No, not once a threshold value for maximum contact resistance has been met (we are referring to test probe resistance here, not the installed ground that is being tested). It is a common fallacy that driving the test probes deeper will lower the readings. Imagine your readings changing as you drive the probes; what would be the correct value?
It’s true that the resistance of the ground under test does change as the test probes are moved, but the standardized procedures deal with this and provide a means of determining when the correct reading has been achieved.
With respect to the probes, this is not necessary. They need only make a minimum amount of contact with the soil, the attainment of which can be recognized by merely observing the display indicators. Once contact is achieved, the test may proceed.
Does it matter how deep I drive the probes?
My testing is all on concrete and macadam; how can I drive probes?
The good news is that you probably don’t have to. Some ground tester models have uncommonly high resistance tolerances in the test circuits (typically 4, 40, and 400 k for the current circuit, 75 k for the potential). Any surface contact of a resistance less than these high thresholds is enough.
Therefore, you may only need to lay the test probes flat on the surface and establish contact by wetting the area. Concrete conducts current fairly well and chances are good that you’ll have an acceptable test. Macadam is not as good because of the non-conductive tar, but you may still be able to achieve enough contact.
You’ll know because indicator lights on the tester warn you if the contact threshold has not been met. If that becomes a problem, you can improve your chances by using a contact mat instead of the provided probes. Mats are flexible metallized conductive pads that mate with the surface contours. They are readily available from ground materials suppliers.
