ProSoundWeb Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1] 2  All   Go Down

Author Topic: NCV testing positive on IEC cables  (Read 1843 times)

Steve Litscher

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 512
    • MixMasters Podcast
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}

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 16552
  • Hickory, Mississippi, USA
    • Resotune
Re: NCV testing positive on IEC cables
« Reply #1 on: March 13, 2020, 03:27:41 pm »

no

JR
Logged
Don't tune your drums half-ass. Listen to what a properly "cleared" drum sounds like.   http://circularscience.com/

Steve Litscher

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 512
    • MixMasters Podcast
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

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3096
  • Southwest Washington (state, not DC)
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 »
Logged
Stop confusing the issue with facts and logic!

Steve Litscher

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 512
    • MixMasters Podcast
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

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1333
  • Helena, Montana USA
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.
Logged
"Good tools are expensive, but cheap tools are damned expensive."

Scott Helmke

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 1585
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.
Logged

Kemper Watson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 513
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 .
Logged

dave briar

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 355
  • Helena Montana
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.
Logged
..db

Jonathan Johnson

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 3096
  • Southwest Washington (state, not DC)
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.
Logged
Stop confusing the issue with facts and logic!

ProSoundWeb Community

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


Pages: [1] 2  All   Go Up
 



Page created in 0.05 seconds with 23 queries.