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Old 07-11-2011, 04:43 AM   #1
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Fried electronics after earth ground accidentally disconnected


Hi all,

I would like to solicit some advice on how to prevent damage in the future and ask for physics lessons on how exactly an earth ground disconnect caused a power surge, and some other issues that came up in today's troubleshooting...

What happened:

Was working on removing foundation near the main panel. Was unaware that part of the buried ground wire from the main was actually surrounded by concrete about 2 ft underground. Not the rod, just some of the excess buried cable (old house, sloppy pour apparently, and sloppy wiring. The excess should have been trimmed, not buried along with the rod...). When the concrete came out, so did the wire, which disconnected from the grounding rod. This apparently caused a power surge to race back up (down?) my electrical system. This in turn fried a bunch of the electronic gear that was connected to a sub panel that had no grounding rod. Subsequent sub-panels all have grounding rods and their circuits experienced no problems:

from main lug to fried sub panel (no ground rod) - 30ft.
from main lug to next ground rod sub panel - 60ft

Questions:

1. Can someone explain (in simple words) why a disconnect in the main's earth ground generated a power surge? I can't wrap my head around the physics. And explain why no breakers tripped, and none of the fuses internal to the fried gear blew either.

2. If the sub-panel that fried had had a ground rod would that have been sufficient to have saved the electronics?

3. We have ground rods at every panel on the property (4 buildings, 5 subs + 1 main), except the one where stuff got fried. It *had* a rod, but it was temporarily disconnected for the foundation work. We had figured that with more ground rods all of another 40ft, 50ft, and 90ft away that the electrons would go thataways instead of frying the stuff under the non-rod sub. Why were we wrong?

4. Do all grounding rods = earth grounds? Not sure that I understand the nomenclature in the threads I've been reading.

5. Do all earth grounds require that the panel connect the neutral to the ground? If so, if I use an ohm meter on such a panel I should get conductivity between the neutral and the ground lugs, right? If I don't have conductivity does that mean that the grounding rod isn't going to work properly? Is there a reason NOT to have the ground rod connected to both the ground and the neutral?

6. We have a persistent 1-3 volts of stray voltage measured on all grounds. It was originally 40 volts but apparently there was a bad circuit that was somehow feeding it. I don't pretend to understand how, all readings of the line showed NO shorts of any sort. Anyway, is 1-3 volts of stray voltage acceptable or should I keep plugging away trying to find the source? If so, what are likely culprits if shorts have been eliminated?

7. We have one of those no-contact voltage detector pens. Inside one of the buildings the pen goes off near almost all metallic objects. I mean everything - completely disconnected BX cables, the individual copper wires therein, simpson strong ties in the ceiling joists, even piles of nails in stud bays. The readings are stronger the higher up vertically the objects are. The same objects, taken out of the building, stop giving readings after a bit. Other similar or identical objects elsewhere do not generate a reading. This was funny at first, but now it's just frustrating, and more than a little creepy. I mean, how would you feel if you thought you finally found where your short was, disconnected the cable, can see *both* disconnected ends, and the pen is still going off like mad?? What the heck could be causing this? And no, there are no large electric motors in operation anywhere near by. And according to my compass, North is exactly where it should be...

Thanks for your time,

-o
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Old 07-11-2011, 06:35 AM   #2
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Fried electronics after earth ground accidentally disconnected


1. Some piece of equipment, possible cable TV equipment up in a utility pole, had a defect causing some line power to go to ground. As long as the ground bonding was intact the voltage was not excessive and also did not trip the breaker coincidentally. When the ground wire was disconnected the voltage leakage was high enough to fry something else. This is probably true if you measure significant voltage betweent he broken end of the ground wire up to the equipment/panel and a known ground or between the broken end of the ground wire up to the equipment/panel and a known neutral. For example the 1 to 3 volts you used to measure jumped up to 40 when the ground wire got broken.

2. There was a lightning strike some time before you found the equipment fried.
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Last edited by AllanJ; 07-11-2011 at 06:40 AM.
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Old 07-11-2011, 08:26 AM   #3
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Fried electronics after earth ground accidentally disconnected


You have a loose or disconnected neutral between the main and that subpanel. While the ground rod was connected, you had sufficient conduction to maintain a balance between the hots. Once the rod was disconnected, the hots "floated" resulting in high voltage on one leg and low voltage on the other.
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Old 07-11-2011, 09:03 AM   #4
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Fried electronics after earth ground accidentally disconnected


To add a little more..
In a single panel configuration:
Normally the earth ground does not come into play for normal distribution of current. The GEC must be connected to the neutral bar in the main panel to dissipate any transient voltages like lightning. BUT, if the neutral line from the transformer to the main panel becomes open or resistive, the grounding system will provide an alternate "Neutral" because at the transformer, the center tap connection has its own ground.

This was a sub panel. Did it have a three wire feed or four wire feed? It sounds to me like it could have a three wire feed.
What probably happened is you had a bad neutral at this panel. You disconnected the GEC from the rod but you still had a UFER - intentional or hap-hazard. This concrete encased ground wire would have been providing the alternate neutral until you ripped it out. Now this panel has no way to balance the voltage on the two bus bars.. Not good.

Just my guessing.......
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Old 07-11-2011, 09:07 AM   #5
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Fried electronics after earth ground accidentally disconnected


Quote:
Originally Posted by oukachiru View Post
6. We have a persistent 1-3 volts of stray voltage measured on all grounds. It was originally 40 volts but apparently there was a bad circuit that was somehow feeding it. I don't pretend to understand how, all readings of the line showed NO shorts of any sort. Anyway, is 1-3 volts of stray voltage acceptable or should I keep plugging away trying to find the source? If so, what are likely culprits if shorts have been eliminated?
How are you measuring this stray voltage? What tool are you using? What is the reference point?
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Old 07-11-2011, 09:38 AM   #6
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Fried electronics after earth ground accidentally disconnected


Quote:
Originally Posted by a7ecorsair View Post
To add a little more..
In a single panel configuration:
Normally the earth ground does not come into play for normal distribution of current. The GEC must be connected to the neutral bar in the main panel to dissipate any transient voltages like lightning. BUT, if the neutral line from the transformer to the main panel becomes open or resistive, the grounding system will provide an alternate "Neutral" because at the transformer, the center tap connection has its own ground.

This was a sub panel. Did it have a three wire feed or four wire feed? It sounds to me like it could have a three wire feed.
What probably happened is you had a bad neutral at this panel. You disconnected the GEC from the rod but you still had a UFER - intentional or hap-hazard. This concrete encased ground wire would have been providing the alternate neutral until you ripped it out. Now this panel has no way to balance the voltage on the two bus bars.. Not good.

Just my guessing.......
Agreed. You have an issue other than just a diconnected ground. Current on the ground is a problem.
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Old 07-12-2011, 02:08 PM   #7
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Fried electronics after earth ground accidentally disconnected


Quote:
Originally Posted by a7ecorsair View Post
How are you measuring this stray voltage? What tool are you using? What is the reference point?
Just a standard Ohmmeter, set to the 10v AC setting. In any given line, or at the ground bars at other sub panels, if I disconnect the ground, then reconnect using the ohmmeter between ends, I get 1-3 volts.

Quote:
This was a sub panel. Did it have a three wire feed or four wire feed? It sounds to me like it could have a three wire feed.
Yes, the sub that fried was a 3-wire, literally: it is old enough not to have a ground bar, just the 2 hots and a neutral. All ground wires are tied together and then attached (by me) to the housing. When I found the panel, the ground wires were all rolled up and not attached to anything =/ I've since re-attached a lead that goes to an outdoor grounding rod. Whole panel will be replaced by something a little more modern when we get to renovating the building..

I found something new in the wall last night - there is an outdoor light on the front of the building that I had thought went straight back to the sub. I was wrong, there was a standard (albeit very old) light switch on the *neutral* line, in the wall, behind the drywall. Switch was set to "on". Looks like there used to be door where I found the switch, and instead of removing the switch and putting in a junction, they just bent the switch box sideways and covered with drywall. I don't know if this would have contributed to the power surge problem at this panel, but it has since been corrected.

I'll keep plugging away, but I still haven't found any shorts of any sort that would put current on the ground. To test I tripped all the breakers in the sub, then used the ohmmeter to test each hot wire to the ground and to the neutral. If no reading, then no short that is causing the shortage, right?

Thus far all of the neutrals have checked out. Not done yet though. To check the neutrals, I have a long wire that is connected to the neutral bar. At each outlet/junction I touch the ohmmeter lead to the neutral wire there and to the wire I'm pulling with me. A solid reading means the neutral is good, ya?
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Old 07-12-2011, 03:05 PM   #8
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Fried electronics after earth ground accidentally disconnected


1. How did you verify that the neutral connection from the main to this sub is good at both ends?
2. Is the neutral bar isolated from the panel case?
3. You don't have a short.
4. Your "stray voltage" is about right if the neutral bar is bonded to the case.
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