The object of an earthing system is to provide a system of conductors, as nearly as possible at a uniform and a zero, or earth, potential. The purpose of this is to ensure that, in general, all parts of the equipment and installation other than live parts shall be at earth potential, thus ensuring that persons coming in contact with these parts shall also be at earth potential at all times.
Circuit and system earthing
The purpose of circuit and system earthing is to limit excessive voltage from line surges, from cross-overs with higher voltage lines, or form lightning, and to keep non current carrying enclosures and equipment at zero potential with respect to earth. Earthing the system helps facilitates the opening of overcurrent protection devise in case of earth faults.
Earthing asspciated with current carrying conductor is normally essential for the security of the system and is generally known as system earthing, while earthing of non-current carrying metal work and conductor is essential for the safety of human life, animals, and property and it is generally known as equipment earthing.
The earthing arrangements shall be such that:
The value of resistance from the consumer’s main earthing terminal to the earthed point of the supply, or to earth, is in accordance with the protective and functional requirements of the installation, and expected to be continuously effective,
- Earth fault currents and earth leakage currents likely to occur are carried without danger, particularlyfrom the point of view of thermal, thermomechanical and electromechanical stresses.
Precaution shall be taken against the risk of damage to other metallic parts through electrolysis.
Where a number of installations have separate earthing arrangements, protective conductors running between any two of the separate installations shall either be capable of carrying the maximum fault current likely to flow through them, or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.
Methods of earthing
The usual method of earthing is to join the exposed metal work