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I was working installing a 2 pole 50A 240V GFCI breaker and trying to debug an appliance. I turned off the GFCI breaker I was working on, but the main breaker was on. About the time I had one of the wires undone I put my hand on the metal enclosure of the breaker box and received a shock. What could possibly have caused this? The main was on, and the GFCI breaker was connected to the box via the pigtail to the neutral terminal. Any heuristics/diagnostics to determine why the external enclosure became electrified?

The subpanel (in a detached garage) has 4 wires coming in: 2 hot to the main breaker, one to the grounding bus, and neutral to the neutral bus. In addition, there is a very thick copper wire running outside the conduit that appears to terminate in the ground below the deck on the outside of the building.

There is continuity between the neutral and ground bus, which I understand is not normal for a subpanel.

Am I right in suspecting that the grounding electrode is not grounded? How would I test this?

Walrus the Cat
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First I would like to say, thank god you weren't seriously injured. Second this is a very serious situation. The reason someone is shocked when touching a metal enclosure is because you are grounded and the enclosure is not.

You're neutrals and grounds are tied together at the first means of disconnect. It will be bonded at the main breaker and if there is no separate main it will be done at the Panel's main breaker. Check to make sure the bonding screw is properly installed and the grounding bus is bonded with the neutral bus. Then check to see if your grounding electrode conductor actually runs to a proper grounding electrode. Check for corrosion and loose connections. If everything is properly done, if you have a problem you should get a trip on the breaker having a problem.

If you're not getting a trip then you probably have a piece of 240V equipment that is missing a phase, the neutral, or the ground is not connected properly and you are getting one phase trying to seek a path back to the Panel and the breaker. I noticed that you said you were trying to debug an appliance. I would start there.

Under no circumstances should you be trying to troubleshoot this problem under power. Disconnect all power and troubleshoot with a continuity tester or an ohmmeter. If all of this seems to technical I would advise you seek professional help.

Retired Master Electrician
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  • I looked in the main panel (with the main breaker off) and there are bare grounding wires running awfully close to stripped back (still live) wires feeding the main breaker. Is it possible they are touching it and could have caused this problem? – Walrus the Cat Jun 24 '17 at 18:19
  • If a hot wire were to touch a neutral or a ground you would have an immediate short circuit which would trip an overcurrent protector (circuit breaker) or burn through one of the conductors. You should look for something loose or disconnected or corrosion or even burnt through. – Retired Master Electrician Jun 24 '17 at 18:38
  • there is no evidence of burning or corrosion in the main or sub panels. Check out the updates to the question, and thank you for your help / concern. – Walrus the Cat Jun 24 '17 at 20:16
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I put my hand on the metal enclosure of the breaker box and received a shock. What could possibly have caused this?

The enclosure was energized and you were grounded and completed the path back to ground. (This is only true on a grounded system).

If the enclosure is properly grounded and it accidentally becomes energized then it should trip the breaker feeding it. A wire could have been rubbed bare and come in contact with the enclosure or have been stripped improperly and come in contact. If the enclosure is not properly grounded it becomes energized and DOESN'T trip the breaker. Then it just sits and waits for someone to complete the circuit.

This is why proper grounding of a grounded system is extremely important.

Re-check all the wiring involved check the enclosure to a known ground with a volt meter to see if it is indeed energized. Make sure this problem is resolved before proceeding.

Good luck and stay safe!

ArchonOSX
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  • It's not possible that anything "downstream" of the switched-off GFCI breaker (including its pigtail neutral) could have had anything to do with this, right? – Walrus the Cat Jun 24 '17 at 16:13
  • Probably not. If the GFCI Is switched off then there is another wire energizing the panel. – ArchonOSX Jun 24 '17 at 16:23
  • how should i check the enclosure with a known ground if I suspect that the (possibly ungrounded) grounding electrode / ground rod may be the cause of this? can i put an alligator clamp on a piece of metal and stick it in the ground? – Walrus the Cat Jun 24 '17 at 20:17
  • You should be able to check between the enclosure and the equipment ground to the panel or the neutral to the panel or the grounding electrode to the ground rod. – ArchonOSX Jun 25 '17 at 17:57
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Ok so I kind of figured this out. The subpanel is actually grounded at 2 points - a small grounding wire coming in through the conduit from the house, and a large copper grounding wire going to the grounding electrode outside.

Due to differences in potential between the house's panel and the subpanel, the difference between the grounding bus and the outside of the subpanel with only its main breaker on is about 2v. A small amount of current is being carried in from the neutral / ground from the house.

Walrus the Cat
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  • neutral/ground is not a thing. It's two things. There should be a volt-ish or two of difference between neutral and ground. If you understand how a beam torque wrench works, it's like that. – Harper - Reinstate Monica Jun 25 '17 at 02:38
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It seems like the feeder neutral is busted. Fix it and then pull the errant bonding strap

These symptoms are a sign that the feeder ground was improperly used to route around a broken feeder neutral. Turn off the feeder at the house, find the broken neutral, and fix it (you may need a UF splice kit, or even to dig up and rerun the cable). Then you can pull the errant subpanel bonding strap out, and enjoy a shock-free life.

ThreePhaseEel
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