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Author Topic: What was happening in this situation?  (Read 446 times)

Jeffery Foster

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What was happening in this situation?
« on: June 29, 2014, 10:22:02 pm »

At an outdoor event this weekend. Power was brought to stage right by the organizers by way of a breaker box distributing four separate 20A circuits. All neatly built on-site to a wooden wall.

Of course it rained. Because that's what happens to us PSW forum people.

The couple of SOOW power cables that were already run in their troughs simply got unplugged and the ends covered until the rains passed. The distro boxes that housed the 14-20 plugs were the plastic weatherproof type that we closed up until after the rain.

Sun is coming out and it's time to set the stage....again.  I take my Fluke Volt-detector and start poking around the distro. All seems to be fine. HOWEVER, once I plugged our SOOW cable back in, I checked everything again. I found this; the rubber jacket of the cable would alert the Fluke Voltdetector. And here's the clue; it would alert all along the outer jacket of the (now plugged in cord) as I traveled down the wall.  Once the cable began its lay in the wet grass, the Fluke detected nothing.
Is there an easy explanation for what I was observing?
Many thanks in advance!
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Mike Sokol

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Re: What was happening in this situation?
« Reply #1 on: June 29, 2014, 10:31:20 pm »

I found this; the rubber jacket of the cable would alert the Fluke Voltdetector. And here's the clue; it would alert all along the outer jacket of the (now plugged in cord) as I traveled down the wall.  Once the cable began its lay in the wet grass, the Fluke detected nothing.
Is there an easy explanation for what I was observing?
Many thanks in advance!

Any of the NCVTs (including the Fluke VoltAlert) works by capacitively coupling between your hand holding the tester, and the tip of the tester. So they will generally beep when held on the outside of an energized power cord. However, sometimes they won't beep if you're holding them on the neutral wire side of the extension cord. When there's water on the outside of the power cord I'm sure it's acting like a capacitor which is wrapping the field all around the wire, increasing the capacitor surface area and making it easier for a VoltAlert to beep.

Jeffery Foster

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Re: What was happening in this situation?
« Reply #2 on: June 29, 2014, 11:00:14 pm »

Outstanding explanation Mike, thanks. Why do you think the Fluke VoltAlert would *quit* reading hot as I moved down the cable from dry to in-the-grass wet.

And lastly, does the alert reading of the outside of a cable present a dangerous situation in your opinion?

Thanks again!

Any of the NCVTs (including the Fluke VoltAlert) works by capacitively coupling between your hand holding the tester, and the tip of the tester. So they will generally beep when held on the outside of an energized power cord. However, sometimes they won't beep if you're holding them on the neutral wire side of the extension cord. When there's water on the outside of the power cord I'm sure it's acting like a capacitor which is wrapping the field all around the wire, increasing the capacitor surface area and making it easier for a VoltAlert to beep.
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Jonathan Johnson

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Re: What was happening in this situation?
« Reply #3 on: June 29, 2014, 11:17:37 pm »

And lastly, does the alert reading of the outside of a cable present a dangerous situation in your opinion?

Eeek! There's electricity leaking through the intact, insulating jacket of the cable! Everyone run for your lives!

No, just kidding. The alerting on the outside of the cable is normal and expected behavior. (Or behaviour, if you are in a different British colony.)
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Stop confusing the issue with facts and logic!

Stephen Swaffer

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Re: What was happening in this situation?
« Reply #4 on: June 29, 2014, 11:33:22 pm »

I can't explain the theory like Mike, but I have a fair amount of field experience using them.  They are a handy tool,  but if for any reason I am not 100% sure they are working correctly I won't rely on them if I will be doing something hazardous. Actually since I learned that I am very allergic to electricity, I rarely rely on them if I plan to handle parts that are normally energized in use!   
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Steve Swaffer

jasonfinnigan

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Re: What was happening in this situation?
« Reply #5 on: June 29, 2014, 11:46:37 pm »

The non-contact voltage detector generally will go off with any voltage even low voltage. They are great items to have for quick hot checking but they, aren't the tool to tell if a circuit is properly working, they won't tell you if there is high voltage, low voltage, no neutral, no ground etc.

And yes, it is 100% normal to detect through cables, heck even the casing of electronic devices will set them off depending on the level you have it set to.
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Mike Sokol

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Re: What was happening in this situation?
« Reply #6 on: June 30, 2014, 07:13:08 am »

And lastly, does the alert reading of the outside of a cable present a dangerous situation in your opinion?

That's perfectly normal when reading the outside of a power cable, but as noted earlier in the thread it's not a 100% reliable test of the cable being powered. That's because you could be sensing on the neutral side of the cable.

However, if you place a NCVT on the outside of an XLR cable and detect voltage, that means the cable shield has at least 40 volts AC on it (and up to 120 volts). That suggest that your mixing console has lost its "ground" EGC if you're powered from house AC, or your generator frame isn't grounded/earthed when running. From all my experiments that's as close to a 100% reliable test as I've found in the field.

Don't do what one of my live-sound students did last summer. I gave out free VoltAlert testers to my graduating students, and one reported that he used it for testing power at a barn dance. He used it during a stage check finding voltage on the microphones, traced it back the XLR cables to the mixing console (of course), then had the VoltAlert beep a foot away from the mixing console surface (30 cm for our UK readers). However, he really didn't believe what it told him, so he touched the console with a finger and got "a pretty good shock". He found that the extension cord the farmer supplied from the barn power had the ground pin broken off. Yikes  :o

Of course, don't use your own body as a backup voltage test. If there's any question at all, the gold standard is a voltmeter checking between a known earth ground and the chassis of the gear or microphone in question. However, you have to be sure there's no paint or corrosion beneath the meter leads when using a DMM. But a NCVT doesn't care about rust or paint and will easily find a hot mic from inches away which is why it's a great pre-show check for "hot" mics and guitars. Again, I've done a lot of experimenting and testing with various NCVT products on sound gear biased to different voltage levels, and this seems to be a solid application for them. See pic below for my NO~Shock~Zone demonstration of measuring a hot mic with a NCVT.   
« Last Edit: June 30, 2014, 10:14:10 am by Mike Sokol »
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Mike Sokol

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Re: What was happening in this situation?
« Reply #7 on: June 30, 2014, 07:34:03 am »

Outstanding explanation Mike, thanks. Why do you think the Fluke VoltAlert would *quit* reading hot as I moved down the cable from dry to in-the-grass wet.

The water on the outside of the cable created a voltage divider capacitor between the interior of the cable and the earth. These testers operate by listening for 50 or 60 Hz "hum" via capacitive coupling.

Mike Sokol

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Re: What was happening in this situation?
« Reply #8 on: June 30, 2014, 11:28:02 am »

FYi: Here's video of my testing a miniature VW microbus for a hot skin condition at various voltages. Pretty much the same thing happens with a hot mic on stage. http://www.youtube.com/watch?v=rtT3te_XNBM#t=53
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