Arc fault protection is a relatively new technique employed for the fast clearance of arcing faults on BUS bars & within metal clad switchgear & associated cable boxes. The arc is detected using an optical sensor & the signal input to a protection device which also monitors the load current on the system. When an arc fault start current is coincident with a signal from the optical sensor a trip command is output. A trip signal can be achieved within 10ms which can be considerably faster than that provided by a traditional IDMT overcurrent relay. This is because the onset of an arcing fault can occur at relatively low fault currents. Arguably the greatest risk of arc fault damage exists at the CB cable termination & in the CB chamber itself due to the slow clearance times of the IDMT feeder protection.