The purpose of our study is to clarify how the switching devices start fire, that is the first subject of our research on the fire hazards of wiring devices. We have experienced several cases of fire in which the burning presumably started from switches. It is generally assumed that the ignition of a switch may be caused by a short-circuit in the switch or local overheating of the switch due to the contact resistance.
Despite the above assumption, while making the investigation of fire cause of those fires, we have noticed that some burnt switches showed no evidence of short-circuit or local overheating due to contact resistance and there must be some other cause of ignition.
Then we have started our study to determine: (1) whether or not the repeated operation of a switch starts fire: (2) what process the ignition of a switch takes: (3) how the electric current through a switch is related to the starting of ignition of the switch.
From the results of our study we have found that in case of a pullswitch made of bakelite, minute sparks produced by repeated switching may carbonize the bakelite, and this carbonized part may, in some cases, be conductive of electricity. In such case the fixed contact pieces and the movable contact piece of the switch will be bridged, allowing the electricity flow through the bakelite even when the switch is in off position, and in such condition a switch may start fire. It was also noticed that the more electric current and the less power factor, the more chances of ignition there would be for the switches.
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