Arcing due to physical damage in electrical wiring system. Such as:
Wires are accidentally damaged by nails or screws
Animals and vermin chewing the wire insulation layer
Damage on insulation layer due to Heat, humidity and voltage stress
Arcing due to loose connection
Arcing due to improper joints in the circuit or at outlets
Arcing due to extension cords
Arcing due to damaged appliances or electrical devices