The advantage of the earth clamp is that it is quick and easy to set up: simply clamp the cable connected to the earth in order to measure the earth value and the currents flowing in the earth.
An earth clamp comprises two windings: a "generator" winding and a "receiver" winding. - The clamp's "generator" winding develops an AC voltage with a constant value E around the conductor clamped; a current I = E / Rloop then flows through the resistive loop. - The "receiver" winding measures this current. - As E and I are now known, the loop resistance can be deduced from them