When a fault occurs on an electrical system, electrical current usually flows to earth, tripping protection systems. On overhead line wood pole networks (between 6.6kV and 33 kV), only sites with a transformer, switchgear or pole termination are directly bonded to earth (using a wire down the pole) for safety and lightning protection.
Typically, when the system is operating normally, there will be a minimal flow of current down the earth wire (Earth leakage). At sites with local earth, on breakdown of insulation, i.e. occurrence of fault, the current will flow down the local earth wire into the ground. As the breakdown of insulation increases, the flow of current into the ground rises, eventually reaching a critical point. Beyond this point, the flow of current can be great enough to cause asset failure and trip upstream protection systems, resulting in loss of supply to customers.