Fire performance of Electric Cables

The following article is featured with the kind permission of MDM Publishing

Electrical cables are frequently blamed by the media and fire authorities as the cause of building fires however it is often not the failure of the cable which starts a fire but the misuse of the cable by frayed or damaged insulation, overloading due to incorrect or insufficient circuit protection, short circuit or over voltage. These situations can cause high temperatures in the cable conductors or electrical arcing which may heat the cable insulation and any surrounding combustible materials to start a fire.

Cable manufacturers generally endeavour to manufacture electric cables which under the above situations, or in cases where a fire is started by another unrelated cause, will not burn or at least will not help spread a fire through the building.

Today there are various cable flame retardance test standards written by technical standards committees in Europe and USA. These common standards propose test methods intended to determine if the electric cables or materials they are made from are self-extinguishing (Flame Retardant). These test methods may also be embedded by Authorities into mandatory building design codes.

This article takes a look at the common test methods and questions if the test protocols employed do in fact provide the implied level of flame retardance performance when cables are installed and used in buildings.

Making flexible electric cables
Most common flexible cables are made from hydrocarbon based polymers. These base polymers are not usually flame retardant and have a high calorific value so chemicals are added to make them more suited to electrical cable use. Halogens like Chlorine are particularly good additives which help retard flame propagation and don’t significantly impact the dielectric properties of the polymer so Halogens can be used in both cable insulations and in cable sheaths. These halogenated polymers (example: PVC & CSP) also have a negative side effect that in fire they can release the halogens which are extremely toxic and when combined with the moisture in eyes, mouth and lungs are very irritant.

For cables which need to be ‘Halogen Free’ and ‘Flame Retardant’ other non-halogen flame retarding elements like alumina-trihydrate (ATH) can be used instead of Halogens, but while effective in retarding flame propagation these fillers often negatively affect the polymer by reducing dielectric performance or affecting mechanical and water resistance. For this reason additives like ATH are mostly used only in cable jackets. Halogen Free Flame Retardant cables most often use a more pure polymer like PE, XLPE or EPR for the insulation which has good dielectric and mechanical properties but may not be very flame retardant.

Electric Cables: Propagation performance in fire
Often the best flame retardant cables are halogenated because both the insulation and outer Jacket are flame retardant but when we need Halogen Free cables we find it is often only the outer jacket which is flame retardant and the inner insulation is not.

This has significance because while cables with a flame retardant outer jacket will often pass flame retardance tests with external flame, the same cables when subjected to high overload or prolonged short circuits have proved in university tests to be highly flammable and can even start a fire. This effect is known and published (8th International Conference on Insulated Power Cables (Jicable’11 – June 2011) held in Versailles, France) so it is perhaps surprising that there are no common test protocols for this seemingly common event and one cited by both authorities and media as cause of building fires.

Further, in Flame Retardant test methods such as IEC60332 parts 1 & 3 which employ an external flame source, the cable samples are not pre-conditioned to normal operating temperature but tested at room temperature. This oversight is important especially for power circuits because the temperature index of the cable (the temperature at which the cable material will self-support combustion in normal air) will be significantly affected by its starting temperature i.e.: The hotter the cable is, the more easily it will propagate fire.

It would seem that a need exists to re-evaluate current cable flame retardance test methods as these are commonly understood by consultants and consumers alike to provide a reliable indication of a cables ability to retard the propagation of fire.

If we can’t trust the Standards what do we do?
In the USA many building standards do not require halogen free cables. Certainly this is not because Americans are not wisely informed of the dangers; rather the approach taken is that: “It is better to have highly flame retardant cables which do not propagate fire than minimally flame retardant cables which may spread a fire” – (a small fire with some halogen may be better than a large fire without halogens). One of the best ways to make a cable insulation and cable jacket highly flame retardant is by using halogens.

Europe and many countries around the world adopt a different mentality: Halogen Free and Flame Retardant. Whilst this is an admirable mandate the reality is rather different: Flame propagation tests for cables as adopted in UK and Europe can arguably be said to be less stringent than some of the flame propagation tests for cables in USA leading to the conclusion that common tests in UK and Europe may simply be tests the cables can pass rather than tests the cables should pass.

Fire performance of Electric Cables_APF_Magazine

For most flexible polymeric cables the choice remains today between high flame propagation performance with halogens or reduced flame propagation performance without halogens.

Enclosing cables in steel conduit will reduce propagation at the point of fire but hydrocarbon based combustion gasses from decomposing polymers are likely propagate through the conduits to switchboards, distribution boards and junction boxes in other parts of the building. Any spark such as the opening or closing of circuit breakers, or contactors is likely to ignite the combustible gasses leading to explosion and spreading the fire to another location.

While MICC (Mineral Insulated Metal Sheathed) cables would provide a solution, there is often no singe perfect answer for every installation so designers need to evaluate the required performance on a “project-by-project” basis to decide which technology is optimal.

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