ABSTRACT
Contact with metalwork made live by a fault is clearly undesirable. One popular method of providing some measure of protection against such contact is by protective earthing, protective equipotential bonding and automatic disconnection of supply. The regulations recommend therefore that the preferred method of earth fault protection for installations in a TT systems be achieved by a residual current device (RCD), such that the product of its residual operating current and the loop impedance will not exceed a figure of 50 V. In a faulty circuit, either line to earth or neutral to earth, these currents are no longer equal; therefore the out-of-balance current produces some residual magnetism in the core. Certain appliances such as cookers, water heaters and freezers tend to have, by the nature of their construction and use, some leakage currents to earth. These are quite normal, but could cause the operation of an RCD protecting an entire installation.
