How did my ground and neutral become energized by a short in the buried main line?

Question

Long post, because I'm not sure what information is critical. The ultimate question is what, if anything, I need to do to ensure there are no hazardous conditions in my home.

Friday night, the lights flickered for 10 seconds and then the house lost power. (Mostly, it turns out.) No other house on the block lost power. I went out to the service panel. I could see the meter was not on.

When I touched the service panel, I got a shock and dropped the door. I got my no contact tester (not a volt meter but the light up stick) and it lit up and beeped when I touched the panel box. I turned the main breaker off with a wooden spoon. I touched the box again and it again showed voltage even with the main breaker off. I then started testing other things. It lit up on the ground wire. It lit up on the hose bib. I yelled into the house and told everyone not to move. I tested the door handle and it was ok. I got everyone and the dog out of the house and into the car for warmth and called the power company.

I went back in with the tester, testing everything before I touched it. Pretty much every thing I tested in the house that was connected to the electrical system showed a positive test. The outside of the metal oven. The frame of the tv. Metal lamps (the actual lamp, not just the cord.) A laptop that was plugged in (the laptop itself.) Each switch cover. Most peculiar, low power led lights in our range hood were on at about 20 percent their usual strength. (They are not on a batter back up.) The GFIs in my kitchen were tripped, but their lights were on, showing red (not green, as usual). The refigerator was getting some power, very weakly.

The guy from the power company got there a few hours later. He was initially confused about what was going on. He asked whether we had any solar or batteries or generators that could be "backfeeding" the electrical system. (We don't.) He unscrewed the lines feeding the house from the meter. As soon as he disconnected the last of them, everything stopped testing positive with the non-contact voltage meter. The lights in the stove went out. He tested each of the wires that were coming in from the street, which had been disconnected from the meter. It appeared to me, but I'm not sure because I was standing a respectful distance away, that he got a voltage reading on the neutral. He said it appeared that the buried cable coming in from the street had degraded or been damaged and that the "hot must have nipped the neutral." I wanted to ask more, but I also wanted to let him do his thing.

He ran a temporary above ground cable that restored power and said the company would install a new underground cable from the street this week. He cleared us to go back in the house. I tested things with my voltage meter and it was all ok. Everything was working fine, except the high speed internet. The guy from cox came yesterday. He found that the connector from the box across the street, which feeds the line to my house, was completely fried and melted. He said he had never seen that, but that it had "done its job." I think he was saying that the coaxial cable is grounded at my house and so current must have zapped it along the ground? I don't really understand. I looked at where the coax enters my house and there is a box on the outside of my house with a ground wire that comes out of the box and is clamped to a copper water line.

Anyway, seeing the melted cable freaked me out a little.

I did a bunch of reading on what could cause a house to have power if the main breaker is off. None of it makes sense. And how would my copper plumbing show positive? And why didn't any of my breakers trip?

Let's say that the insulation between the hot and neutral wore out beneath the ground, feeding my house. Could this explain it? That would put current on the neutral coming into the house. Could that serve as power to everything the house and the grounding system that flows independent of the breakers? Why wouldn't that be a short circuit that would trip a fuse or breaker in the transformer box at the street?

Anyway, the "why" is less important to me than understanding if there are likely to be unseen dangers in the house as a result of this incident and what, if anything, I can do about that. I guess the power company guys will be back to re-install cable to my house and I can ask them, but I'd like to be educated. I can't find anything like this on the internet. Many thanks.

Answer 1

I think the power guy's assessment was spot-on

He said it appeared that the buried cable coming in from the street had degraded or been damaged and that the "hot must have nipped the neutral."

What was happening is that the incoming hot probably contacted the ground indirectly via the insulation failing in some fashion. Your neutral bonds with the ground in your main service panel, and are fed into a series of grounding rods near your panel. As such, your panel and all electrical switches and receptacles attach to this point (via grounding wire) and the powerful draw of the service neutral prevents it from flowing through you instead (which is why you don't bond ground and neutral anywhere else).

What it sounds like is that the shielding on the incoming service wire degraded and was able to put that power into the ground nearby, which was then picked up by the grounding rods (this process would be amplified if the ground was wet from recent rains). This then reversed the process, where your ground and neutral now became live. This is evidenced by

• Your being shocked by the panel (which is bonded to the ground)
• The melted cable device (which they bond to your electrical panel ground)
• The problem being rectified by a new service wire, with the old being disconnected

What, if anything, I need to do to ensure there are no hazardous conditions in my home

It wouldn't hurt to have an electrician come out and inspect the panel. I wouldn't open a main panel myself, since the incoming hots from your power company cannot be turned off by you (that main breaker just cuts power to the rest of the panel). Specifically, make sure nothing in the panel was damaged, and that the ground and neutral bond look OK.

Next, take your contactless tester and try them against a ground and neutral wire. You shouldn't get any reading from them (unless electricity is flowing through your receptacle from a downstream source). A wiring tester would also help in this case.

Once the power company has permanently replaced the wire, you should be just fine.

Answer 2

ThreePhaseEel, as usual, blew my mind with a notion that the below trouble might not even be at your house, but rather, a neighbor's. This neighbor is sending dozens of amps from his house to the pole transformer, and your house happens to be along that route. So it's trying to hop on your Grounding Electrode System, your cable TV hookup, and your neutral-ground bond, to get back to transformer neutral (which is both shared and unfused).

Current takes every path available to it, in proportion to its conductance (1/resistance). That means such current is pursuing every path at once. So this could be from a neighbor, and the cable damage could be affecting several houses, even. That's why turning off your panel did nothing.

Lost neutral, but 1 hot survived

It sounds to me like a) you lost neutral, and b) your grounding electrode system was not up to the task and/or completely missing.

I suspect both 1 hot and neutral were lost, probably because they came in contact with each other and burned each other up, leaving only the other hot(s).

I don't think you lost only neutral; that has other symptoms.

Suppose we call actual Earth "zero volts".

Every house, and every transformer, has a Grounding Electrode System or is supposed to. This thing's job is to get good contact with actual earth (which is made of dirt, mind you). This "grounds" the transformer chassis, the metal service panel, and "grounded" stuff in your house (that third pin on the socket).

At the transformer, one of the "hot" wires is bonded to the transformer's Grounding Electrode System. This supply wire is called "Neutral" because it's supposed to be near 0 volts, ha ha! Notice they still put insulation on it.

Every house (except in Philippines) gets neutral from the transformer along with 1-3 hots. (2 in North America). Leaving nothing to chance, there's also supposed to also be a neutral-ground bond in your house's main service panel. You're about to read why.

Snip the neutral. Now what?

So we think neutral + 1 hot got taken out. Now what?

There are appliances connected between the functioning hot and the neutral. Current is trying to flow from the working hot to the neutral. Neutral is disconnected, but the neutral-ground bond and grounding electrode system are still there.

So the current is flowing from the good hot, through the appliance, to the neutral wire to your service panel, through the neutral-ground bond. Through your Grounding Electrode system (if it exists), through the dirt, to the transformer's Grounding Electrode System, through its N-G bond and back to the transformer neutral.

If the Grounding Electrode System is tip-top, then some current returns, but it is limited by the high resistance of the dirt. It actually creates a voltage gradient on the dirt between your house and transformer, where the ground is 105V at your house but 0V at the transformer, varying across the yard. That can interact amusingly with metal fences, dog runs, and the like.

If your Grounding Electrode is rubbish, then all your house's grounds simply light up at near 120V.

And - here's what happened - If your Grounding Electrode System is rubbish, and it's also bonded to some other grounded thing like your Cable TV drop, then the mains current will seek to return via the Cable TV drop. Now you know why that burned up!

The poor performance of the Grounding Electrode (through all that dirt) is why the range hood lighting performed so poorly. If you had turned everything off but a couple of lights, the dirt could have easily handled that low current and they would have acted almost normal.

I turned off the main breaker, though

This person (from yesterday) thought so too. But maybe you didn't actually. The main breaker in this panel doesn't switch neutral or graound. So if there's another panel, then a load fed from that other panel could be doing this to neutral and ground for all your panels.

Fix it. Properly.

First, someone should eyeball your power distribution and see if there are any alternate panels or illegal taps that you don't know about, that may have affected your ability to shut off power.

Then, you should revisit your house's Grounding Electrode System. It didn't perform well. They are often poorly installed to begin with, often legacies from the pre-1960s when nobody cared, and often depend on a metal water pipe (that somebody replaces with plastic later, e.g. the "smart water meters" that are plastic, if you're in Flint, your water pipe GES is no good now, because they're on a crusade to replace all leaded pipe). So the GES should be rebuilt in your home.

A modern US Grounding Electrode System consists of at least two, 8-foot-long grounding rods at least 6' apart (farther is better). You can get by with one if it passes a special test, but for a DIYer, the test costs more than another ground rod. It also grabs any other grounding-point-of-opportunity (but not gas pipe), i.e. the water supply pipe. Really, you can't overbuild your Grounding Electrode System, so if you feel inspired to use 4 rods, or use metal conduit for your next run to an outbuilding, go for it. The ideal system is an Ufer, where it ties into the reinforcing rods in your basement or pad, because that has lots and lots of ground contact. But an Ufer has to be built in at the time of concrete pour; it cannot be retrofitted.

Answer 3

Your breakers are on the hot side , from your description your neutral was connected to one of the hot wires underground. Your ground rod or grounding electrode system must be at a fairly high level.

I have seen 2 ea 10’ ground rods have over 100 ohms code allows 1 rod if the value is 25 ohms or less and no measurement is required when you have 2. When the hot connected the neutral every thing that was grounded was now live. Remember the neutral is not switched this is why everything was hot. A ground value of 6 ohms is only going to be pulling ~20 amps the power company would not even notice the short at that level.

The one thing that surprises me is that the utility ground at the transformer must be at a high level also because if it was a good low value it would have blown the utility fuse.

Think I covered the areas you asked about. If you want more specific answers, we can go there, but did not want to waste your time if this meet your needs.