Current in grounding electrode conductor

[Barry Kann]

In a residence, when a 120V-10A heater is on, 1.5A flows in the Grounding Electrode Conductor (GEC). I isolated the current to the ground in the old unused copper telephone cable. Since there is no connection at all to this cable, I disconnected this phone ground from the GEC. The 1.5A in the GEC went away. The phone cable ground was providing a parallel path for neutral current. Who caused the problem, the phone company or the utility company?

Barry Kann
Fox Mill Electric Service

 

[G._S._Ohm]

Seems like one interpretation of your posted data is that your ground impedance was 120v/1.5A = 80 ohms and your heater was still drawing 10A.


To answer your question, IMHO more measurements are needed, using power resistors
and a 4-1/2 digit DVM or a clamp-on ammeter.

 

[don_resqcapt19]

There are many parallel paths for grounded conductor current in a code compliant installation. If the phone cable is an underground cable with a metallic shield, the shield will be bonded to the electrical grounding system at each building. This creates a path that is in parallel with the service grounded conductor and there will be current on this path.
The only time this is a problem is if you lose the sevice neutral and the parallel path(s) try to carry all of the grounded conductor current. In this case the ampacity of the parallel path(s) many be too small to carry the load and could result in a fire.

 

[Barry Kann]

Parallel path for neutral current

Parallel path for neutral current

In my experiment of using a 120V-10A heater, if 1.5A flows in the shield of the underground phone cable, that is 15% of the current, does that imply that their is higher than normal impedance in the actual neutral from the house to the utility transformer?

 

[Smart $]

In my experiment of using a 120V-10A heater, if 1.5A flows in the shield of the underground phone cable, that is 15% of the current, does that imply that their is higher than normal impedance in the actual neutral from the house to the utility transformer?

Not necessarily. Could just as easily imply that the the underground phone cable has relatively low impedance bonding to utility ground.

For example, if the service neutral and the phone cable ground were the only two return paths for neutral current, and the service neutral impedance was 1 ohm, the impedance of the phone cable ground would be 5.6 ohm for 15% of the current.

 

[G._S._Ohm]

In my experiment of using a 120V-10A heater, if 1.5A flows in the shield of the underground phone cable, that is 15% of the current, does that imply that their is higher than normal impedance in the actual neutral from the house to the utility transformer?

I forgot to mention: you know the current in the ground when it was connected but I'd also measure the voltage drop between the now disconnected ground and the point at which it was connected.

These kind of problems are really good. From a very few measurements you have to infer what is going on in a semiconducting earth. Thevenin's theorem comes in handy here. Same goes for people getting shocks from swimming pools except it's worse because the wet skin/body resistance varies somewhat.