Non-Contact Meter Lights up on Grounded Metal Surfaces and in the Soil

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Master_HOBrFINCl

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Location
Pueblo, CO
Occupation
Electrical PE
I am commissioning a single axis tracker on a 4.4MVA central inverter and got shocked while performing routine tests on a 24V instrumentation wire which shares a right-of-way with a 1500V DC line and a 480V three-phase AC line. Volt-meter said only 20V AC which doesn't seem like enough to cause a shock. Fluke VoltAlert non-contact meter lights up on the conductors at the rack instrument box side but not at the PLC side back at the central station. It lights up on steel and aluminum grounded surfaces everywhere within the rack too. It even lights up directly in the soil, tens of feet away from any buried conductors. I have never seen a non-contact meter light up in the dirt before.

At the same rack it does not light up at night, even when the inverter runs in VARs at night mode (DC contactors open). There is a 345kV line about a mile away, I don't think it could be capacitively coupled this far away. It's not happening on other racks. The inverter is running and says good insulation resistance on both positive and negative. The 480V transformer is grounded wye - grounded wye though there is not a good zero-sequence source from the MV-AC system (ungrounded wye on the low side). The inverters are negatively grounded, so maybe that could serve ground fault current. All the 480V systems have been functionally tested and verified working 100%.

What's going on here?
 
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When you say it 'lights up directly in the soil', are you holding the thing, sticking the tip into the soil and getting a response, or are you sticking it into the ground, letting go, and still getting a response?

-Jonathan
 
You could try using an oscilloscope to measure the voltage between various points such as the rack, more distant grounded objects, a probe in the soil, etc. That way you could see if there's a significant voltage at higher frequencies caused by the switching of the inverter, or if there's a voltage primarily at 60 Hz.
 
You probably know this but the Fluke non-contact meter only works on AC. You shouldn't rely on it when you have 1500VDC in the mix! That said it could conceivably measure the DC when then inverter is switching it, like Synchro hypothesizes. I was inclined to say you've got an AC ground fault until you told us about the day/night difference.

Relatedly, why did you tell us a 20V AC reading but not tell us any DC reading?
 
Master_HOBrFINCl said:
I am commissioning a single axis tracker on a 4.4MVA central inverter and got shocked while performing routine tests on a 24V instrumentation wire which shares a right-of-way with a 1500V DC line and a 480V three-phase AC line. Volt-meter said only 20V AC which doesn't seem like enough to cause a shock. Fluke VoltAlert non-contact meter lights up on the conductors at the rack instrument box side but not at the PLC side back at the central station. It lights up on steel and aluminum grounded surfaces everywhere within the rack too. It even lights up directly in the soil, tens of feet away from any buried conductors. I have never seen a non-contact meter light up in the dirt before.

At the same rack it does not light up at night, even when the inverter runs in VARs at night mode (DC contactors open). There is a 345kV line about a mile away, I don't think it could be capacitively coupled this far away. It's not happening on other racks. The inverter is running and says good insulation resistance on both positive and negative. The 480V transformer is grounded wye - grounded wye though there is not a good zero-sequence source from the MV-AC system (ungrounded wye on the low side). The inverters are negatively grounded, so maybe that could serve ground fault current. All the 480V systems have been functionally tested and verified working 100%.

What's going on here?
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What is a Electrical p.e.? I havent heard of that...
 
Hand held Non-contact Voltage detectors are very good at detecting AC voltages. However, they are not very good at detecting No voltages. If you need to be sure of Zero voltage use a low impedance voltmeter.

The OP said they were getting 'shocked'. This needs to be properly investigated.
Was the shock from the outer insulation of the 24V cable, from any one conductor to ground, or between the conductors themselves?
 
winnie said:
When you say it 'lights up directly in the soil', are you holding the thing, sticking the tip into the soil and getting a response, or are you sticking it into the ground, letting go, and still getting a response?

-Jonathan
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It lights up when I put the tip about 1/8" into the soil.
 
jim dungar said:
Hand held Non-contact Voltage detectors are very good at detecting AC voltages. However, they are not very good at detecting No voltages. If you need to be sure of Zero voltage use a low impedance voltmeter.

The OP said they were getting 'shocked'. This needs to be properly investigated.
Was the shock from the outer insulation of the 24V cable, from any one conductor to ground, or between the conductors themselves?
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The shock was between the metal screw-on connector - which I believe is connected to shield - to the grounded metal bracket for an inclinometer. Will try the low impedance voltmeter next. Why would a high impedance voltmeter not detect the voltage while a low impedance voltmeter would? I am not sure about the theory on that one.
 
synchro said:
You could try using an oscilloscope to measure the voltage between various points such as the rack, more distant grounded objects, a probe in the soil, etc. That way you could see if there's a significant voltage at higher frequencies caused by the switching of the inverter, or if there's a voltage primarily at 60 Hz.
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I was afraid of this. We may decide to do this. The inverter switches in the kHz range so I would not be surprised to find that's why my AC current clamp measures 0A of ripple current anywhere on the DC system.

Interestingly my Fluke pen glows nearly 2 feet away from any modules in the facility. It's got to be the inverter ripple current.
 
Master_HOBrFINCl said:
It lights up when I put the tip about 1/8" into the soil.
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If you are still touching the meter you may get a false reading. For example it is possible for a high impedance meter to show voltage when you hold the leads in your fingers while standing under a fluorescent light.

This is why I said they are not good for detecting Zero volts.
 
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Master_HOBrFINCl said:
I have read this as well. I guess the lower impedance increases sensitivity?
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Low impedance shunts 'phantom' voltage, meaning capacitively coupled apparent high voltages.

In fact to diagnose what you are examining you want both high impedance measurements. The high impedance measurement will tell you the voltage of the initial shock, the low impedance measurement will help you understand how much current capacity is 'behind' that shock.

An extreme example is the measurement you would get (with appropriate instruments) after you walk across a carpet on a cold dry day. The high impedance measurement might show 25,000V ( you would actually need a special very high impedance meter for this, not a typical electricians multimeter) but the low impedance measurement would show 0V (after you discharged and said 'ouch')
 
Master_HOBrFINCl said:
It lights up when I put the tip about 1/8" into the soil.
Click to expand...

And it was just sitting there or were you holding it?

The 'non-contact' sensor actually detects voltage between two electrodes, the tip capacitively coupled to whatever you are sensing and the body electrode capacitively coupled to your hand.
 
It sounds like the shield on both sides of the 24V cable may have some continuity to its local earth ground. If so, then there would be a closed loop including the earth that multi-kHz magnetic fields from the "DC" lines could couple into. Because this loop would have some inductance, the induced loop current might be causing some voltage to be developed on the shield at the rack end of the cable and the local earth ground.

Can you temporarily disconnect all 24V instrumentation cables, and then see if your noncontact tester is still lighting up in the soil when the inverter is operating during daylight?
Then try the noncontact tester on the end of the disconnected cable to see if it still lights up, and if so, does it have to be closer or is it just about the same as before. With the cable disconnected, there could still be some electrostatic coupling going on, but I think it could have less impact than the magnetic coupling.
There could also be some resonance in the loop mentioned above at kHz frequencies that is amplifying the effects observed.
 
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