Technical

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44 Supports for cables in escape routes 49 Changes to the recommendations for fire detectors 52 An introduction to the requirements for auxiliary circuits 57 Changes to electrical installation certificates 60 Limiting values of measured earth fault loop impedance 63 Fire protection for electrical equipment in lofts 66 Snags and solutions: A practical guide to everyday electrical problems, now updated to Amendment No 3 of BS 7671

Supports for cables in escape routes

There have been a number of instances at times of fire where firefighters have been placed at risk, injured or killed due, in part, to entanglement in cables that have fallen down causing an obstruction. For example, the investigation carried out by Hampshire Fire and Rescue Service into the deaths of two firefighters in 2010, identified one of the factors that led to their deaths as a failure of cable supports. One specific conclusion in the report states: ‘Surface mounted electrical cabling was encased in plastic trunking which failed when exposed to heat, so releasing the cables.’

Plastic trunking melted in a fire and released cables that obstructed escape

Evacuating a building in the event of fi re should be an unhindered transition; however, it is self-evident that with increasing fl oor area and/or number of fl oor levels, a longer evacuation route will generally be created and the potential for hazards along that route increased.

Experience has shown that even with adequate evacuation planning (including rehearsal), hazards may be present in an escape route at times of fi re, and regardless of the familiarity a building user might have with the particular escape route and building, panic can still occur during the evacuation process, such as, for example, from an unexpected obstacle, such as cables which are dangling and/or have fallen to the fl oor in the escape route following heat from fi re.

In addition to fl ame, fi re produces other hot products, such as smoke and gases which will generally rise and circulate in the building and the

Fig 1 Example of cables within plastic

trunking with metallic supports

increased temperature might, for example, subject non-metallic support systems (such as trunking) mounted at high level to a degree of deformation, resulting in sagging and the lid becoming dislodged leading to cables hanging down.

In the light of such risks, for many years both BS 5839-1 (Fire detection and fi re alarm systems for buildings – Part 1: Code of practice for design, installation, commissioning and maintenance of systems in non-domestic premises) and BS 5266-1 (Emergency lighting – Part 1: Code of practice for the emergency escape lighting of premises) have recommended the use of metallic cable support systems, clauses 26.2(f) and 8.2.3 refer respectively.

These clauses are predominately concerned with maintaining circuit integrity of these important safety services; however, it should be noted that Note 9 to clause 26.2 of BS 5839-1 mentions that cables supported only by plastic cable trunking can create an entanglement hazard to fi refi ghters.

Until the publication of Amendment 3 to BS 7671, there was no similar requirement to support cables in escape routes with metallic cable support systems. However, in response to the recognised risks associated with entanglement, Amendment 3 to BS 7671: 2008 has introduced Regulation 521.11.201 which aims to prevent wiring systems in escape routes becoming an entanglement hazard to building users evacuating a building and fi refi ghters entering a building. 1

Requirements of BS 7671

The requirements of BS 7671 apply to all wiring systems typically used in and about a building, including systems used for: • low voltage distribution circuits and fi nal circuits, • safety services (such as emergency lighting and fi re detection), • data, voice and television.

Regulation 521.11.201 requires all wiring systems in escape routes to be supported such that they will not be liable to premature collapse at times of fi re. Consequently, cable supports will need to have a high melting point, such as steel or copper (melting points in the range of 1450 ºC and 1050 ºC respectively).

This eff ectively prohibits cables in escape routes being supported solely by materials having a low melting point, such as non-metallic clips, ties, conduit and trunking, unless the cables are given additional fi re-resistant means of support/retention. For example, fi xing metallic ‘P’ clips to the structure for security of cables within non-metallic trunking (see Fig 1).

It should be noted that in an escape route the sole use of certain metallic support systems, such as a cable basket or cable tray may not ensure Regulation 521.11.201 is met. Metallic ties may need to be used (where appropriate) to secure cables to the support, to avoid, at times of fi re, non-metallic supports disintegrating and subsequent dangling cables (see Fig 2).

Also, Section 422 (Precautions where particular risks of fi re exist) of BS 7671 should not be overlooked. This Section includes certain requirements regarding the fl ammability and fi re resistance of wiring systems and therefore places implications on the supports for the wiring system.

Escape routes; a brief overview

Although electrical contractors are familiar with working in and about buildings, they may be unfamiliar with the term ‘escape route’ when applied to a building and therefore unsure of the application of Regulation 521.11.201.

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Fig 2 Example of cables dangling where

plastic supports have disintegrated

BS 7671 defi nes an escape route as ‘a path to follow for access to a safe area in the event of an emergency’, and such routes in buildings are routes intended to provide a safe unobstructed means of escape from any location in a building to a place of safety, typically outside the building. In non-domestic buildings, escape routes are normally identifi ed as part of the design process and are generally apparent by the siting of escape route direction signs and emergency exit signs in and around the building.

Contractors should beware that an absence of signage does not necessarily mean that a location is not an escape route. For example, signage is typically not installed in domestic premises; however, these premises will be constructed having a means of escape in accordance with the requirements of the building regulations in the United Kingdom (UK) jurisdictions, namely, England, Wales, Scotland and Northern Ireland.

Determining an escape route in domestic premises, like any building, may at times appear complex, but as a minimum it should be assumed that the hall, stairway and landing form part of an escape route. Information issued by governments for the UK jurisdictions in the form of Approved Document B – Fire safety (England and Wales), the Technical Handbook – Fire (Scotland) and Technical Booklet E – Fire safety (Northern Ireland) provides guidance on the means of escape.

Alternatively, where contractors are unsure whether their particular working location is an escape route, they could seek guidance from: • in non-domestic premises the person responsible for safety, or • in domestic premises the local Building

Control Body.

Also, for an escape route where contractors carrying out periodic inspection and testing observe cables supported solely by non-metallic cable supports, a Classifi cation code C3 (Improvement recommended) should generally be recorded on an Electrical Installation Condition Report.

1 Electrical Safety First issued a Safety Bulletin on supporting heavy cables in Switched On

Edition 33 Summer 2014