To protect cables from moisture it is used longitudinal sealing.
Ways of sealing:
- filling the whole space – void space – in the core with hydrophobic compound;
- compound hydrostable corks, which periodically placed through the cable length;
- dry powder (cellulose-based, for example), which is applied on insulated cores. The powder bloats when there is some wet inside, and it prevents the longitudinal propagation of wet;
- appending water-absorbing elements into a cable core.
As a cable’s filler it is used composition of products of oil’s distillation – petroleum jelly with polyethylene addition. For example, there is water-blocking filler, which consists of paraffin, ceresin and mineral oil with polyethylene addition.
The level of filler and polyethylene insulation compatibility depends on swell value. The higher insulation swelling, the less polyethylene’s density. Level of insulation swelling ΔH is defined by weighing samples before and after ageing in filler.
Service capacity rises from C’ to C” when the core free space filling is 100% (solid insulation δ‘, thickness d). Insulation’s thickness can be increased to save service capacity on C’ level (δ”>δ‘), but an outer cable’s diameter will be also increase at least by 15%. The mass of solid filled cables is bigger than the mass of unfilled cables in average 35%.
It is possible to leave the insulation thickness unchanged under C’=const condition: just replace solid insulation with foam or solid-foamed insulation. But the cables’ mass will increase by 5…10%. (table below)
Silicon rubber-based compound is used to create water-blocking corks while periodic fractionary core filling. Compound is injected in dose into the stranded core by port injection. The length of cork is about 0.2 m, and the interval is near 4 m. Because of the correlation, the level of free core space filling is 5%, which practically has no effect to form factors and electrical parameters of cables. This method is usable for solid, foamed and solid-foamed insulation.
Over the last years, other compositions are used as water-blocking filler.