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Author Topic: NCV testing positive on IEC cables  (Read 1845 times)

Steve Litscher

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NCV testing positive on IEC cables
« on: March 13, 2020, 03:14:24 pm »

I'm tracking down an unusual issue with a mixer, and after exhausting a bunch of potential theories, grabbed my NCV tester.

I was surprised to find a couple of things:

- The tester lights positive near any IEC cable jacket
- At this venue, I'm measuring 0.155V between neutral and ground

Are any of these items things to worry about?

Thanks in advance.

John Roberts {JR}

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Re: NCV testing positive on IEC cables
« Reply #1 on: March 13, 2020, 03:27:41 pm »

no

JR
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Steve Litscher

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Re: NCV testing positive on IEC cables
« Reply #2 on: March 13, 2020, 03:35:01 pm »

no

JR

Ok, thanks John. And rats... I was hoping it was an easy fix. :-)

Jonathan Johnson

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Re: NCV testing positive on IEC cables
« Reply #3 on: March 13, 2020, 07:25:35 pm »

I'm tracking down an unusual issue with a mixer, and after exhausting a bunch of potential theories, grabbed my NCV tester.

I was surprised to find a couple of things:

- The tester lights positive near any IEC cable jacket
- At this venue, I'm measuring 0.155V between neutral and ground

Are any of these items things to worry about?

Thanks in advance.

A slight voltage difference between neutral and ground is expected when there is a load on the neutral. The reason is that the resistance on the neutral causes a voltage drop which increases with load; the greater the load, the greater the voltage differential between neutral and ground. In fact, if the voltage measures zero (especially when there is a load), it's an indication of a bootleg ground -- where the grounding terminal of a receptacle is connected to the neutral conductor instead of using a separate equipment grounding conductor back to the panel.

An non-contact voltage sensor is proximity sensitive to the electric field that exists around any AC source. (Current flows produce magnetic fields.) https://slt.co/Education/ACElectromagneticFields.aspx

(Bootleg grounds are, unfortunately, common in older buildings which were originally wired before separate grounding was common. For example, a two-prong receptacle will be replaced with a three-prong receptacle, but the installer doesn't want to go to the labor or expense of rewiring the circuit. So they'll jumper the ground to the neutral, and that will "satisfy" a typical 3-light tester. Where it gets dangerous is when hot and neutral get switched around -- so they jumper the ground to the hot instead of the neutral. This is known as "reverse polarity bootleg ground" and is especially dangerous. If one audio component is plugged into a properly wired receptacle and another is plugged in to an RPBG, you can get equipment-destroying currents on the shield of the equipment. Personnel touching both systems -- such as a performer touching steel guitar strings and putting their lips on a metal microphone grille -- can receive a shock. The nature of an RPBG receptacle is that a 3-light tester will incorrectly indicate proper wiring.)
« Last Edit: March 13, 2020, 07:34:00 pm by Jonathan Johnson »
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Steve Litscher

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Re: NCV testing positive on IEC cables
« Reply #4 on: March 13, 2020, 10:04:20 pm »

A slight voltage difference between neutral and ground is expected when there is a load on the neutral. The reason is that the resistance on the neutral causes a voltage drop which increases with load; the greater the load, the greater the voltage differential between neutral and ground. In fact, if the voltage measures zero (especially when there is a load), it's an indication of a bootleg ground -- where the grounding terminal of a receptacle is connected to the neutral conductor instead of using a separate equipment grounding conductor back to the panel.

An non-contact voltage sensor is proximity sensitive to the electric field that exists around any AC source. (Current flows produce magnetic fields.) https://slt.co/Education/ACElectromagneticFields.aspx

(Bootleg grounds are, unfortunately, common in older buildings which were originally wired before separate grounding was common. For example, a two-prong receptacle will be replaced with a three-prong receptacle, but the installer doesn't want to go to the labor or expense of rewiring the circuit. So they'll jumper the ground to the neutral, and that will "satisfy" a typical 3-light tester. Where it gets dangerous is when hot and neutral get switched around -- so they jumper the ground to the hot instead of the neutral. This is known as "reverse polarity bootleg ground" and is especially dangerous. If one audio component is plugged into a properly wired receptacle and another is plugged in to an RPBG, you can get equipment-destroying currents on the shield of the equipment. Personnel touching both systems -- such as a performer touching steel guitar strings and putting their lips on a metal microphone grille -- can receive a shock. The nature of an RPBG receptacle is that a 3-light tester will incorrectly indicate proper wiring.)

This is both super helpful and intuitive - thanks for sharing the extra info. It's much appreciated.

Mark Cadwallader

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Re: NCV testing positive on IEC cables
« Reply #5 on: March 15, 2020, 12:46:57 am »

The sad state of affairs is that it took me a moment or two to recognize the topic as pertaining to Non Contact Voltage Tester (NCVT), not Novel Corona Virus.  My day job must be getting to me.
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Scott Helmke

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Re: NCV testing positive on IEC cables
« Reply #6 on: March 16, 2020, 02:46:36 pm »

Make sure the IEC cable is wired correctly. I do find miswired ones, factory molded, on occasion.
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Kemper Watson

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Re: NCV testing positive on IEC cables
« Reply #7 on: April 09, 2020, 04:36:26 pm »

Make sure the IEC cable is wired correctly. I do find miswired ones, factory molded, on occasion.

Exactly. I've found two in the last year. Shipped with lighting fixtures .
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dave briar

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Re: NCV testing positive on IEC cables
« Reply #8 on: April 09, 2020, 06:07:41 pm »

Exactly. I've found two in the last year. Shipped with lighting fixtures .
Same here. Three of our five FBT monitors came with reverse wired 14x3 IEC power cables.
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Jonathan Johnson

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Re: NCV testing positive on IEC cables
« Reply #9 on: April 10, 2020, 01:32:35 pm »

Same here. Three of our five FBT monitors came with reverse wired 14x3 IEC power cables.
Being reverse wired usually isn't a problem. An AC load doesn't care which line is hot and which is neutral

If the power cord is directly supplying a power switch that only interrupts the "hot" line, leaving the neutral connected, or if it somehow references chassis ground to neutral, or if it has a circuit breaker or fuse that interrupts only one line, then reverse wired could be a problem. If the switch or circuit breaker interrupts both lines, a reverse-wired cord isn't unsafe.
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dave briar

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Re: NCV testing positive on IEC cables
« Reply #10 on: April 10, 2020, 03:00:52 pm »

Being reverse wired usually isn't a problem. An AC load doesn't care which line is hot and which is neutral

If the power cord is directly supplying a power switch that only interrupts the "hot" line, leaving the neutral connected, or if it somehow references chassis ground to neutral, or if it has a circuit breaker or fuse that interrupts only one line, then reverse wired could be a problem. If the switch or circuit breaker interrupts both lines, a reverse-wired cord isn't unsafe.
Right, and to whit, the five monitors all worked fine for a few years until one day one didn’t. Just went inert. My “helper” at the time went to the stage and upon disconnecting the power cord (terminated with a True1 connector) noticed that the connector was loose on the cable and so twisted it almost 180 degrees to demonstrate it to me — while it was still plugged in no less.  Sparks and a blown breaker were the result. It was while replacing the True1 connector that I discovered the reversed hot and neutral. I asked the FBT repair guy whether based on his knowledge of the internals of the monitor that might have been in any way related to the monitor dying.  He didn’t think so right off but would take a closer look and let me know. Nope was his reply an hour later, just a coincidence that both issues occurred on the same monitor. Still, I rewired the two other cables so that the hot wire connected to the “L”ine terminal on the True1. All five monitors are still working fine.
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Daniel Levi

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Re: NCV testing positive on IEC cables
« Reply #11 on: April 10, 2020, 03:18:06 pm »

Being reverse wired usually isn't a problem. An AC load doesn't care which line is hot and which is neutral

If the power cord is directly supplying a power switch that only interrupts the "hot" line, leaving the neutral connected, or if it somehow references chassis ground to neutral, or if it has a circuit breaker or fuse that interrupts only one line, then reverse wired could be a problem. If the switch or circuit breaker interrupts both lines, a reverse-wired cord isn't unsafe.

And, of course, in Europe Schuko, Italian or un-earthed 2 pin plugs will go in either way, it's only earthed French/Danish/Swiss/UK plugs that are polarised (although I believe originally France had no standard as regards to polarity), plus anything with a "figure-8" connector will also have random polarisation.
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Robert Lofgren

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Re: NCV testing positive on IEC cables
« Reply #12 on: April 11, 2020, 10:01:45 am »

Here is an interesting video explaining why leds glows even when turned off. It is kind of related to this issue...

https://www.youtube.com/watch?v=jqTOrSDFVg0
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Ivan Beaver

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Re: NCV testing positive on IEC cables
« Reply #13 on: April 15, 2020, 03:40:56 pm »

Being reverse wired usually isn't a problem. An AC load doesn't care which line is hot and which is neutral

If the power cord is directly supplying a power switch that only interrupts the "hot" line, leaving the neutral connected, or if it somehow references chassis ground to neutral, or if it has a circuit breaker or fuse that interrupts only one line, then reverse wired could be a problem. If the switch or circuit breaker interrupts both lines, a reverse-wired cord isn't unsafe.
Yes, the gear may "work" fine, but does it sound fine? with the polarity of the AC line reversed?

Just ask any true hifi guy who can "hear" when the power company changes the polarity of the AC.  It makes their systems unlistenable.  So they have to wait until the power company flips the polarity back to the "correct" position in order to listen to their music.

NO, I do NOT make this stuff up-----------------------
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Jonathan Johnson

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Re: NCV testing positive on IEC cables
« Reply #14 on: April 15, 2020, 05:34:01 pm »

Yes, the gear may "work" fine, but does it sound fine? with the polarity of the AC line reversed?

Just ask any true hifi guy who can "hear" when the power company changes the polarity of the AC.  It makes their systems unlistenable.  So they have to wait until the power company flips the polarity back to the "correct" position in order to listen to their music.

NO, I do NOT make this stuff up-----------------------

Do they use audiophile batteries in their IR remote controllers, too?
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Ivan Beaver

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Re: NCV testing positive on IEC cables
« Reply #15 on: April 15, 2020, 08:44:10 pm »

Do they use audiophile batteries in their IR remote controllers, too?
Nothing digital for those guys.  Pure analog baby!!!!!!!

The fewer the controls the better.
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Jonathan Johnson

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Re: NCV testing positive on IEC cables
« Reply #16 on: April 15, 2020, 08:56:06 pm »

Nothing digital for those guys.  Pure analog baby!!!!!!!

The fewer the controls the better.

So you're saying they'd cut off their fingers to spite their ears?
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Kemper Watson

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Re: NCV testing positive on IEC cables
« Reply #17 on: May 02, 2020, 10:30:59 am »

Yes, the gear may "work" fine, but does it sound fine? with the polarity of the AC line reversed?

Just ask any true hifi guy who can "hear" when the power company changes the polarity of the AC.  It makes their systems unlistenable.  So they have to wait until the power company flips the polarity back to the "correct" position in order to listen to their music.

NO, I do NOT make this stuff up-----------------------

 How and why I knew it was reversed? Every time I plugged into the drop snakes with the PA muted there was a small pop. Less than phantom power but enough to know something wasn't right. DMX was run down snake lines. When I unplugged the DMX lines popping quit. I have these plugs that glow red when the hot/ neutral is reverse. I used this to chase down the issue . Once the reversed cable was found and removed the popping quit.
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John Roberts {JR}

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Re: NCV testing positive on IEC cables
« Reply #18 on: May 02, 2020, 11:54:09 am »

Yes, the gear may "work" fine, but does it sound fine? with the polarity of the AC line reversed?

Just ask any true hifi guy who can "hear" when the power company changes the polarity of the AC.  It makes their systems unlistenable.  So they have to wait until the power company flips the polarity back to the "correct" position in order to listen to their music.

NO, I do NOT make this stuff up-----------------------
some semi-pro (code for not pro) 2 wire line cord gear, grabs a psuedo ground from the nuetral (that should be 0V and grounded at the service drop).

These not pro SKUs sometimes capacitor couple neutral to chassis ground to provide some shielding. Of course reversed line and neutral is now dumping AC current into the chassis shield. These caps are sized to avoid creating a safety hazard, but can corrupt audio integrity.

I have encountered audio issues last century trying to pass audio through prosumer/semi-pro gear using two wire line cords.

JR
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Re: NCV testing positive on IEC cables
« Reply #18 on: May 02, 2020, 11:54:09 am »


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