Complex sentence: In the event of a fault, a current is allowed to flow to earth by the earthing system which if it is an excessive current will operate the fuse or circuit breaker, thereby protecting the circuit and removing hazardous voltages from the exposed conductive surfaces.
Split into simpler sentences: In the event of a fault, a current is allowed to flow to earth by the earthing system. If this is excessive the overcurrent protection of a fuse or circuit breaker will operate, thereby protecting the circuit and removing hazardous voltages from the exposed-conductive surfaces.