How clamp-ons work? The jaws of the clamp-on contain two windings. When clamped over a ground rod and energised, a current transformer (CT) induces the test current by electro-magnetic coupling. The current travels through the soil and returns to complete the circuit through all available system grounds. Since these are normally plentiful, their total resistance is negligible, and principally, the test measures the resistance of the intervening soil. The tester accurately measures the current flow, and the second winding senses voltage drop around the loop. Ohm’s law does the rest. The method can be an enormous time saver, but unlike traditional 3-point testing, it cannot be used everywhere.

Both types of testing—traditional and clamp-on have own unique strengths. They are by no means competing or mutually exclusive technologies. For their effective utilisation their differences must be understood. Merely clamping onto any accessible site does not constitute effective ground testing. The contrast between the two methods can be drawn as -

A well equipped ground testing program will take advantage of both technologies. A clamp-on will quickly pay for itself in man hours. A traditional tester will equip the operator for all possible situations.

To perform any test where a client or third party require conformance to a reference standard as the method has not been incorporated into any independent standard. However the clamp-on should be used for the following -

Courtesy: Kusam Electrical Industries Ltd (Kusam-Meco)