I have 120v when measuring between true ground and the screw on my outlet plate

Question

In the process of going to add some low voltage landscape lights to an existing low voltage transformer I found out that when I touched the metal base of the transformer I got a bit of a shock. I originally figured it was something shorted in the transformer box itself but as I traced it out further to where it was connected to the outside 120v outlet that if I put my meter leads on the metal part of the outlet box and the other end directly into the ground ( the dirt) I read 120v. Kept backing up and found the same thing on 2 other patio outlets thats on the same circuit.

Im in the low voltage business so I know how to use a meter and have some knowledge of electricity but I cant figure out how I have the 120v between the metal outlet box and earth, and again, this means sticking one lead directly into the dirt near the box.

Any thoughts or advise very much appreciated.

Answer 1

Unfortunately there could be several causes. At a minimum you don't have an effective grounding conductor, to say the least. That's one thing that needs to be fixed. A way to find the problem is to unplug everything on that circuit and see if you still have the problem. If not, you have a defective (ground faulting) appliance or other piece of equipment. If you still have 120 between ground and the real ground (the Earth!), then you have a short someplace. I'd try removing all the outlets and examine them for shorts, loose wires.

I recently fixed a true short (tripped the breaker when a outlet controlled by a switch was turned on). Turned out over time the ground wire in the box came into contact with the hot side of the outlet.

If you are comfortable working in a main panel, I'd pull the cover off, determine which breaker controls the "shocking" circuit and follow the wire from the breaker to where the cable enters the panel, then follow it's ground and see if it's connected and in good shape.

You could also have a break in the ground connections anywhere from the main panel throughout the circuit. Like Ed, ph3, Harp and others have said here, wire and cable failures inside a wall, once installed are extremely rare. By far most often these issues are in an outlet or fixture box someplace. One more reason the code requires fixture and outlet boxes to be permanently accessible!

Please be careful when working inside a live panel. One of my rules personally is only one hand in the panel at a time whenever possible.

Answer 2

Well, it's tricky because dirt is not a very good conductor. So this could mean all sorts of things. This could just as easily be "phantom voltage"; most DVMs (and every cheap DVM) are very vulnerable to that type of misread.

Anytime you're dealing with ground, you need a canonical reference for what ground even is. The first place you go is the Grounding Electrode System - that's the ground rods, water main or embedded Ufer ground. You need to make sure that exists, is reasonable for Code (not interrupted by a plastic smart meter, for instance), and is connected to your service panel.

Then you need to look at the bonding in the service panel - is neutral bonded to ground exactly and only at the main panel?

Then you look at the Equipment Grounding Conductors on each branch circuit and feeder coming out of that panel. Are they intact? Are they hooked up correctly? Are they NOT bootlegged to neutral anywhere?

Rather than sticking a probe in dirt, you should be referencing your ground measurements to that Grounding Electrode System. Referencing to something else is just going to confuse the matter.

This type of problem is a good excuse for a thorough work-over of the above, oft-neglected systems. If they are anything less than tip-top, stop right there. That explains your trouble. Fix it and the problem will either go away, or become readily apparent. Of course people often want to fix the problem without fixing the grounding; that's the harder path. You're on a bug hunt for a ground fault at that point.

You just have to keep shutting off breakers then isolating loads and wire segments until the problem disappears. Like I say, it goes a lot faster if you fix any grounding system defects, and then, you finish with working grounds!