Internationally, it has been agreed to classify the earthing systems as TN System, TT System and IT System. They are: a) TN system — has one or more points of the source of energy directly earthed, and the exposed and extraneous conductive parts of the installation are connected by means of protective conductors to the earthed point(s) of the source, that is, there is a metallic path for earth fault currents to flow from the installation to the earthed point(s) of the source. TN systems are further sub-divided into TN-C, TN-S and TN-C-S systems. b) TT system — has one or more points of the source of energy directly earthed and the exposed and extraneous conductive parts of the installation are connected to a local earth electrode or electrodes are electrically independent of the source earth(s). c) IT system — has the source either unear- thed or earthed through a high impedance and the exposed conductive parts of the installation are connected to electrically independent earth electrodes.