Why starting current of self regulating cable is higher than operating current?


Why starting current of self regulating cable is higher than operating current?

Self-regulating cables have the characteristic of changing resistance and therefore power, in response to the environment surrounding them. This is referred to as having a positive temperature coefficient (PTC) of resistance. When the heater is unpowered its resistance is determined by the ambient temperature of its surroundings. When the heater is energized it begins to heat up and its resistance increases rapidly reducing the current being drawn. This initial change in current, referred to a “start-up current”, involves heating the cable itself, to overcome the thermal inertia of the heating cable itself and little heat energy is transferred to the gutter or other surface. As this is a relatively small mass, this initial start up period is quite short (usually under 5 minutes). After this, the heater resistance continues to increase (current decreases) as the gutter or other surface being heated raises in temperature. It is necessary to understand the relationship between the start-up current and the operating current to properly design a self-regulating heating system. You need to know what's the start-up current is for a given minimum start-up temperature in order to properly size the circuit breaker and prevent possible nuisance tripping.

The SR heating cable, which has a polymeric heating core, blended and loaded with carbon particles extruded in between with parallel bus bars, and that the heating takes place between the busbars by the current flow through the polymeric heating core. This provides the property of cut-to-any-length feature to the SR heating cable. 

Written by Gabriel Taut
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