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Author Topic: NCV testing positive on IEC cables  (Read 5362 times)

Steve Litcher

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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 Litcher

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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 Litcher

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


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