Tray bien

Cables are vital links in a building’s safety critical systems during a fire. They supply power to emergency lighting, exits and smoke ventilation equipment; they also carry data between fire alarm panels and detectors, call points and sounders.

But cable containment systems play an equally important role in maintaining pathways for power and data in an emergency. This was acknowledged in July 2003 when BSI published BS EN 5839, an updated version of the UK’s code of practice for fire alarm installation. This refers to the importance of specifying fire-resistant cable systems, a term that includes the cabling, tray, brackets and other supporting accessories.

Since the publication of BS EN 5839 there has been plenty of talk about the fire resistance of cables, but surprisingly little about the crucial role of cable containment. How often have fire-rated cables been specified, but contained in products that collapse, burn or emit poisonous gases and smoke?

The standards

“Don’t be confused by the terminology,” says Matthew Way, managing director of Cablofil UK. “There’s a world of difference between low smoke and fume and non-combustible cable management products. Cable management should remain mechanically stable in a fire, supporting the fire-rated cables.”

Specifiers or contractors should choose cabling and containment products that will let safety critical systems operate for as long as possible in a fire, giving occupants time to escape. Their first step should be to find products that comply with appropriate standards. Sadly, if they look for a standard that covers the fire resistance of cable trays, they will be out of luck. There are no European standards for fire resistance of cable trays. So what should specifiers do?

“Fortunately, there is a standard that covers the combined fire performance of cables and their support systems – it comes from Germany,” explains Way. DIN 4102: Fire behaviour of building materials and building components sets requirements for materials and components used in buildings to guarantee the best possible security in case of a fire.

Part 12 of the standard – Circuit integrity maintenance of electric cable systems; requirements and testing – is designed to ensure the reliability in fires of cable systems with a nominal voltage up to 1 kV. It also sets out tests for combinations of cables, trays and accessories to ensure that safety critical systems will continue to operate for 30, 60 or 90 minutes after a fire starts.

Testing systems

As part of its ongoing commitment to testing its products’ performance, Cablofil has taken the unusual step of having its steel wire mesh cable tray tested to DIN 4102-12.

The company turned to the Institut für Baustoffe, Massivbau & Brandschutz (IBMB) at Braunschweig in Germany to carry out the tests. “IBMB has an enviable reputation in fire protection, disaster control and smoke propagation research. Its laboratories are certified to perform tests and issue certificates to DIN 4102,” says Way.

IBMB does not certify the cable tray alone, but a combination of tray, accessories and cables. It tested Cablofil tray with cables from Eupen, Dätwyler and Alcatel. Typically, for each type of cable two pairs were installed in the tray: one with the smallest and one with the largest diameter of cable.

Technicians at the laboratory carried out three sets of tests to Part 12 of DIN 4102. The standard says that the cables and trays under test must be at least three metres long; all measurements must take place in an oven that is at least two metres wide, three metres long and 2·5 metres high. Once the cable system is installed in the oven the temperature is increased to 1000ºC.

Cablofil’s CF 54 tray was used in all the tests. CF 54 is a wire mesh tray that is 54 mm deep and available in widths from 50 to 600 mm.

In the first set of tests, cables were fitted in CF 54 that was 100-400 mm wide, mounted on Cablofil’s 41 x 41 heavy-duty struts and brackets. The strut is mounted vertically and the horizontal bracket extends to one side and supports the tray. The brackets were wider than the tray to accept a threaded rod for extra support. Use of any other accessories would have required separate tests. The cables and trays spanned 1·25 m, and the load on the tray could not exceed 10 kg/m.

Technicians then increased the temperature of the oven in line with the standard, ensuring at all times that the cables and cable containment were performing properly.

A further set of tests followed using CF 54 tray in 100 and 200 mm widths. This time the tray and its contents were supported by light-duty CSN brackets. These are L-shaped, with the vertical section attached to a wall and the horizontal section supporting the tray; again, a threaded rod provided extra support. For these tests, the span was reduced to 800 mm, as defined in the standard.

A final set of tests was conducted with CF 54 tray supported by ceiling-mounted CSNC light-duty brackets and threaded rods.

The test results

At the end of the tests the fire-rated cables had charred and the tray distorted. But the wire mesh tray and its accessories still supported the cables, and the installation survived the temperatures. As a result, the Cablofil wire mesh tray is now certified to DIN 4102-12.

“With cables and containment intact, critical building services such as fire alarms and emergency lighting and ventilation would still operate, alerting the fire brigade and giving the occupants a good chance to escape,” says Way.

“When you next specify cable containment products, remember that the latest codes of practice – including BS EN 5839 – make it your responsibility to assess the fire risks and choose products that will not compromise the performance of safety critical systems in the event of a fire,” he adds.

Also, remember that cable containment products, not just cabling, play a vital safety role in any installation.