TABLE OF CONTENTS The purpose of a substation earthing system is to ensure safe conditions for personnel and plant in and around the site during normal and earth fault conditions. To achieve this, it needs to perform the following functions:
Provide equi-potential grading by electrodes or similar, to control touch and step potentials.
Enable connection (bonding) of necessary exposed extraneous conductive parts to earth.
Provide a route for the passage of fault current which does not result in any thermal or mechanical damage to connected plant and allows protective equipment to operate.
Provide an earth connection for transformer neutrals, sometimes via an impedance to restrict the fault current magnitude.
Minimise electromagnetic interference between power and other systems such as control or communication cables or pipelines.
This Chapter is an over-view of the subject, dealing mainly with the design limits and the general procedure followed to design an electrode system, including a computer analysis.
Small power and lighting installation earthing is covered in Chapter 7.
Substation earthed metal work (consisting of switchgear enclosures, supports, fencing, etc.) and overhead line steel supports all have an impedance to true earth. When fault current flows through them to earth, a voltage rise will occur. This earth potential rise (EPR) is the maximum voltage that the earthing system of an installation may attain relative to a remote point assumed to be at true (zero) earth potential. The EPR is the product of...
