8.1 Introduction

Three-phase power systems are earthed by connecting one or more selected neutral points to buried earth electrode systems. Such earths are referred to as system earths. At electrical installations, all non-live conductive metallic parts are interconnected and also earthed to protect people against electric shock, and in this role, the earth is referred to as a protective earth. Under normal conditions, there is only a residual current or no current at all in the earth path. However, very high magnitudes of current return to source via the earth path under fault conditions. The earth also conducts lightning currents and the current path may involve part of a power system either directly or by induction. The earthing system, or part of it, may therefore also be specifically designed to act as a lightning protective earth [1].

The earth is a poor conductor and, therefore, when it carries high magnitude current, a large potential gradient will result and the earthing system will exhibit an earth potential rise (EPR). Earth potential rise is defined in the recently published CENELEC document HD 637 SI: 1999 [1] as the voltage between an earthing system and reference earth. In the UK, this quantity has previously been referred to as the rise of earth potential (ROEP) or the earth electrode potential. In the US, it is referred to as the ground potential rise (GPR). Here, this quantity will be referred to as the earth potential rise (EPR).

Soil and rock...