guide to fire resistant cable fixings · 18th edition wiring regulations and cable supports...

GUIDE TO FIRE RESISTANT CABLE FIXINGS

© MIDLAND FIXINGS LTD.


This guide is given as helpful information for specifiers and installers of electrical systems in the context of cable supports and fixings that satisfy the requirements of the 18th Edition Wiring Regulations.

18th Edition Wiring Regulations and Cable SupportsBS7671:2018 is the 18th edition of the IET wiring regulations and the British Standard covering the design, installation, testing and inspection of electrical installations. The 18th edition came into force from 1st January 2019 and applies to all fixed wiring installations designed from this date.

BackgroundIn response to a number of fatal, fire related incidents where persons, including fire fighters, have been entangled by falling cables, the 18th edition has introduced significant changes to the way that cables are fixed. The intent of this regulation is to prevent the possibility of cables collapsing prematurely in the event of a fire and becoming a hazard to evacuation and fire-fighting.

The RegulationChapter 52 Selection and Erection of Wiring Systems

Regulation 521.10.202 states ‘Wiring systems shall be supported such that they will not be liable to premature collapse in the event of a fire’ - the 18th edition introduction makes clear this now applies throughout the installation and not just in escape routes as in the previous edition.

The following notes to the regulation explain:

Note 2: Cables installed in or on steel cable containment systems are deemed to meet the requirements of this regulation.

Note 3: This regulation precludes, for example, the use of non-metallic cable clips or cable ties as the sole means of support where cables are clipped direct to exposed surfaces or suspended under cable tray, and the use of non-metallic cable trunking as the sole means of support of the cables therein.

Note 4: Suitably spaced steel or copper clips, saddles or ties are examples that will meet the requirements of this regulation.



How to Apply the Regulation in Practice

Are Plastics Banned?

Plastic cable fixings have not been banned, it is just that they can no longer be used as the sole method of supporting cables. Plastic cable ties, clips and cable supports must all be adequately supplemented with metal supports. The practical difficulty with this is deciding how often and where to position metal supports and how to supervise that adequate metal cable fixings are being used.

ADVICE: To be sure of compliance with the regulation it is advisable to use metal cable supports throughout the installation.

Selection of Suitable Cable Supports

The regulation cites cable supports made from steel or copper as examples of products that meet the requirements of the regulation. A copper fixing will melt at the same point as the copper cable while a steel fixing can survive to temperatures well in excess of this giving an additional margin of safety.

ADVICE: By selecting a steel cable support it provides a margin of safety in excess of the point at which the cable melts.

0

200

400

600

800

1000

1200

1400

1600

Melting Points of Wiring System Materials

Tem

pera

ture

°C

Rigid PVC Aluminium Copper Steel

1400

1084

660

170



Spacing of Cable Supports

Note 4 of regulation 521.10.202 which relates to the prevention of risks from cable entanglement in the event of fire states that cable supports should be ‘suitably spaced’

‘Suitably spaced’ should be assessed in the context of each installation and consider the sag effect on cables during fires to make sure they will not become a hazard.

In addition, other regulations within BS7671:2018 that address the suitable support of cables from the aspect of circuit integrity should be taken into consideration.

ADVICE: A suggested approach to deciding what ‘suitably spaced’ means:

1. Determine the correct cable support spacings for circuit integrity following the cable manufacturers recommendations and the IET ‘Selection and Erection Guidance Notes’ - These should be the maximum cable support spacings for the cables.

2. Then for the purpose of cable entanglement risks, ensure that suitable fire-resistant supports are placed at the following maximum centres:

Cables installed horizontally across doorways and windows: 250mm

Cables installed vertically: 400mm Cables installed to ceilings: 1000mm

These spacings should be reduced if necessary in consideration of the specific installation conditions and load carrying capacity of the supports.

Which Fixings? Having selected the appropriate fire-resistant supports for the installation it is essential that these are fixed using all-steel fixings. Plastic and nylon wall plugs and similar fixings cannot be relied upon to secure the installation from premature collapse in the event of fire.

ADVICE: Plastic and nylon wall plugs and all similar fixings should not be used for fixing fire-resistant cable supports. Use all-steel fixings such as masonry screws, steel expansion plugs and cordless nail fixings.



Product Selection Guide

Product Application Fixed With

Firesafe Tie Mounts Cables And Cable Bundles Cordless Nailing Tools

Metal Push Mounts Cables And Cable Bundles Push In - No Fixings Required

Steel Cable Ties Cables And Cable Bundles Tensioning Tool Recommended

Metal Cable Harness Cable Bundles Blue Screws/ Wall Screws/ Steel Plugs

Conduit Clips Cables in Plastic Conduit Blue Screws/ Wall Screws/ Steel Plugs

Steel Conduit Saddles Cables in Plastic Conduit Blue Screws/ Wall Screws/ Steel Plugs



Product Application Fixed With

U-Clips Cables in Plastic Trunking Blue Screws/ Wall Screws/ Steel Plugs

Steel-P-Clips Plastic conduit and single cables Cordless Nailing Tools

Firefix Clips Fire-rated Cables Cordless Nailing Tools

Stag Clips Single and Double Cables Push-in - No Fixings Required

Metal Cable Clips Twin & Earth Cables Traditional Cable Clip with Nail

Product Selection Guide

guide to fire resistant cable fixings · 18th edition wiring regulations and cable supports...

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