It is extremely important that engineers designing electronic and electrical systems are aware of the flammability, smoke generation and toxicity of cables that are being specified, particularly in systems where a fire could endanger personnel or damage critical control equipment: it really could be a matter of life and death!
Cables specifically designed for use in hazardous areas fall into clearly defined categories. In the event of fire, standard cables emit heavy smoke and toxic and corrosive gases that are a hazard to personnel and cause corrosion to expensive equipment. To reduce this hazard many cable manufacturers produce Low Smoke Zero Halogen (LSZH or LS0H) cables that increase the time for safe evacuation of the area and reduce corrosion damage.
Flame Retardant (FR) cables have much less chance of igniting when a flame is applied to the casing material and most importantly, the fire will not spread along its length. It is this resistance to fire propagation that is so critical in confined spaces where there are high-density cable runs. It is also possible to specify a fire resistant cable, such as Habiaflame2 from Habia Cable that not only resists burning and propagation, but will continue to operate under fire conditions.
Many types of cable are available in the categories mentioned above but it is vital to understand that compliance to one does not automatically mean compliance to the others. For example, some cable materials contain halogens to provide their flame retardant properties. These Low Fire Hazard (LFH) cables may be flame retardant and have low smoke generation but they will not be zero halogen.
Some materials offer low smoke and zero halogen properties, but without the flame retardant properties. These are commonly referred to as Low Smoke Fume (LSF) cables.
Other cable products use materials such as Habia Cable’s HFI 140 insulation and HFS 80 sheathing are able to comply with all categories. It is always advisable to specify the requirement for FR, LFH, LSF and/or LSZH to be sure of the right cable performance.
Setting the standard
There are a number of international standards that regulate flame retardant cables such as UL 94 – Flammability, a test that measures and grades the flammability of the jacket material but not the insulated wire or cable. IEC 60332-1 measures the flame retardant properties of a single, vertical insulated wire or cable where a pass or fail is decided by the distance over which the charred portion of the cable extends after fire conditions. IEC 60332-2 is similar to IEC 60332-1 but applies to small wires or cables while IEC 60332-3 tests bundles of wires and cables and is divided into a number of different categories, each covering a different volume of combustible material. A pass is awarded if the charred portion does not exceed 2.5m above the height of the burner. Finally, IEC 60331 measures the fire resisting properties of a cable and requires the cable to function normally for the duration of 120 minutes at 820oC.
FR cables are for use in applications where there is a high fire risk such as in an engine bay and in many types of vehicles as they slow the spread of the fire and give people vital extra time to escape.
Cables that are both FR and LSZH should be used in areas where large numbers need to be evacuated as the lack of smoke will not obscure exit signs and there are no toxic fumes that could cause lasting health issues. Perhaps the most well known example of this is underground transport systems where FR and LSZH cables have become essential in both infrastructure and rolling-stock following the King’s Cross fire.
Fire resistant cables are vital for use in safety equipment and can be found in many safety critical applications such as emergency lighting, sprinkler systems, fire alarms and emergency alarm and announcing equipment.
When personal safety and corporate responsibility is a key factor in product selection, the benefit of using FR and LSZH cables becomes clear. These products offer high performance that is certified to external test standards for ‘peace of mind’ use in a wide range of industries.