AC Circuits and Power Systems in Practice

11Earthing Systems

Low voltage distribution systems are generally based on a star connected set of windings where each phase voltage is referenced to the neutral terminal. The system of earthing the neutral determines the fault characteristics of the network, including the touch voltages to which people will be exposed in the event of a fault, and the magnitude of the current flowing. A solidly earthed neutral, for example, will result in large fault currents, but it will limit the extent to which over‐voltages can exist. On the other hand, while earthing the neutral through an impedance reduces the magnitude of earth fault current, it does so at the expense of permitting considerable over‐voltages to occur. Therefore electrical equipment must be capable of withstanding either high fault currents or large over‐voltages, until such time as a fault can be detected and cleared.

LV Electrical systems are earthed for the following reasons:

• To provide a current path to earth in the event of a phase‐to‐ground fault
• To provide a limit on the voltage that exposed metallic parts of an installation may attain during a fault
• To permit the operation of protective devices that remove the risk of electric shock to personnel in the vicinity of a fault
• To limit the rise of the LV potentials due to a fault within the MV network

Earthing system design is based on mitigating the effects of electricity on the human body that can occur during an earth fault. Therefore before we begin a discussion ...