Earth Electrode Resistance Measurement

Good morning everybody,

I want to measure the earth-resistance on a primary substation with separate N and PE grounds.

I have measured it with 3-wire connection earth resistance loop test on the LV panel. I am using a FLUKE 1653B tester. The result was over 40 Ohm.

I have also connected the two grounds and, around the connecting wire, I have used an earth ground Clamp. The result as around 5 Ohm. I didn't cut off the power to perform the earth ground clamp test.

Any explanation to the different results? I've suspected there was a problem with the substation transformer, maybe on the offload voltage tap changer. Some type of rust that could justify the high earth resistance loop test result.

I have inspected the transformer thoroughly and it was all fine.

I am not very familiar with earth ground Clamps. Am I doing something wrong?

I am looking forward to hearing from you. Any help will be greatly appreciated.

Best regards.
Carlos Melim
You need to provide a description or sketch of how you are making your ground measurement (preferred), and how you are connecting the two grounds together and that measurement. Also, where you are located, what electrical code you come under, and what type of power system you have (US, IEC (TN, .TN-C, TN-S, etc))?

I assume that when you refer to a “earth ground Clamp” you are referring to a earth ground clamp type ground tester. Is that correct?

Also, could you tell us why you are trying to test your grounds in this way, e.g. what you main concern, etc..

All in all, a good sketch would help out a lot.

William (Bill) L. Mostia, Jr. PE
ISA Fellow, FS Engineer (TUV Rheinland)
Winner of the 2018 ISA Raymond D. Molloy Award
Principle Engineer
WLM Engineering Co.