Grounding test lead lengths

4-probe testing,

What are you testing? I'm sorry, but that isn't clear in your question.

If electronics, why such long leads?

Are you doing 4-wire testing like for a low value resistor or in circuit component?

I'm testing for the soil resistance at a new substation site. I am using the Biddle DET 2/2 tester. A very good value is around .5 ohms. A few of the additional threads have some pretty good answers to the lead length question. Thanks.

Do you mean a Kelvin bridge configuration which has two current wires and two voltage sensing wires to make very low ohms resistance measurement such as contact resistance in switch gear for power distribution systems..

Current is the same in all parts of a series circuit so the current leads are not critical. However, it would be better to have the voltage leads of equal lenght. In theory that is.

In practical real world situations, you run the risk of introducing inductive pickup of voltage in the voltage sensing leads.

Why such long leads. Even big switch gear seldom needs moe thantwo ten foot lead sets. That is what I used on my test gear. And yes I di duse shielded cables to minimize noise pickup.

It is not the Kelvin bridge configuration... but close. It is the "Line Traverse" method of measuring for soil resistivity. The probes (or stakes) have distances as large as 300 to 800 feet apart according of an example of a Nomogram for resistivity calculations in the DET 2/2 tester manual. Later threads have some good answers to my question I think. thanks.

I call your attention to the 2nd line

http://cr4.globalspec.com/edit/newthread?pt=1

-rfg

For other readers of the thread The Wenner formula is used to calculate soil resistance:- here's a link to a general description:-

http://www.netaworld.org/files/ItemFilea837.pdf

Mejic,

The reason for using four probe measurements is to eliminate the resistance in the test leads from the measured result.

Wires A and B are attached to one probe C and D to the other. (When I say probe I mean stake in the ground: in normal electronic applications you need to use four probes to eliminate the contact resistance).

...............A,B.................................................C,D.....................

Drive a known current from A to D

Measure the voltage from B to C

Calculate the resistance using Ohms law (V=IR, so R = V/I). Apply Wenner's formula.

Now as long as the resistance in the A and D leads (and probably much more significantly in your case: the sample) does not prevent you from driving the required current, and the resistance in the B and C leads is considerably less than the input impedance of the voltmeter you can't really go wrong.

Digital multimeters typically have an input impedance of 10 MΩ. So a few Ohms in the test leads won't make any difference.

Does this make sense?

Excellent answer. The tester is apparently a "current source" which means the test lead resistance doesn't matter for the two "C" probes. The tester senses the resulting voltage from the "P" probes and the lengths also do not matter since the ohmage of the tester is "infinite" and all of the voltage will be sensed by the tester. This thread business is very good. Thanks everyone.

The test leads that come with your test set have been calibrated to it at the specified length.. I would check with mfg of test set before modifying..

Here is a good article on ground testing..

http://www.netaworld.org/files/ItemFilea837.pdf

What mfg of ground tester are you using??

The tester is the Biddle DET 2/2 and it did not come with test leads. We had to make our own. I called the mfgr and from them and of the thread answers here, it appears the lengths do not matter. The tester is apparently a current sources and will pump out a certain current regardless of the resistance of the lead lengths. Thanks to all.