Various thermoplastic and thermo-set based
polymers, blended with acetylene carbon blacks and graphite fillers to form
electrically conductive polymers, were investigated to determine the
switching mechanism under high power short-circuit fault conditions.
Presently, a positive temperature coefficient (PTC) effect is the accepted
switching mechanism for low power overcurrent devices. However, for high
power, distribution level devices, this mechanism has been assumed to be the
same under high fault conditions. This work proposes another possible
switching mechanism responsible for a steep change in resistance under high
power faults. The proposed mechanism of resistance change comes, not from
the PTC change in the material, but rather from the vaporization of the
conducting polymer contact areas at the polymer/electrode interface. A
comparison between low power polypropylene based devices with a positive
temperature coefficient (PTC) effect (thermal expansion of the polymer bulk)
and high power devices was done to support this theory. A model was
developed along with numerous short-circuit tests to support this theory.
Modifications with e-beam irradiation were made to alter the PTC
characteristics of thermoplastics but had little effect on high fault
short-circuit switching. Also, numerous short-circuit testing on thermo-set
based materials without any PTC transition still switched under high fault
currents. These all supported the proposed theory that switching, under high
fault currents, was from vaporization of the contact areas at the polymer
surface rather than thermal expansion in the bulk of the conductive polymer
as previously accepted.