At pre-determined distances, the insulation is removed from alternate conductors, exposing the base conductor. A heating wire (or element) is wrapped around the two conductors, and where the wire meets these two areas, a heating ‘zone’ is created.
Sometimes the heating element is strengthened with fiberglass thread to absorb the high stresses created (especially at higher voltages) in the relatively thin heating wire.
A layer of insulation is then extruded over the cable. Similar to the self-regulating cable, a further optional layer of metallic braid and a final thermoplastic or fluoropolymer insulation can be extruded over the cable to provide a finished product.
The heating cables in this range of heating cables are considered power-limiting as the alloys used in the heating element have a relatively high Temperature coefficient of resistance (also known as the alpha-coefficient). What this means in laymen’s terms, is that when the element temperature increases, its resistance also increases.
This effect is much less than in self-regulating cables discussed earlier allowing for lower start-up currents and slightly longer circuit lengths.
The disadvantage of this type of cable is the zoned construction. If the installer isn’t careful (or not aware) of ‘notch’ locations, then there is a high likelihood that a cold section of cable will be installed without realising.
Also, high voltage cables (~500VAC or higher) create very, very high tension in the heating element and joints, and failures are common-place due to the element simply snapping or tearing away from the notch. These problems are not common in modern cables at lower voltages.