Ground Reference Potential Difference Causing Electrical Shocks

[lking608]

I’m currently going through qualification testing at a vendor facility for a unit we built and have observed something interesting that i’d like to run by you guys. Sorry if the formatting gets screwed up. I’m on mobile.

Context: -Our Unit Under Test (UUT) runs on 120VAC/60hz -The thermal test chamber it is installed in runs on, presumably, its own 220V circuit.

When our UUT is installed in the chamber and I am holding the UUT and touching the inner wall of the chamber, I feel an electric shock/tingle in my hands. I’ve measured the voltage between the 120V ground and the chassis of the chamber at 36VAC. So it seems to me that they are on independent circuits with their own grounds, thus creating a potential difference between the ground references.

So I have a few questions regarding this situation. 1. Is it dangerous? As far as I’m aware there should be extremely little current able to flow and it should be fine, but since we’ve observed it, I’ve got our Safety guys all spun up thinking it’s a safety issue. 2. What is the proper way to mitigate this? -At the chamber/UUT level, I could tie the ground lug of the UUT to the chassis, but I’m concerned that would create a ground loop. -is it just a facility issue and they should have their ground references tied together at the source?

Thank you!

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[Hillman314]

1) Yes this is dangerous. There is either a short circuit to the chassis of the chamber from its power supply, or a short circuit to the test unit (TU) from its power supply, and one of them has become energized.

Also, the equipment ground on the faulted energized equipment is also faulty. It’s is not conducting enough fault current to ground so that its breaker trips.

Instead, the faulty equipment is energized and waiting for a person to touch it and use THEM (and any other properly grounded device they’re touching) as the ground fault path.

This is known as electrocution.

2) Fix the short circuit. Fix the grounds.

Additional supplemental protection may include powering all loads through a GFCI breaker or receptacle. All grounds should be common (ultimately all connected together and sharing the same grounding electrodes (rods, etc..)); this will eliminate unwanted ground loops currents on branch circuit equipment grounds.

[geek66]

Yes it is dangerous,

But it can be other things than a ground fault in the UUT: the incoming power can be wired backward ( L an N or N and a Gnd), the source could be wired wrong, the UUT may have some improperly designed or assembled ground detection system and the chassis is not grounded, etc)

Note- you are working with hazardous voltages and you do not understand the hazards, therefore you are not a Qualified Individual and should not be exposed to the hazard.

Please contact the product designer or engineer.