Earth Resistance Testing – Techniques Part IV

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Clamp-on/Stakeless Method

This measurement method is innovative and quite unique.  It offers the ability to measure the earth resistance without disconnecting the ground rod under test from the grounding system.  This type of measurement also offers the advantage of including the bonding to ground and the overall grounding connection resistance.

Illustration 1 shows a simplified ground distribution system.  The equivalent circuit is shown in Figure A.  If R1, R2, R3, … Rn are simplified to Re, then only Rg and Req are left in the circuit (refer to Figure B).  If a constant voltage is applied to the circuit, the following equation is true.Earth1


If Rg and R1, R2, … Rn are similar values, and n is a large number (such as 200), then Req will be much less than Rg and approaches zero.

Rg>>Req (Reg → 0)


If Rg and R1, R2, … Rn are all 10 ohms, respectively and n = 200.  Then Req by calculation equalsEarth3


In this example, we can see that as long as the number of multiple electrodes is large enough, the equivalent resistance is negligible with respect to the ground resistance to be measured.


Earth6Illustration 1

Since current must flow through the ground, at least two ground rods must be in the circuit.  Caution should also be taken on the placement of the tester.  If ground resistance is to be measured, connections are typically made under the bond point.  In many situations connections above the bond point introduces another wire carrying the measurement current rather than through ground.  Connection above the bond point is useful for measuring bond integrity.  Refer to the illustration 2.

Earth7Illustration 2

In certain applications it will be necessary to connect the clamp-on tester above the bond point.  This is common when testing utility pole grounds.  As the illustration shows there may be both a butt plate under the pole and a ground rod.  In this type of circuit, the tester must be placed above the bond so that both grounds are included in the test.

Earth8Illustration 3

Protec Equipment Resources both rents and sells clamp-on earth resistance test sets from premier manufacturers like Megger and AEMC.  As always our experienced staff is ready to answer questions and set you up with the right tools for your next job.