Chapter 9 : measuring earth resistance and continuity testing

by E.Hering

Measuring earth resistance

Earth resistance is a key parameter in determining the efficiency of earthing systems.

In this article, measurement of earth resistance fundamentals are explained:

• parameters,
• standards for measuring instruments,
• selection of measuring equipment,
• how to ensure measurment accuracy and report the measures.

Then a comprehensive overview of measurement methods is given for RE:

• the balanced-bridge method,
• the current-voltage techniques,
• measurement methods with current from the supply.

Current-voltage techniques, equipments and methods that draw no supply current are presented in details:

• earth resistance meters,
• methods using a probe and an auxiliary earth electrode,
• method using a probe but no auxiliary earth electrode,
• method without a probe and an auxiliary earth electrode ("stakeless method"),
• stakeless methods (no probe, no auxiliary earth electrode) using a clamp-on ohmmeter,
• selective earth resistance measurements using a probe, an auxiliary earth electrode and a clamp-on ohmmeter.

An inventory of measurement equipments and methods that draw current from supply is done:

• methods using a probe
• methods using the PEN conductor or the neutral conductor instead of a probe
• method without a probe
• selective earth resistance measurements using a probe and a clamp-on ammeter

Continuity testing in earthing systems

After a brief introduction, testing methods are introduced:

• continuity testing in ring earth electrodes that can be opened,
• continuity testing in ring earth systems that cannot be opened

together with advices to write the testing report.