Lost Neutral and Ground Rod Step/Touch Potential

Status
Not open for further replies.
J

JPinVA

Senior Member
Location
Virginia
If the supply side neutral is open and there is a short to EGC (or high load) in the residence, does this create a step and touch potential hazard around the residence ground rod? I say yes, but I'm just a theory guy and know that theory and reality don't always match due to something I've failed to account for in the theory.

Anyone have any "real life" experience with this situation? Lessons learned? Safety protocols?
 
Is the ground rod the only grounding electrode? I've seen open neutrals operate without problems for a long time just off of the water pipe.
 
As infinity noted, the critical factors are the size of the effective electrode (the bigger it is the less likely there will be dangerous touch/step potential at any point) and the contact impedance of the electrode (hazardous potential will be a function of voltage drop which is current time resistance.)
If the resistance is high enough that the OCPD does not trip you can be pretty sure than the earth to electrode potential will be half or more the normal line potential.
The risk then strongly depends on where the accessible parts of the electrode are.
 
Let's assume we have one rod that meets the 25ohms requirement, and no other paths other than earth.

I'm interested in three things:

1. Can a dangerous potential exist at the rod with an otherwise compliant configurations (other than than the lost neutral).

2. If (1) is correct, what (if any) code exists to eliminate the threat to normal Joe Blow stepping next to the rod?

3. And what, if any, safety protocols (above and beyond the normal safety protocols) does one follow when arriving on site to troubleshoot.
 
Last edited:
Yes, you would have step-potential. Think of it like this, when the neutral is no longer connected the ground rod just becomes one big worm getter. And if you know anything about a worm getter rod you would also know how your hands,fingers,etc felt when you approached the rod touching the soil.

For those of you not familiar with a worm getter, it’s nothing more than taking a live wire connected to a steel rod and sticking it in the ground to make worms come up to the surface.
 
JPinVA said:
Let's assume we have one rod that meets the 25ohms requirement, and no other paths other than earth.

I'm interested in three things:

1. Can a dangerous potential exist at the rod with an otherwise compliant configurations (other than than the lost neutral).
Click to expand...

Not really. I believe one would have to posit multiple problems to have a serious danger. E.G. hot-to-ground fault plus lack of effective ground fault current path. Or open neutral plus lack of proper grounding electrode. I believe that generally speaking, having the rod, and being closer to it, makes things safer than not having it or being farther away. I can't think of how a properly connected electrode makes a situation worse. An open neutral without a grounding electrode (or perhaps worse with a disconnected grounding electrode) strikes me as a much more dangerous situation than an open neutral on a properly grounded and bonded service.

Of course, one cannot assume that there are no other problems. If you are being called to the site because there is one known problem, the chances are not zero that there's another, unknown problem. I'm sure someone has developed a protocol for being careful when approach an unknown situation, but I'll see what others suggest in terms of specifics. Common sense tells me: wear insulating gloves and test for voltage.
 
jaggedben said:
Not really. I believe one would have to posit multiple problems to have a serious danger. E.G. hot-to-ground fault plus lack of effective ground fault current path.
Click to expand...

That was my assumption. A short (e.g., hot to EGC) or high load...anything to put the majority of the phase voltage on the GEC. The loss of the neutral eliminates the effective ground fault current path, making the GEC path (to rod) the only option to close the circuit.
 
The grounding electrode and everything connected to it will be energized where the service neutral is open. The IEEE Green Book says that resistance between the rod and the earth is ~25% at 1' from the rod, ~68% at 1', ~86% at 5' and 100% at 25'. From that you can look at the expected step/touch potential around a a single driven rod.
 
Status
Not open for further replies.
Top