Measuring earth resistance with the clamp-on method — no auxiliary electrodes

The guide on the 3-point method covers the classic fall-of-potential measurement using two auxiliary electrodes in the ground. This article covers an alternative technique that requires no auxiliary electrode at all: the clamp-on method (also called the "stakeless" method), measured with a clamp-shaped instrument around the earth conductor or electrode itself.

Operating principle

A clamp-on earth resistance tester induces a known AC voltage onto the earth conductor under test via one clamp, and measures with the same or a second clamp the resulting current that flows via the earth electrode and back via all other, parallel earth paths present. The ratio between the induced voltage and the measured current gives the resistance of the electrode under test — without needing to drive auxiliary electrodes into the ground.

The crucial condition: multiple parallel earth paths

The clamp-on method relies on an assumption that does not hold in every situation: the electrode under test must be connected in parallel, via the earthing network, to at least one other, independent earth electrode or connection, so that a closed current loop exists through which the induced test current can return. The more parallel earth paths are present, the closer the measured value approaches the true resistance of the individual electrode (the combined resistance of the other paths then becomes negligibly small compared to the electrode under test).

For a single, isolated earth electrode with no parallel path — as is common with an individual TT domestic installation with just one earth rod — that closed loop is entirely absent. In that situation the clamp-on method gives no usable reading: for such a single electrode, the 3-point (fall-of-potential) method remains the only suitable technique.

When the clamp-on method is suitable

The clamp-on method is mainly useful in situations that naturally have multiple parallel earth connections, for example:

Networks with multiple, interconnected earth electrodes (such as a series of low-voltage poles or cabinets connected to each other via the PEN conductor or a shared earthing network).
Situations where it is practically difficult or impossible to drive auxiliary electrodes into the ground (paved surfaces, limited space around the measurement location).

Practical relevance

Before using a clamp-on meter, it must be established whether the electrode under test actually forms part of a network with multiple parallel earth paths. If not, the 3-point method is required — the convenience of the clamp-on method (no auxiliary electrodes, faster measurement) does not outweigh a reading that does not correctly represent the true earth electrode resistance.

Common mistakes

Using the clamp-on method on a single, isolated earth electrode (for example an individual TT domestic installation) — without a parallel earth path this does not give a reliable value.
Not checking whether multiple parallel earth paths are actually present before the measurement, and still treating the result as representative.
Choosing the clamp-on method purely for convenience (no auxiliary electrodes needed) without assessing whether the situation is actually suited to it.
Directly comparing clamp-on results with 3-point measurements without realising that both methods measure a slightly different aspect of the earthing configuration — especially with a limited number of parallel paths, the results can diverge.