5.6.2 - Increased fire risk

As with other systems of earth-fault protection, PME does not prevent a fault occurring, but will ensure that the fault protection device operates quickly when that fault appears. For example, if a fault of 2 Ohms resistance occurs in a 240 V circuit protected by a 20 A semi-enclosed fuse in a system with an earth-fault loop impedance of 6 Ohms, the fault current will be 240/(2 + 6) A = 240/8 A = 30 A. The fuse would not blow unless the circuit were already loaded, when load current would add to fault current. If the circuit were fully loaded with a load current of 20 A, total current would be 50 A and the fuse would blow after about 18 s. During this time, the power produced in the fault would be:

This could easily start a fire. If, however, the earth-fault loop impedance were I Ohm, current would be 80 A and the fuse would blow in about 1.6 s and limit the energy in the fault circuit.