Tony Mayo, Project Director at consulting engineers, Hilson Moran Partnership believes there is an aspect of fire-rated cable testing that needs to be re-visited to ensure cable integrity under fire conditions. He argues that it is a topic for serious debate.
Tony Mayo, Project Director at consulting engineers, Hilson Moran Partnership believes there is an aspect of fire-rated cable testing that needs to be re-visited to ensure cable integrity under fire conditions. He argues that it is a topic for serious debate.
The advent of buildings designed around fire engineered solutions rather than to prescribed standards means that, increasingly, more reliance is being placed on fire alarm and detection measures to ensure adequate protection of the building’s occupants and to safeguard the building itself. This move was no doubt fuelled by the enactment of the Regulatory Reform (Fire Safety) Order 2005 that, in addition to shifting the onus for fire safety onto the shoulders of building owners and occupiers, underlined the need to take what the Order describes as “reasonable” measures.
While we could spend many hours discussing precisely what constitutes a “reasonable” decision or action – surely it will vary from project to project – it is clear that the new fire safety landscape heralded in a greater awareness of the importance of system reliability. Traditionally, concerns over fire detection and system performance have centred around the dependability, accuracy and connectivity of the sensors and detectors, and the integrity of the system’s control panels. The increased importance of these systems has, however, also turned attention to the fire performance of the power and detection cables upon which the majority of these systems are totally dependant.
A CASE IN POINT.
This was acknowledged in Approved Document B (Fire safety) of the Building Regulations that came into effect in April 2007, where Clause 5.38 of Volume Two (buildings other than dwelling houses) requires that, under certain circumstances, in large or complex buildings there may be systems that need to operate for an extended period during a fire. These include: automatic fire suppression installations; fire detection and alarm systems; fire compartmentation; smoke control and ventilation; sprinklers and wet risers; ventilation and shutters; and firefighting lifts, all of which require a secure power supply that will retain its integrity in the event of fire.
The World Trade Centre horror will be an everlasting reminder that evacuation is a major issue in high-rise buildings. Even in buildings a fraction of the height of the twin towers, the critical importance of cables retaining their integrity for two hours cannot be overstressed.
Approved Document B states that guidance on the selection of cable for such systems is given in three British Standards. These are: BS 5839-1 (Fire detection and fire alarm systems for buildings. Code of practice for system design, installation, commissioning and maintenance); BS 5266-1 (Emergency lighting. Code of practice for the emergency lighting of premises); and BS 7346-6 (Components for smoke and heat control systems. Specifications for cable systems).
BS 7346-6 includes details of the flame irradiation exposure, direct impact and high pressure water spray tests. However, last year, these tests were published as a stand-alone Standard, as BS 8491 (Method for assessment of fire integrity of large diameter power cables for use as components for smoke and heat control systems and certain other active fire safety systems).
The tests in BS 8491 were a clear raising of the bar in terms of fire performance. The Standard calls for a fire test that incorporates 115 minutes of direct mechanical impact followed by five minutes of the application of water – in five-second bursts – at a pressure that equates to that of a fireman’s hose. It goes beyond the requirements of BS 5839, which originally defined the “standard” and “enhanced” categories of cables.
INTEGRATED TESTING PROTOCOLS.
The requirement for an integrated fire, impact and water spray test protocol was first enshrined in a British Standard way back in 1983 with the publishing of BS 6387 (Performance requirements for cables required to maintain circuit integrity under fire conditions). However, an actual sprinkler head was used in the test, which led to criticism that some of the water failed to make contact with the cable.
A similar test was adopted in BS 5839, via BS EN 50200 [Method of test for resistance to fire of unprotected small cables for use in emergency circuits] where “standard” cable, for example, is exposed for 15 minutes to fire and indirect mechanical shock, followed by a 15-minute fire, mechanical shock and water-drench test. This test though differs from the earlier BS 6387, as the water is applied via a purpose-designed “spray bar” directed at the cable at a rate that represents the output of a sprinkler head. The “enhanced” category test increased the test times to two 60-minute periods, which illustrates the significant difference between “standard” and “enhanced” cable performance under fire conditions.
Interestingly, the European standard, BS EN 50200 has two categories, PH30 and PH120, which correspond closely with the “standard” and “enhanced” categories in the UK. Significantly though, the European standard does not call for the application of water in Europe. However this element of the test has been incorporated into Annex E of the standard for applications within the UK.
TWO-HOUR TEST VALIDITY.
This prompts an important question that we ought to be debating. Perhaps the issue is not whether water application should be part of the test, but whether the protocol in the British Standards is representative of a fire condition. The grounds for questioning this is that the application of water will completely cool the cable in the first few minutes, so there is little prospect of failure from then on, with equally little value in continuing the test
What is really needed is a two-hour test that genuinely differentiates between the performance of various soft skin alarm cables. A more realistic test regime would be to burn the cable for the full two hours before applying water for just the last five minutes as the protocol adopted for the fire resistant power cables in BS 8491. This would impart the maximum damage to the cable before assessing its capability to remain watertight.
There is no disputing that this is promoting a very demanding protocol. However, there are at least five cable manufacturers that have third-party approved cables currently on the market that satisfy these requirements, and there are other manufacturers that are keen to follow suite to demonstrate the performance of their own cables. True, aside from the city-centre avant-garde architectural projects, it could be argued that there is not a great call for a “super enhanced” cable. Well, not yet perhaps; but that will quickly change if we experience a high-rise fire where a more reliable two-hour cable rating would have saved lives. So, perhaps it is time to get that debate started? What do you think?
AUTHOR:
Tony Mayo BSc Eng MCIBSE MIEE is Project Director with Hilson Moran, the consultancy responsible for many of Canary Wharf’s landmark buildings, the Swiss Re building and The Pinnacle in the City of London. He is a member of Hilson Moran’s Technical Committee, was a member of the BSI Committee FSH 25/3, is a member of FSH/1 and has been intimately involved in establishing higher standards for fire-rated cables for more than a decade. He can be contacted on +44 (0) 0 7940 8888, via email at [email protected]. The consultancy’s website is at www.hilsonmoran.com.