for new video review projects, perhaps I'll do a bit of a comparison/shootout of these meters.
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Sorry to press on this but hypothetically a neutral with <1 ohm of resistance to ground and perhaps 1000pF of capacitance between line and neutral, at 60 Hz and 120VAC will not generate much current (<50 uA). This current times 1 ohm is <50 uV.
The impedance of the NCVT is not important but it's voltage sensitivity, or how small of a voltage it will detect is. Even if you scale up both my estimates by 10x we are still talking only mV of neutral voltage which IMO is below the threshold for even the most sensitive NCVT.
OTOH if that neutral wire were completely floating we wouldn't have 1 ohm to ground, but more like a capacitive divider with perhaps 1/2 the mains voltage present to a high impedance VOM. This might be detected by a NCVT. However if something is plugged into the outlet (like the OP mentioned) with a fully floating neutral (my speculation), there could be a significant current path between line and neutral, and measurable voltage on the outlet neutral lead.
Of course this is speculation, but i don't think my hypotheticals are that far from reality.
JR
I can't express the math and physics to back up my assertions. But here's another mystery to consider:
In my family room, I have a ceiling fan with a light kit and night light function. The fan motor and the lights are controlled by separate wall switches, in addition to the pull-chains on the unit itself. I have installed four C7 LED bulbs in the sockets for the night light.
If I pull the chains so the night light function is "on" -- the position of the pull chain for the speed of the fan motor seems irrelevant -- and have both wall switches off, the night lights glow very dimly. If I turn on the wall switch for the fan, (again, the fan pull chain doesn't make a difference) the night lights glow slightly brighter (but not full brightness).
In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
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I can't express the math and physics to back up my assertions. But here's another mystery to consider:
In my family room, I have a ceiling fan with a light kit and night light function. The fan motor and the lights are controlled by separate wall switches, in addition to the pull-chains on the unit itself. I have installed four C7 LED bulbs in the sockets for the night light.
If I pull the chains so the night light function is "on" -- the position of the pull chain for the speed of the fan motor seems irrelevant -- and have both wall switches off, the night lights glow very dimly. If I turn on the wall switch for the fan, (again, the fan pull chain doesn't make a difference) the night lights glow slightly brighter (but not full brightness).
In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
I have a small collection of LED lights for home use. More than a dozen types as they each have features and oddities for the different uses I have in mined. Several exhibit this glow your talking about. Typically the ones that may glow when "off" are dimmable and I have them on some kind of low end dimmer. Moving them to a hard off or to a dimmer with some kind of filtering and they work different.
LED's work on current, not voltage. They do have a forward voltage rating of 1.5 to 4 volts depending on the construction. The dimmable ones have some kind of circuit to translate 0 to 120 volts chopped AC to 0 to full current at the DC forward voltage of the unit. Any kind of noise or phantom voltage on the line may get them to glow, they only need a few ma to light.
As for the OP's observation and the discussion on floating neutrals. Floating neutrals come in shades of gray. A neutral may be high impedance from corrosion, under sizing, or lose connections. Varying loads on on different legs may "pull" that voltage around. See:
https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws (https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws)
EDIT, Forgot to add that it is not just what's plugged into the outlet. It's all the loads on the whole electrical system down stream of the transformer and the location of the bad neutral that will determine the neutral voltage.
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In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
Always remember Occam's razor for troubleshooting strange occurrences. entia non sunt multiplicanda praeter necessitatem (entities must not be multiplied beyond necessity) or as I like to think of it - KISS (Keep It Stupid Simple). Yes, that's backwards of Keep It Simple Stupid, but hey it's my quote.
There will usually be some really Stupid-Simple explanation for this phenomenon if you can get your mind wrapped about the various impedances, leakages, capacitive and inductive couplings, etc... It's typically one obvious cause once you separate out all the complex explanations.
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I can't express the math and physics to back up my assertions. But here's another mystery to consider:
In my family room, I have a ceiling fan with a light kit and night light function. The fan motor and the lights are controlled by separate wall switches, in addition to the pull-chains on the unit itself. I have installed four C7 LED bulbs in the sockets for the night light.
If I pull the chains so the night light function is "on" -- the position of the pull chain for the speed of the fan motor seems irrelevant -- and have both wall switches off, the night lights glow very dimly. If I turn on the wall switch for the fan, (again, the fan pull chain doesn't make a difference) the night lights glow slightly brighter (but not full brightness).
In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
A perhaps simpler explanation, if line and neutral are reversed, such that the light switch is breaking the neutral side instead of breaking the hot, line side. When switched off the lamp is energized and swinging 120VAC just with a floating return. Any capacitance on the return side may draw enough current to explain the glow coming from very efficient LED lamps. I have heard similar reports about CFL lamps that glow briefly after being turned off.
Caveat, this is just speculation but logical to me.
JR
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My favorite DMM for production - $150, does high and low impedance AC voltage measurements, has built-in NCVT:
http://en-us.fluke.com/products/digital-multimeters/fluke-117-digital-multimeter.html
(off topic)
Here's the tag line for the Fluke 117:
The Fluke 117 digital multimeter is accurate even in low light. Check out the best non-contact multimeter for efficiency in demanding settings today!
This struck me as a strange/funny description. Especially the first sentence. Are they saying that many meters are not accurate in low light? Does this meter allow me to measure things without contacting them?
I know what they're trying to say but they're not saying it well.
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In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
I think the simple answer lies in the switch loop. There is an unswitched hot running in parallel to the switched feeds to the light and fan. When the switches are open, the hot capacitively couples to the switched feeds. When turning on the fan switch, now there are two hots capacitively coupling to the light feed. The voltage induced by the coupling is enough to light the LEDs; the impedance of the LEDs is high enough to not draw the voltage down to zero.
Basically, the LED lamps in conjunction with the switch wiring are acting as a NCVT.
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I think the simple answer lies in the switch loop. There is an unswitched hot running in parallel to the switched feeds to the light and fan. When the switches are open, the hot capacitively couples to the switched feeds. When turning on the fan switch, now there are two hots capacitively coupling to the light feed. The voltage induced by the coupling is enough to light the LEDs; the impedance of the LEDs is high enough to not draw the voltage down to zero.
Basically, the LED lamps in conjunction with the switch wiring are acting as a NCVT.
That is logical too, I have heard reports of glowing CFL/LED lamps in circuits that didn't have an extra live wire in the bundle.
One obvious difference between a NCVT and your situation is the parallel wire run, tens (?) of feet long could have significant coupling.
JR
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I can't express the math and physics to back up my assertions. But here's another mystery to consider:
In my family room, I have a ceiling fan with a light kit and night light function. The fan motor and the lights are controlled by separate wall switches, in addition to the pull-chains on the unit itself. I have installed four C7 LED bulbs in the sockets for the night light.
If I pull the chains so the night light function is "on" -- the position of the pull chain for the speed of the fan motor seems irrelevant -- and have both wall switches off, the night lights glow very dimly. If I turn on the wall switch for the fan, (again, the fan pull chain doesn't make a difference) the night lights glow slightly brighter (but not full brightness).
In summary, with supposedly no power going to the fan/light unit, the LED night lights still glow. Hmmm.
Is it safe to assume that you are being precise is using the term switches? The lights are not controlled by a dimmer, correct?
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Is it safe to assume that you are being precise is using the term switches? The lights are not controlled by a dimmer, correct?
Correct. No dimmer in the circuit. (In fact, now that I think of it, there are no dimmers anywhere in my house. I haven't needed one and the wife hasn't asked for one, so maybe I'd better keep my mouth shut. ;) )
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Some older switches have a neon bulb. That might be enough leakage for an LED or sensitive meter.
Some nit-picking electricians say that a switch w/neon changes a switch wall-box to an outlet box.
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I generally take along a SureTest Analyzer which tests EVERYTHING at once including voltage, frequency, voltage drop at 15 and 20amp loads, ground impedance, etc... However, it still can't find an RPBG, so I always use a NCVT to check for a hot ground. http://www.idealindustries.ca/products/test_measurement/circuit_analyzers/suretest_circuit_analyzers.php
At $300 a SureTest Analyzer is pretty expensive for casual testing, but maybe worth it for touring where you don't want to plug a $100,000 console into an unknown receptacle. Even if you bring along your own distro, you never really know what it's plugged into unless you meter it first.
Now that I think about it, I've never plugged a SureTest Analyzer into an outlet miswired with 240-volts, so I'm going to talk to their engineers to see if it will survive that kind of over-voltage before I blow up a $300 tester.
Sorry to wake this zombie thread, but I wanted to add some info that I found when doing some research and looked at the link Mike provided. According to the Ideal site for the SureTest, the AC Voltage Range is 85.0 - 265.00 VAC. So it appears that it will be fine if you happen to use it on an outlet that is miswired. I can think of one time when I plugged my distro, which had a 50 amp range plug on it (NEMA 14-50) into a receptacle that was wired 3 phase. This would have been extremely handy to have. >:(
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Sorry to wake this zombie thread, but I wanted to add some info that I found when doing some research and looked at the link Mike provided. According to the Ideal site for the SureTest, the AC Voltage Range is 85.0 - 265.00 VAC. So it appears that it will be fine if you happen to use it on an outlet that is miswired. I can think of one time when I plugged my distro, which had a 50 amp range plug on it (NEMA 14-50) into a receptacle that was wired 3 phase. This would have been extremely handy to have. >:(
Back when I was still optimistically pursuing my outlet tester I bench tested a neon lamp driven from a resistive voltage divider that would illuminate for 240V but remain dark for 120V.
A decent VOM can confirm AC voltage pretty competently.
JR
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Back when I was still optimistically pursuing my outlet tester I bench tested a neon lamp driven from a resistive voltage divider that would illuminate for 240V but remain dark for 120V.
A decent VOM can confirm AC voltage pretty competently.
JR
Completely agree. I'm looking into the SureTest for the voltage drop under load ability as well as the frequency measurement, as well as some of the other test functions that the $2 outlet tester can't do.
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Sorry to wake this zombie thread, but I wanted to add some info that I found when doing some research and looked at the link Mike provided. According to the Ideal site for the SureTest, the AC Voltage Range is 85.0 - 265.00 VAC. So it appears that it will be fine if you happen to use it on an outlet that is miswired. I can think of one time when I plugged my distro, which had a 50 amp range plug on it (NEMA 14-50) into a receptacle that was wired 3 phase. This would have been extremely handy to have. >:(
I recently spoke to an application engineer at Ideal (the makes of the SureTest Analyzer), and they confirmed that it won't be damaged by plugging it into 240 volts. In fact at my request they plugged on into 240 volts and let it run all day without damage. I love this meter...
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Completely agree. I'm looking into the SureTest for the voltage drop under load ability as well as the frequency measurement, as well as some of the other test functions that the $2 outlet tester can't do.
I also use it on a lot of ground-loop hum testing since it will identify hot, neutral and ground impedance, as well as predicted voltage drop under 15 and 20 amp loads. I've used it in a parallel test with my big-honking 10kW load bank, and they both agree within a few percent of each other. Between my SureTest Analyzer, Fluke 117 and a NCVT I can measure most anything I need to at any installation or show. Of course, you need to understand what the reading really mean, but being able to tell how stiff of a 20-amp circuit you're plugging into can save a lot of grief when the EDM starts....