testing integrity of buried egc?

Status
Not open for further replies.
R

ricry

Member
Location
USA
Power goes to garage underground with galvanized rmc. Its 50+ years old and also serves as the egc and I fear it might be shot.

Do I need special equipment to test the integrity of the egc?

I tested impendence from where it exits my main panel and to an outlet in the garage and it was jumping all over the place... 100-200 ohm. Im assuming its compromised but I also suspect I am unable to test in this manner accurately with a standard dmm.
 
ricry said:
Power goes to garage underground with galvanized rmc. Its 50+ years old and also serves as the egc and I fear it might be shot.

Do I need special equipment to test the integrity of the egc?

I tested impendence from where it exits my main panel and to an outlet in the garage and it was jumping all over the place... 100-200 ohm. Im assuming its compromised but I also suspect I am unable to test in this manner accurately with a standard dmm.
Click to expand...
What procedure did you use to perform this impedance test? Temp wire back to beginning of circuit and test continuity? Turn loads off and use the feeder neutral for said temp wire is an easy way, but make sure there is no N-G bonding beyond beginning of the feeder, which is hopefully service panel if bonding is done there.
 
ricry said:
I tested between garage outlet ground prong and main panel with extension cord.
Click to expand...
Not sounding so good then. Can try to see if the EGC will carry any significant load - but be careful. If it can't carry say a 1000 watt heater, it will never trip on a ground fault.
 
Would it not be possible to run some #14 thhn from the ground bar in the main panel out to the garage and test continuity between the rmc and the thhn?

Should be able to test off the existing neutral too, that is ran out to the garage, assuming it's in good shape.
 
Cow said:
Would it not be possible to run some #14 thhn from the ground bar in the main panel out to the garage and test continuity between the rmc and the thhn?

Should be able to test off the existing neutral too, that is ran out to the garage, assuming it's in good shape.
Click to expand...
He said he used an extension cord, have to assume but maybe remind him to make sure the cord used was in good condition.
 
kwired said:
But you were 50 years too late for the one in the OP, what do you do when you run into something existing and it is questionable?
Click to expand...
If you can't push a fishtape through the tube of rust, you have to use the neutral and re-ground it.
 
LarryFine said:
If you can't push a fishtape through the tube of rust, you have to use the neutral and re-ground it.
Click to expand...
Of course, bond it and say that was originally done that way - still compliant right?

Actually I'd be a little surprised to not find neutral bonded at the second structure in this installation.
 
kwired said:
Actually I'd be a little surprised to not find neutral bonded at the second structure in this installation.
Click to expand...

It has a poorly done subpanel. 3 wires feed 2 20a breakers. A few years ago i unbonded the neutral and add a ground bar and a ground rod.

Would that account for my high impendance readings? (approx 175 ohm).?
 
ricry said:
It has a poorly done subpanel. 3 wires feed 2 20a breakers. A few years ago i unbonded the neutral and add a ground bar and a ground rod.

Would that account for my high impendance readings? (approx 175 ohm).?
Click to expand...
If your test path was a good extension cord plugged into something that is bonded at the supply end plus the steel conduit in question, then your steel conduit is not electrically continuous nor is there an alternate low resistance path back to the point where that cord was connected.

quite honestly, I'd rather see the neutral bonded then to leave it like it is regardless of what code might say, an ungrounded conductor faulting to EGC will raise voltage on everything connected to it and is not low enough resistance to cause enough current to flow and open an OCPD.
 
Status
Not open for further replies.
Top