FIRE SAFETY Technical commentary
Comment from Al Rufaie,
B.Sc. CEng. MIET, FIHEEM, medical electrical consultant (and former head of electrical engineering guidance for Department of Health Estates 1985 – 1997), a member of Panel D (Medical Locations Expert) of the Joint IET/BSI Technical Committee JPEL/64 responsible for the publication and maintenance of BS 7671 (Requirements for Electrical Installations – the IET Wiring Regulations), and the author of IET Guidance Note 7/ Chapter 9 Medical Locations, published in November 2015. Al Rufaie is also involved with IEC and CENELEC Technical Committees.
1 Part B Approved Document of the Building Regulations (Fire Safety) – Applicable in England “Although I am not qualified to fully comment on the above, I have undertaken some research on this topic. Part B contains two volumes, both of which came into effect in 2007, with some Amendments, published in 2010 and 2013. They reference some 45 entries of various BS EN and BS ISO standards, including BS 476. BS 476: Fire Tests on Building Materials and Structures, contains 17 parts. Only part BS 476-4: ‘Non Combustibility Tests for Materials’, dates from 1970; the remaining 16 parts date from 1987–2009.
“On reflection, I am not at all sure that the fire test methods used date back to 100 years, as asserted. Even if they do, the parameters (limits on temperature / time) could well have changed. Having said this, it is well reported after the recent Grenfell Tower fire that some of the fire tests carried out on particular cladding materials were not to the latest British Standards.
2 Comparisons of fire-resistant cables, polymeric or mineral insulated copper clad (MICC) “As far as (2) above is concerned, I would like to first explain the terms ‘Flame Retardant Cables’ and ‘Fire Resistance Cables’. Flame Retardant Cables are designed to restrict the spread of a fire by restraining combustion, but functionality can be impaired in the presence of a fire for a specified time and temperature. In the UK these cables generally comply with BS EN 60332-22 and -23 and -24, Tests on electric and optical cables under fire conditions. Vertical flame spread of vertically-mounted bunched cables.
“Fire-resistant cables are designed to continue their full operation (Circuit Integrity) without loss of functionality for a specified time and temperature. In the UK polymeric cables generally comply with BS EN 60331-1 and -2 and -3, Tests for electric cables under fire conditions. Circuit integrity. Initially the main designs were based on copper conductors wrapped with mica tape(s) and cross-linked
and samples are subjected to the fire time/ temperature protocol of ASTM E-119-75, which is virtually identical to ISO 834-1/ EN1363-1. During testing the cables, fixings, and supports experience significant mechanical stresses caused by expansion and contraction. After two hours at a final temperature of 1,020˚C the cables are immediately subjected to a fire hose stream test, which not only imparts huge thermal stresses on the wiring system, but also significant mechanical stresses. All five samples
polyethylene (XLPE) insulation. The second generation was based on silicone-rubber-insulated conductors. This material has the property of forming a ceramic shield when burning, which maintains high electrical resistance. It is the most common solution for building applications.
Time/temperature parameters
“The test time/temperature parameters are of the order of 90 minutes/850˚C. They are mainly used in providing power for critical systems (safety circuits) in buildings, such as fireman’s lifts, emergency fire pumps, emergency alarms, and voice-alarm communications etc. MICC cables are also fire-resistant, but tested to a higher circuit integrity of time/temperature parameters (up-to 180 minutes/950˚C) than for polymeric cables. In the UK the recognised standard is BS 6207, Specification for mineral-insulated cables. Copper-sheathed cables with copper conductors. They are mainly used in highly flammable atmospheres such as the petrochemical and gas industries, power stations, process industries handling flammable fluids, and nuclear reactor instrumentation etc. – in other words, mainly in industrial applications.
“There are advantages and disadvantages in the choice of polymeric or MICC cables, related to the intended application. Polymeric cables are less expensive than MICC cables, which additionally require greater specialist skill to install. The UK standards shadow the IEC and CENELEC equivalents, which are recognised throughout the world and, in the latter case, within Europe.
“Richard Hosier’s article suggests we should use cables as specified by the American Underwriters Laboratories (UL) 2196 standard for MICC fire-resistant cables for multi-rise accommodation and other buildings. This standard adopts a test protocol to a maximum of 120 minutes/1020˚C, somewhat similar to BS 6207.
‘Horses for courses’ principle
“Designers in the UK always follow the principle of ‘horses for courses’, by specifying the required fire-resistant cables for the intended use as directed by the Building Regulations, the various codes of practice, and BS 7671, The IET Wiring Regulations. The same principle is adopted in the design of healthcare premises. HTM 06-01, Electrical Services Supply and Distribution, refers to the use of ‘Safety Circuits’, as indicated in BS 7671. As an example, a designer might specify MICC cables for the instrumentation and control in an ‘Energy Centre’ of a hospital, but polymeric types for other safety circuits, i.e. a risk assessment is called for. To suggest that MICC cables should be used instead of polymeric cables for fire-resistant applications everywhere in a public building is, in my view, slightly overstating things.”
must survive in working condition, and certification is given independently for horizontal and/or vertical mounting.
A more ‘real-world’ testing environment
It is well established that fire testing the electrical integrity of cables in full scale is significantly more demanding and representative than testing only short lengths of horizontally mounted circuits, because the sheath and insulation of flexible polymeric cables will burn away
in fire so that the cable supports holding the cables in vertical installations can no longer support them. Bare mineral insulated cables do not have this problem. While the time/temperature regimes of ASTM E 119-75 (as used in UL2196) and ISO834-1 (as used in BS476 pts 20-24) may not be fully representative for all built environments today where modern building materials with high calorific values are used, nor for the potentially higher/faster time temperature profiles of fires in ‘areas of special risk’, the
August 2018 Health Estate Journal 29
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