Obviously heat will have caused this and a resistive load will have generated the heat.
A high resistance junction in the switch may have been caused by tightening the screw terminal through the insulation of the wire or the screw may have worked loose over time as the switch was operated resulting in sparks and sooting on the conductors.
At 30 Amps a resistance of just 0.1 Ohm is 90Watts of heating at the junction - Power in Watts is current squared multiplied by resistance (P=I²R)
Peccavi
April 2009
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