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Author Topic: Interesting outlet at show last night  (Read 6913 times)

Jonathan Johnson

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Re: Interesting outlet at show last night
« Reply #10 on: March 20, 2016, 10:41:31 pm »

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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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #11 on: March 21, 2016, 07:02:32 am »

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. 
« Last Edit: March 21, 2016, 07:40:57 am by Mike Sokol »
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John Roberts {JR}

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Re: Interesting outlet at show last night
« Reply #12 on: March 21, 2016, 11:19:37 am »

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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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #13 on: March 21, 2016, 11:45:28 am »

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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TJ (Tom) Cornish

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Re: Interesting outlet at show last night
« Reply #14 on: March 21, 2016, 12:05:58 pm »

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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Tim McCulloch

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Re: Interesting outlet at show last night
« Reply #15 on: March 21, 2016, 12:33:13 pm »

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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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #16 on: March 21, 2016, 01:19:46 pm »

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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Jonathan Johnson

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Re: Interesting outlet at show last night
« Reply #17 on: March 22, 2016, 01:09:47 am »

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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Tom Bourke

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Re: Interesting outlet at show last night
« Reply #18 on: March 22, 2016, 08:12:03 am »

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

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.
« Last Edit: March 22, 2016, 08:16:17 am by Tom Bourke »
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Mike Sokol

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Re: Interesting outlet at show last night
« Reply #19 on: March 22, 2016, 08:13:47 am »

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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