DESCRIPTION – SELF-LIMITING HEATING CABLES
Self-limiting cables can be bought in running metre for make-up on the site. The cable has a varying output depending on the ambient temperature, which guard against overheating even if the cable crosses itself. This also allows installation in Ex-surfaces (all Värmekabeltekniks self-limiting cable types are Ex-rated).
The self-limiting cable have a unique capacity in proportion to the sheath temperature of the cable, reduce the emitted output. These cables are often mentioned as self-regulated cables but this is a wrong denomination, as a required temperature not can be guaranteed without temperature control.
On the other hand, the cables make it possible to give an even temperature on a pipe even if the ambient temperature varies along pipings.
Värmekabeltekniks self-limiting cables are approved within Ex-surfaces when the cables have a stated T-rate, i.e. a maximum temperature that the cable reach. The T-rate varies for different outputs/m.
The self-limiting cables are built up round a semi-conductive bed embedded in the outer edges. The bed between the conductors has a capacity to lead current. In conducting capacity is in proportion to the temperature. At rising temperature the conducting capacity decreases, overheating is prevented. At sinking temperature, the current increases and the increasing heat requirement is satisfied.
The emitted outputs of the self-limiting cables are depending on the ambient temperature and the method of installation, a good dissipation of heat increase the emitted output. (Aluminium tape that covers all the length of the cable and is attached against the pipe, cool down the side of the cable, which borders against the insulation and thereby, increase the emitted output with 30-50%).
All self-limiting cables have a starting current which is higher than the operating current. This must be taken into consideration when designing circuit-/connection lengths, (see data sheet).