Current on grounding electrode conductor

[Andy32821]

Hi guys,

I have tested fifty 75 kva (480 to 208/120) transformers at the facility I work out for the amount of current returning to the transformer via the grounding electrode conductor.

39 of the transformers had no current on the GEC.

4 transformers had between 0.2 and 0.5 amps on the GEC.

4 transformers had between 0.5 and 1.0 amps on the GEC.

1 transformer had 2.5 amps on the GEC.

1 transformer had 6.0 amps on the GEC.

Is it worth the time to chase down what appears to be fault current on the units with less than one amp?

What is an acceptable fault amperage threshold ?

Is this even a valid preventative maintenance test ?

Thanks,
Andy

 

[davidbeach]

It would be difficult to find the lower current "faults" as they are probably leakage rather than a true fault. You might be able to find the higher currents by individually switching off circuits and seeing when the current changes. Good Luck.

 

[Skogsgurra]

A very good question to ask! (If it is a useful test).

I wish more people would ask themselves if they are doing the right thing. My answer is not very definitive; It probably is a useful test. It may prevent a fire - but it is usually more efficient to wait for the fuse to blow or the breaker to trip.

If you cannot accept that a fuse blows or a breaker trips, then it is definitely a valid and useful test.

Gunnar Englund

http://www.gke.org

 

[RonShap]

Depending on the impedance of the neutral conductor path, current on a grounding electrode conductor is normal. Remember that current is seeking its source, and will take all parallel paths back to it's source. The GEC is a path back to the source, albeit a high impedance, but a parallel path none the less, and will carry current.
So I think your test is worth it, as a benchmark, to monitor the situation.

 

[alehman]

A few tenths of an amp is probably explained by unbalanced capacitive charging current. I would want to investigate the 2.5 and 6 amp cases. These are most likely caused by an equipment ground being used as a neutral somewhere.

The GEC should not be a parallel path unless the neutral and ground are reversed somewhere or there is a N-G connection downstream.

 

[resqcapt19]

Ron,

Depending on the impedance of the neutral conductor path, current on a grounding electrode conductor is normal. Remember that current is seeking its source, and will take all parallel paths back to it's source.

Given that these transformers are in a "facility" they are most likely separately derived systems. There should be no parallel paths as the code only permits the bonding of the grounded conductor and the grounding conductors at a single point. The lower currents are most likely just normal system "leakage", but I would be looking for an illegal grounded to grounding bond on the load side of the transformers.
Don

 

[Andy32821]

Thanks for the help guys.

Although safety is always first, my hope is to improve power quality and reliability.

The 6 amp and 2.5 amps are probably faults without a good equipment grounding conductor and may pose a serious safety problem.

My main question is do the 0.5 to 1.0 amp currents pose power quality and corrosion problems to the point it’s worth spending a limited maintenance budget on, or will I just be chasing ghosts?

I will chase down a few of these currents and post what I find.

If anyone has already been down this road or has any ideas please let me know.

Andy