burnt ground

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sethas

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Los Banos, CA.
I went to service call at a mobile home park on friday. The customer said that the ground wire, the one bonded to the water main, had become extremely hot to the touch, so hot in fact that it caught a motorcycle cover on fire. An electrician had been called out before me, and had cleaned off the corrosion around the connection, and then proceeded to re-tighten the connection. He then informed her that she needed a new pedestal. The pedestal is a 30A 120V unit. It has an illeagal tap off of it that controls two 15A 120V circuits, with a shared neutral. At the main disconect, the hot conductor and the neutral conductor are the only present wires for the pedestal, no ground to be found. At the mobile home, or rather a poorley converted RV, the panel, on the RV, appears to bonded correctly to the frame. At the pedestal, there are three wires, a hot conductor, a neutral conductor, and a ground wire, which is bonded to the water main. I presently get 120V from the hot to neutral, and from the hot to ground. There is no continuity from the ground to the hot or the neutral, and no voltage from the neutral to the ground. I suspect that the ground wire travels only from the water main, to the pedestal, approx. 2 feet. Any ideas why it would get so hot as to catch fire? What is going on, and why? Thanks for any input.
 
Re: burnt ground

There must be current flowing over the grounding electrode system. This current could be coming from just about anywhere. A neighbor's service, a bad utility connection, the RV itself, the list is endless. What you are suggesting sounds like the situation has become dangerous. I would be getting the POCO out to the site to assist in locating the ground currents.

My guess is that the inside panel has improper neutral to ground bonding and normal operating current is flowing over the RV frame to the water pipe, back up to the sevice pedestal. My second guess would be that a neighbor has developed an open service nuetral and current is flowing through their grounding electrode to your clients grounding electrode, and then back to the source.
 
Re: burnt ground

You could try putting a clamp-on amp meter on the ground wire to see if there is current flowing. Then disconnect the loads one by one to see if the current flow on the ground wire decreases or goes away.
 
Re: burnt ground

I would be getting the POCO out to the site to assist in locating the ground currents.
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Good idea, at least they can verify that it is not on their side. I would bet that it is on the customer's side with all the sloppy wiring you are talking about. :D
 
Re: burnt ground

Thanks for the suggestions. Every once in a while you come across something and your mind stops working. It's nice to know that we can come here and brainstorm like this. Much appreciated guys.
 
Re: burnt ground

Check the RV refer. On electric it works with a heating element which I have seen burn through and short to ground and keep working using ground as nuetral. The only time you will detect the problem is if the refer is "cooling"
 
Re: burnt ground

At the main disconect, the hot conductor and the neutral conductor are the only present wires for the pedestal, no ground to be found.

Are you saying that the supply to the main disconnect is two wire (one hot, and one neutral)? If so, the neutral should be bonded to the panel and a neutral/ground terminal. It appears that the H20 is being utilized as the ground electrode system.

[/QUOTEAt the pedestal, there are three wires, a hot conductor, a neutral conductor, and a ground wire, which is bonded to the water main.]

Are the main disconnect and the pedestal one and the same? If so, are the three wires that you described the ones that are to/from the home?

I presently get 120V from the hot to neutral, and from the hot to ground. There is no continuity from the ground to the hot or the neutral, and no voltage from the neutral to the ground.

The voltage readings do not seem unusual. The lack of continuity between neutral and ground would lead me to suspect that the neutral is missing at the home,and/or neutral bonding is not done at he service pedestal. A short to ground at he home would cause current to return on the grounding conductor. The current path would be to earth through the connection to the water main and would not neccessarily trip a breaker. The coroded connection to the water main may have presented enough resistance to cause the heat.

As you will notice, I am new to this forum, so when you find the flaws to my reasoning, go easy! :confused:
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