EuroWire September 2019

Technical Article

Behaviour of cables in a fire By Nagi Ahmed Abdulmagid, section technical manager, Alfanar Cables Company, Saudi Arabia

Halogen-free materials Halogen-free cable does not contain any of the chemical elements defined as a halogen. Materials used in wire and cable are divided into two categories: one category for materials that contain no halogens, and the other for those that include a high percentage of halogens, with the most common being chlorine, fluorine and bromine. The below table illustrates the halogen content in typical insulation and jacket materials:

After the King’s Cross fire on 18 th November 1987, it became mandatory to use LSHF materials on all London Underground cables. These LS0H cables, which emit non-toxic levels of halogen and low levels of smoke when exposed to fire, are now recommended for use in all highly populated enclosed public areas. LSHF properties are particularly important where smoke and toxic fumes may cause a threat to life. What are halogens? Halogens are non-metallic elements found in the periodic table. The five halogens are fluorine, chlorine, bromine, iodine and astatine. A common material that contains halogens in the chemical structure is PVC. Halogens make cable jackets and insulation highly flame retardant. Some naturally non-halogenated materials have halogen added because of the flame-retardant properties. When burned, halogens emit a thick toxic smoke and become corrosive. These characteristics can cause damage to equipment and pose a safety concern.

There is confusion about the terminology for halogen-free cable because of a lack of an agreed term. The most frequent identifications are found below:

Terminology

Abbreviation meaning Low Smoke, Zero Halogen Low Smoke, Zero Halogen Low Smoke, Halogen Free Low Smoke, Non-Halogen Non-Halogen, Flame Retardant Halogen Free, Flame Retardant Fire Retardant, Zero Halogen

Material

Halogen content weight (%)

LS0H

PE (Polyethylene)

<0.02 <0.02 <0.02 <0.02 13-26

LSZH

XLPE

LSHF

EPR

PU (Polyurethane)

LSNH

CSPE (Chorosulphonated Polyethylene)

NHFR

CPE (Chlorinated Polyethylene)

14-28

HFFR

PVC

22-29 62-78

FEP (Fluorinated Ethylene Propylene)

FRZH

Halogen-free cable tests Measurement of smoke density of cables IEC 61034-2 / EN 61034-2 Test set-up : A cable specimen is burnt in a closed chamber using a flammable liquid. The light transmittance of the resulting smoke is measured optically. Test apparatus according to IEC/EN 61034-1. Flame temperature : Determined by the quantity and composition of the fuel.

Zero halogen and low smoke

Alkalimetals Transitionmetals

Halogens Noblegases

Zero halogen and low smoke do not have the same meaning and cannot be used interchangeably. Zero halogen describes the amount of halogens used to make the compound. These cables may still emit a thick smoke when burned. Low smoke describes the amount of smoke that a compound emits when burned. The cables emit a thin and clear smoke when burning, which makes evacuation and fire fighting much easier and safer. Very often the end user is interested in cables containing both properties.

Halogen-free cables and their application Halogen-free cables and wires are applicable in delicate areas, for example: • Railway and subway stations and cars, buses and bus stations, aeroplanes and airports, and other mass transit facilities • Any public underground or poorly ventilated location • Public entertainment and sports facilities • Apartment buildings and hotels • Hospitals • Computer/data centres

Test duration : 40 minutes.

Compliance criterion : The smoke must transmit the light at the end of the test duration, as stated in individual specifications. The recommendation of light transmission is 60 per cent minimum.

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September 2019