12/02/2003 • Fire prevention

Halogen-free Blister Light Cables

In case of fire, all cables containing PVC emit large amounts of halogens, which form, together with the humidity in the air and the fire extinguishing water, severely toxic chlorides and highly corrosive acids. As long as cables are laid in open spaces along runways and taxiways of airports, the danger of consequential damage is relatively low, because fires are rare in this area and, even in case of a fire, PVC cables are quickly extinguished due to the fire extinguishing properties of the halogens.

Halogen-free cable designs

The draft of the European Standard for primary cables used in circuits for airport blister lighting - ENV 50213 of November 1998 by Cenelec - which is already being applied generally today, defines various different structural designs for these cables. Approved are cables with insulation and sheath made from polyvinyl chloride (PVC), as well as those with an insulation and sheath from crosslinked polyethylene (VPE) as well as ethylene propylene rubber (EPR).
Of these designs, mostly in use today are the structures PVC-PVC for voltage levels 1/2 kV and 3/6 kV, as well as the structure VPE-PVC for voltage level 6/10 kV. These material combinations have been selected based on their compliance with corresponding electrical requirements of the cables, and the low material costs. The potential risk of danger in case of fire is very high, if these cables are routed to larger connection or distribution rooms as well as to feeders (Fig. 1) because of the high content of PVC in these cables. Here, in case of fire, personnel is directly endangered by the toxic gases being created. Considerable property damage may result due to the formation of corrosive gases, which form aggressive acids, such as hydrochloric acid, together with the extinguishing water used to extinguish the fire. In many cases, these consequential damages arising from the corrosiveness of the fire gases and the extinguishing water surpass the value of the actual fire damage many times.
This risk can be avoided by using halogen-free material for the sheathing of cables routed through fire-risk rooms. The standard would allow here VPE and EPR. However, it must be taken into account that VPE-sheath is a very good burner and thus could not prevent fire from spreading out. EPR is an extremely expensive sheathing material. A technically effective and economical alternative to the use of the above-mentioned sheathing materials VPE and EPR is to manufacture cables from halogen-free and flame-resistant sheathing material, based on Etyl-Venyl-Acetat (EVA).

Development of a New Type of Cable

Such a navigation light cable manufactured from halogen-free, flame-resistant sheath was developed and launched onto the Spanish market in 2002. The voltage level of the cable is 6/10 kV, for which a VPE core insulation must be used to guarantee the electrical properties of the cable. In order to ensure a successful extinguishing of the cable in the event of fire despite the fire conducting properties of VPE it was necessary to use a sheathing material with a very high oxygen index (LOI) that prevents the fire from spreading along the cable as quickly as possible.
The fire tests conducted on the sample production of cables has shown that the cable performs very well in fire conditions. The fire extinguishs in just a few minutes and along a length of less than two metres. The transparency of the combustion gases was more than 70 %, which is exceedingly important to people that need to find a way out of a burning room in the event of fire. The ability to achieve the technical requirements described was supported by the standardisation requirements of the cables for the Spanish market. They have a high sheath wall thickness in comparison to the requirements of the draft standard ENV 50213 (wall thickness of the sheath 1.4 mm) on account of the standardised requirement in the Standard UNE 21161-93 of at least 2.0 mm. Nexans Germany has supplied approx. 80 km of this type of cable to the airport in Gerona, and further enquiries have been received.
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