Fall-of-Potential vs. Clamp-on Ground Testing 1

[ASimmons23]

Reference thread:
thread238-231298

Good morning all,

I wanted to get everyone's experience/opinion on using a clamp on meter instead of the traditional fall-of-potential test for ground testing. One of the general contractors is asking for a waiver to our current specs which state a fall-of-potential test shall be used for ground testing. They say that because pavement is already poured in the area and there is heavy work traffic (welding, other electrical testing etc.) a f-o-p test will be cost prohibitive as compared to using the clamp on meter.

The GC referenced this paper..

http://www.aemc.com/techinfo/appnotes/Ground_Resistance_Testers/APP-Ground-ComparisonofTesters.pdf

But I cannot find any reference to a clamp on ground test in IEEE 81 or IEEE 142.

What are your thoughts on the clamp on meter vs fall-of-potential? Will either one give satisfactory results? Is the clamp on method recognized (by IEEE or other governing body) as an acceptable alternative to a fall-of-potential test?

Thanks in advance,
Andrew

 

[jghrist]

I don't know of any industry standard recognizing the method, but I believe that the use of clamp-on ground meters is an acceptable method for small electrodes like one or two ground rods, when connected to a multigrounded neutral. Its use depends on the resistance of the measured electrode being high compared to that of the neutral system. It would not be appropriate for a large ground grid. It cannot be used to measure the resistance of an isolated ground rod (not connected to the utility neutral).

 

[ASimmons23]

Thanks, those are the same conclusions I have come to. I was hoping I had missed a white paper (other than the one I referenced, which is put out by the same company selling the meter) or IEEE reference as it would be much easier to sell to the client who does not like deviating from the spec unless the alternative is well documented and accepted.

 

[Zogzog]

This has been a big debate, AVO which also sold clamp on's is now pushing the ART method. The biggest con of clamp on testing is there is no way to verify your readings are accurate.

Stakeless or Clamp–on method
Advantages

Test is quick and easy.
No disconnection of the ground rod from the system.
No probes need to be driven / cables connected
Measurement includes the bonding and overall connection resistance.
Measures true RMS leakage current flow through the system.

Limitations

Effective only in situations with multiple grounds in parallel (pole grounds).
Cannot be used on isolated grounds (no return path)
Not applicable for installation checks / commissioning new sites
Cannot be used if an alternate lower resistance return exists not involving the soil
– Cellular towers
– Substations
Subject to influence if another part of the ground system is in “resistance area”:
– If there is another part of the ground system in the “resistance area” of the electrode under test, the result will be lower than the true resistance of the electrode – this could lead to a false sense of security.
Requires a good return path:
– Poor return path may give high readings.
Connection must be on the correct part of the loop for the electrode under test:
– Requires thorough understanding of the system.
– Wrong connection can give a faulty result.
Susceptible to noise from nearby substations and transformers (no reading).
There is no built–in proof for the method – results must be accepted on “faith”.

 

[ASimmons23]

Thanks for the response zog, the other hurdle after deciding to use the clamp-on ground test is to make sure a thorough procedure is written/in place and the contractor understands how to properly conduct the test.

For thise interested, I just received an email from an AEMC Instruments Application Engineer stating:

"IEEE is preparing a draft recognizing the clamp on used for ground resistance testing. It will be released in their next publication."

 

[Zogzog]

What I have seen is EC's buying these without any training or understanding on how to use them or what they are actually measuring, it is just "easy". And no matter what IEEE says, there is still no verification of the readings.

 

[dpmac]

I would have to agree with many of the comments regarding the improper use of the clamp on meter. It is very easy to use improperly. Most of our guys will bring a standard 4 terminal test set and a clamp on to a ground test to compare them if they have the time. Over the past few years they have always tried to use both to compare. Some the discussions back at the shop were very informative (and lively). There are situations where they line up very close and some not at all. There are times where the clamp on is the exact tool you want to use but can't and vise versa for the 4 terminal set. You have to understand the equipment your using and the particular site your testing to get reasonable readings.

I once reviewed a test report of over 35 individual ground rods (no connections to any rods when tests were performed) where a clamp on tester was used with outstanding results!

You have to be at least 10% smarter then the equipment your using.

 

[Zogzog]

How do you know if you are within the sphere of influence of another grounding electrode when using a clamp on?

 

[dpmac]

Zog,

There are certain circumstances where it comes in quite handy. Telephone poles, railroad catanary towers, multiple small grids, etc. Providing they are electrically connected and spaced in a fashion that suits the clamp on method.

 

[Zogzog]

dpmac, agreed, they have thier place, but get misused more often than not. Part to blame is the marketing, they make out to be more useful than it is and more simple. Few that use these understand thier limitations.