Prospective Fault Current
Fault current is usually caused by a failure in wiring or other equipment (perhaps due to accident or abuse), resulting in a breakdown or bridging of the electrical insulation. This occurs typically, though not always, at terminations. Fault currents are generally much larger than either load currents or overload currents, and may cause serious damage to wiring or equipment unless appropriate protection is provided.
The fundamental principle of protection against fault current is reproduced as follows:
‘Conductors and any other parts intended to carry a fault current shall be capable of carrying that current without attaining an excessive temperature. Electrical equipment, including conductors, shall be provided with mechanical protection against electromechanical stresses of fault currents as necessary to prevent injury to persons and livestock or damage to property’.
and
‘Persons and livestock shall be protected against injury, and property shall be protected against damage, due to excessive temperatures or electromechanical stresses caused by any overcurrents likely to arise in live conductors.’
Types of fault current
There are two types of fault current: short-circuit current and earth fault current, as referred to in the following definitions:
‘Short-circuit current. An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.’
‘Earth fault current. An overcurrent resulting from a fault of negligible impedance between a line conductor and an exposed-conductive-part or a protective conductor.’
Examples of line-to-line short-circuit current and earth fault current are shown in Figs 1 and 2, respectively. Both figures are based on a TN-C-S system.
An example of line-to-line short-circuit current
Fig 1
An example of earth fault current
Fig 2
Fault current protective devices
Fault current may cause overheating, arcing, fire and even explosion, resulting in injury to persons and damage to property. To reduce the risks of such injury or damage occurring, BS 7671 requires fault protective devices to be provided. The basic requirement is stated as follows:
‘A protective device shall be provided to break any overcurrent in the circuit conductors before such a current could cause a danger due to thermal or mechanical effects detrimental to insulation, connections, joints, terminations or the surroundings of the conductors.’
In addition, BS7671 requires one to take into account minimum and maximum fault current conditions, when selecting a device or devices for the protection of wiring against fault current.
London Electrical Inspection & Testing can design your installation with suitable protective devices so as to comply with the British Wiring Regulations BS7671.
