Grounding airport taxiway lights

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old electron

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I am the inspector on an airport taxiway extension and the contractor is using a AEMC "clamp-on" ground resistance meter and getting extra high readings. (in the 77 ohm range) and I am getting readings in the range of 6.5 to 16 ohms with a three wire Extech 382252 meter. these reading are on a 5/8 X 8' ground rode installed at each Taxiway light. I have been told by a registered PE in Electrical Engineering that the clamp on meter will give false reading without a return path (like two ground rods at several ft. apart) connected to the light. FAA and NEC both require 25 ohms or less. Any input on this subject would be appreciated

Old Electron
 
old electron said:
FAA will not except anything over 25 ohms regardless of ground rods.
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Read the documentation for the clamp meter and you should see that it is at its best when there is a multi-electrode counterpoise such as a utility MGN network or several other good building electrodes from the same GES. This would include water pipes and CEEs.
The PE you are talking to us correct.
If there are no good electrodes and the person testing does not include the wired EGC to the lamp location, then only the three point test will be usable.
 
The clamp-on tester is not an IEEE recognized method of confirming electrode resistance because it only works accurately in very specific circumstances and there is no way to retest in order to check your results.

The 3-point fall of potential is going to be your reliable reading, and you can check it by doing a second test perpendicular to your first and in both cases your remote earth resistance s should be similar (barring any underground influences).
 
Does anyone know how often the ground resistance is tested after initial installation? And if/when the retest indicates a change in resistance above the magical (nonsensical) 25-ohms, then what?

My point is there are so many variables that will change from the time of installation as compared to one day, one week, one month, or one year later...
 
bphgravity said:
And if/when the retest indicates a change in resistance above the magical (nonsensical) 25-ohms, then what?

My point is there are so many variables that will change from the time of installation as compared to one day, one week, one month, or one year later...
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Exactly. You can test the resistance of a rod to your hearts content in the usually stormy and very wet month of April and manage to get below that 25 ohm threshold, only to come back during the usually dry, hot (and miserable) month of August and find twice or better that reading from the same rod.
 
user 100 said:
Exactly. You can test the resistance of a rod to your hearts content in the usually stormy and very wet month of April and manage to get below that 25 ohm threshold, only to come back during the usually dry, hot (and miserable) month of August and find twice or better that reading from the same rod.
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While I agree that electrode resistance can change, and question the logic behind these tests, as a rule-of-thumb soil compaction makes the single biggest difference on electrode resistivity. So if they have bern getting acceptable results since installation those values should only improve with time. If they go up substantially it may indicate a problem.

And while a single electrode like OPs may vary widly , as a rule a larger grid will not, regardless of weather, so again massive deviation could indicate a problem.
 
bphgravity said:
Does anyone know how often the ground resistance is tested after initial installation? And if/when the retest indicates a change in resistance above the magical (nonsensical) 25-ohms, then what?

My point is there are so many variables that will change from the time of installation as compared to one day, one week, one month, or one year later...
Click to expand...

To my knowledge, once the original test has passed, there is never a recheck. I have seen a contractor saturate the area with a water truck and salt to get a test and no one ever has rechecked it. (make you wonder doesn't it.)
MSHA requires 5 OHMS or less at all mining operations, as far as I have been told.
 
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