To eliminate air gaps and give the cable a round shape between the core insulation and the belt insulation, some cables use fillers.
Filler – An element used to fill free spaces in a cable or wire to provide the required mechanical stability, longitudinal tightness, and shape. Fillers made of plastic, rubber and paper tapes are available in the form of harnesses. For stranded cables with belt paper insulation, sulphate paper harnesses are used to fill the gaps between insulated conductors. To fill the gaps between the cores in individual sheaths, harnesses made of impregnated cable yarn or stapled glass yarn are used as filler materials. For cables with plastic insulation, the filling must be:
- in case of insulation made of polyethylene, self-extinguishing, vulcanizing polyethylene – from the insulation material or polyvinyl chloride plastic;
- for insulation made of polyvinyl chloride plastic – of polyvinyl chloride plastic;
- for cables for voltages up to 3 kV – from unimpregnated cable yarn or glass yarn.
Hydrophobic filling is popular, as it is characterized by improved protection against water ingress into the cable and the longitudinal spread of moisture. 
In modern fire-resistant cables, some manufacturers also use halogen-free filling of the gaps between insulated conductors. In the European market, this is necessary to give the cable an aesthetic round appearance. Fire resistance tests of different types of cable products have shown that the filling does not affect the integrity of the circuits under standard temperature conditions (of the cable line) in case of fire, the main structural element being the insulation and its quality.
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Therefore, many types of cables can be made without fillers if necessary.
