Home Improvement Asked by Andy Barnes on May 30, 2021
My garage is not connected to my house and it’s on a circuit that contains a few outlets inside the house as well. I found out the first time I tried to charge my PHEV that the entire circuit isn’t properly grounded. I had an electrician come look at it, and found some old wiring that he replaced and it worked! Until it didn’t… All of a sudden it wasn’t grounded anymore.
The electrician came back over and when he went to test it, it was grounded properly?! Turns out this was just after a rainstorm. Now I’ve found this to be true that every time it rains, the garage circuit is grounded properly and when it’s dry, it is un-grounded again.
Now the electrician want to tear up the underground conduit connecting the garage to the house, but I’m not sure why that would fix it? If anything I would think that it would be un-grounded when it rains, or trip the breaker every time it rains?
Also, there are no outside outlets and the underground wiring is the old metal sheathing style that runs through a 1.5" dia pipe into the house. He has already tried pulling the wire through one end with a stringer but apparently it didn’t work.
To me it sounds like the conduit was used as the grounding conductor, as allowed by code, and it rusted through, therefore it works when the ground is wet and you have a ground connection through the soil and water. If this is the case, then an additional electrode at the garage probably won’t help as the pipe is in the ground.
Your electrician wanting to dig it up makes sense to me because when debris get in the pipe, it becomes impossible to pull new wire. With current code, the ground can be run separately, but that would be pushing the code in my opinion but legal. You can run a ground as long as they originate from the same panel.
Earth is a horrible conductor. I see high resistance quite often; this is why I don’t think another rod will help.
Correct answer by Ed Beal on May 30, 2021
Electricity wants to get back to source, not ground. Source is the supply transformer, or in the case of natural electricity (lightning, ESD), then yeah, it's earth.
The grounding rod is designed to handle natural electricity. Note that natural electricity has VERY high voltage compared to the amperage.
The ground wire handles artificial electricity. It returns wayward fault current to the main panel, where it gets back to the supply transformer via the neutral-ground equipotential bond at that location, and the neutral supply wire.
Your site's problem is lack of a ground wire. That means if there are any ground faults, it will tend to energize all the grounds in the building, and the earth in the vicinity of the building. There will be a "voltage gradient" across the earth along the gap in the grounding. At extremes, an animal with two feet on one side and two feet on the other side could get shocked. Or even a human taking strides. Metal fence lines are particularly dangerous; they could be grounded at one end and present near 120V to ground at the other end.
Your installation goes back to the day when all conduit was metal. Yours was installed one of two ways: either with no thought to grounding at all since it's that old; or intentionally using the metal pipe as the ground "wire". This is a technique I'm normally all in favor of, but underground, it is prone to rusting out. EMT (thin wall) conduit doesn't perform well underground because of this issue.
The electrician's inability to pull or fish through the conduit would support this theory.
So your ground "wire" has broken, and you have all the normal problems from that.
Regardless of what type the conduit was, if it is corroded through in one location, it is likely in very poor condition for the rest of the run.
Answered by Harper - Reinstate Monica on May 30, 2021
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