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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.

Alaska Man
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Andy Barnes
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    Has the actual ground rod been checked? Yours would not be the first installation where corners were cut and the rod is not installed properly. I've seen a 10' rod cut so that only about 6" was actually driven into the ground. – jwh20 Aug 25 '20 at 17:06
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    Hahaha - appreciate the rod puns. But, if the rest of the house is grounded properly from the same breaker box, it couldn't be the grounding rod right? – Andy Barnes Aug 25 '20 at 17:14
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    How do you know the rest of the house is properly grounded? Most stuff doesn't care. It seems the EV charger is attempting to be extra safe here. I guess my point is to check the obvious problem first. The thing says the ground is bad, perhaps it really is. – jwh20 Aug 25 '20 at 17:27
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    When I realized I had the grounding problem in the garage I bought a tester and tested the rest of my outlets. A couple are flipped +/- in a porch addition, but everything except the garage circuit is grounded according to the tester. – Andy Barnes Aug 25 '20 at 18:06
  • @jwh20 Poor impedance to ground through the rod can not cause OP's problem. The path from the ground wire back to the neutral is broken. – J... Aug 26 '20 at 11:05
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    @AndyBarnes I'm amazed that everyone doesn't do this when they get a new house. Flipped outlets can be deadly, and old homes always have a few Joe DIY specials hiding in the walls. – J... Aug 26 '20 at 11:08
  • @J... Why can flipped outlets be deadly? – Andy Barnes Aug 26 '20 at 19:17
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    @AndyBarnes Because ungrounded appliances with 2-prong polarized plugs rely on neutral not being hot to keep you safe. Flip the outlet and the inner pin of your lamp socket becomes neutral and the metal shell outside becomes hot. – J... Aug 26 '20 at 19:34
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    @J...: Do appliances like that (which "rely on neutral not being hot to keep you safe") actually exist in North America? I've never seen or heard of any here in Europe, which makes sense since our plugs are mostly unpolarized. We do use the same kind of Edison screw sockets for light bulbs as you do, and the (supposed) neutral in them is indeed more exposed than the hot, but not enough to create an actual safety hazard when the bulb is in place. Indeed, lamps with symmetric 2-prong [Europlugs](https://en.wikipedia.org/wiki/Europlug) and Edison sockets are still commonly and legally sold here. – Ilmari Karonen Aug 27 '20 at 09:03
  • @IlmariKaronen Well obviously European appliances are designed to be safe with unpolarized plugs because you have unpolarized plugs. Long ago, Europe took the route of making appliances safer for unpolarized outlets. North America instead decided to enforce polarized outlets (with neutral doing double-duty as the grounded conductor) and let appliance code expect that to be the case. North American appliances with polarized, ungrounded plugs do not provide any guarantee of safety if the outlet provides power where a neutral is expected. – J... Aug 27 '20 at 09:41
  • @IlmariKaronen At least here in the Netherlands, ground is seperate from neutral. Our outlets have 3 contacts: hot, neutral and ground. Where current normally flows from hot to neutral, and ground is used as safety. And for example the outside from a washing machine is connected to ground. In that case, when something inside the washing machine breaks, and a loose wire connects to the outside, all current is redirected to ground (and a circuit breaker trips). For that purpose, ground most defnitely needs to be grounded. – Emil Bode Aug 28 '20 at 17:09

2 Answers2

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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.

ThreePhaseEel
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Ed Beal
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    That is a really good answer. I would add that this will eventually fail during rain too. – DMoore Aug 25 '20 at 17:21
  • How deep would a ~20' conduit need to be in order to be a sufficient ground? I'm not sure if it's depth or just surface area that matters for grounding. I'm guessing this conduit is only 3' deep max at the lowest point. But more likely 1-2' deep. – Andy Barnes Aug 25 '20 at 17:22
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    Wait... This just made sense to me... You're saying the conduit has rusted to the point where it's essentially two disconnected pieces that used to fully enclose the wire, but now since they are disconnected, wet ground will act as a conductor when it rains. Is this right? – Andy Barnes Aug 25 '20 at 17:24
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    @AndyBarnes You got it. "he tried pulling a wire through..." this is a good clue that there's a collapsed , rusted conduit. +1 – JACK Aug 25 '20 at 17:31
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    Andy the depth is not as important as the length code allows for a pipe electrode in contact with earth for 10’ to be used. If your charger requires the ground in many cases a solid electrical connection is needed. I have measured several hundred ohms on grounding electrodes even with multiple rods some electronic devices that monitor the system have not been low enough (it sounds like that is your problem). You can test this theory by running a ground wire from the panel or electrode at the house to the garage – Ed Beal Aug 25 '20 at 17:32
  • If the OP were going to bury a ground wire connection, could he use bare #10 copper? On the house side could he connect to the presumably exposed steel conduit? On the garage end? – Jim Stewart Aug 25 '20 at 17:51
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    @JimStewart Heck no, don't bury bare copper! Has to be either in conduit or protected cable suitable for direct burial. – J... Aug 26 '20 at 10:57
  • AFIK code allows galvanized conduit to be buried more shallowly than PVC. – Jim Stewart Aug 26 '20 at 11:14
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    "Earth is a horrible conductor" oh yeah - on a farm with a crushed junction box under surface, we had horses jump when they walked over its location because they provided a better path from just above the junction box to more distant ground. The electrician didn't believe us when we said "the earth is live", until we convinced him to test it with his shoes off and stretch to touch elsewhere. (Australia is 240V domestic & this may even have been 440V industrial.) – Andy Dent Aug 27 '20 at 16:07
  • @andy dent Animals are even more sensitive to stray currents than we are. in the US residential voltage is not more than 120v to ground. So your much higher voltage can travel further with the same conditions. if earth was a good conductor the horse might be dead. in the case of the question when the ground is wet there is a connection, when the ground is dry there was no connection. If the ground was wet in your case the horse would get shocked further away if the ground was wet. Can you touch a 240v live line ? A tingle from earth not electrocution because earth is not a good Conductor. – Ed Beal Aug 27 '20 at 16:26
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An outbuilding needs two kinds of grounds

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.

Metal conduit as the ground "wire" ... underground

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.

Harper - Reinstate Monica
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  • Actually emt is allowed now I remember being surprised by that I will find the code reff I don’t think it was a local thing mug it may have been. – Ed Beal Aug 25 '20 at 18:13
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    I found the reference it was new in 2017NEC. 358.10.A.1 uses permitted: 1) in concrete, in direct contact with earth. The water tight fittings have to be used the same as outdoor but it is allowed in the ground. – Ed Beal Aug 25 '20 at 18:20
  • @EdBeal edited but whoa... that's crazy. I would not do that. Maybe it works in Arizona or something... not in any soil around here. – Harper - Reinstate Monica Aug 25 '20 at 18:33
  • I would almost agree where I see a difference is rigid once threaded the groove thickness is not much different than EMT (this may be why the change was allowed). The difference is the EMT has a uniform galvanized coating. By code threads are supposed to be protected NEC 300.6 in the past I have done this but at my current location the owner did not want the extra expense so I only do it on underground fittings now where I used to use it everywhere. So far I haven’t got hit on an inspection. I have seen other electricians not using corrosion protection similar to switch leg marking missing. – Ed Beal Aug 26 '20 at 14:34