Getting started with 18th edition compliant cable fixings: 5 things to consider

With construction being one of the deadliest industries out there, changes to regulations have been made in recent years in an attempt to reduce the number of deaths and injuries.

Following several fatalities involving people in rescue services becoming entangled in fallen cable work, the BS 7671:2018 18th edition of the Institution of Engineering and Technology (IET) wiring regulations came into force on January 1^st 2019. It applies to any and all fixed wire installations designed from that day on (but does not cover pipework).

Despite this, there still seems to be some confusion regarding the correct and compliant way to select cable supports (also called cable fixings). How can you, as an M&E contractor, apply the regulation in practice? If you share this confusion, hopefully, this article will help you overcome some common hurdles. It will discuss the following:

• Is plastic banned?
• What cable fixings are 18th edition compliant?
• What about when you’re using basket or tray?
• Cable fixings must be suitably spaced
• Do they need to be fire-tested?
• Conclusion

1. Is plastic banned?

A common misconception is that the 18^th edition has banned the use of plastic altogether for supporting cables. This is not true – however, they can’t be used as the sole method. There should be adequate support from metal fixings so that they would be able to handle the load alone, should the plastic ones fail. What makes this difficult in practice, though, is having to decide where and how often to position the metal supports and how much would be enough. To tackle this, you might as well use metal cable supports throughout the entire installation just to be sure that it will be compliant with the standard. Now, you may wonder what supports, in particular, are in line with the 18^th edition.

2. What cable fixings are 18^th edition compliant?

Any cable support made of steel or copper meets the requirements of the wiring regulations. Though, a copper fixing will melt at the same temperature as the copper cable. Therefore, choosing a steel cable fixing instead will provide an extra margin of safety, due to its ability to survive temperatures higher than that which would melt the copper cable. There are many 18^th edition compliant fixings to choose from depending on what’s needed for the application – to name a few, steel cable ties, fire-safe tie mounts, metal screw mounts, fixings bands, safe-d conduit clips, and many more. Once you’ve selected the appropriate cable fixings, keep in mind that the screws/anchors and expansion plugs you use also need to be fire-resistant i.e. all-steel anchors. Examples include masonry screws, steel expansion plugs and cordless nail fixings. Plastic and nylon wall plugs and similar anchors cannot be relied upon to stop the installation from collapsing prematurely in the event of a fire. Find out more about the different applications of cable fixings and when to use each type here.

3. What cable fixings are necessary when you’re already using cable containment systems like basket or tray?

A frequently asked question from customers is “do I still need to use metal cable ties if the cables are already being contained within a basket or tray?”. The answer may surprise you (not) – it depends! As we know, nothing’s ever straightforward in construction. According to the regulations, Note 2 says “Cables installed in or on steel cable containment systems are deemed to meet the requirements of this regulation”. Meaning, that in most cases using cable tray and/or basket is more than enough metallic cable support and it wouldn’t be necessary to use metal fixings. However, in certain circumstances, we do see cables being installed on the underside of the cable tray/basket by virtue of speed or convenience. In such instances, you would definitely require metal fixings. This is where Note 3 of the regulation comes in: “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.” In other words, you shouldn’t use plastic cable clips/ties unless you know the cable is being sufficiently supported by metallic containment. If it is, i.e. it’s sitting in the basket, then there’s no harm in using plastic clips such as CT clips, nylon cable ties or cable tie fixings because it wouldn’t sag down even if the clip/tie melts in the event of a fire. If not, you’d need something like steel cable ties, metal cable clips, Firefix clips, etc.

4. Spacing of cable supports should be suitable

Regulation 521.10.202, Note 4 states that cable supports should be ‘suitably spaced’. Way to be vague, right? Fret not – there’s a reason it’s not super-specific. The suitability is determined by assessing the context of each installation and considering the sag effect on cables and during fires to make sure they will not become a hazard.

You can do this by following the cable manufacturers’ recommendations and the IET ‘Selection and Erection Guidance Notes’ (these should be the maximum cable support spacings). And, to avoid the risk of cable entanglement, ensure that suitable fire-resistant supports are placed at the suitable maximum centres. For example, cables installed horizontally across doorways and windows should have spacings of 250mm, cables installed vertically; 400mm, cables installed to ceilings; 1000mm. But as I said, adjust according to your installation. These spacings should be reduced if necessary in consideration of the specific installation conditions and load-carrying capacity of the supports.

5. Should cable fixings and supports be fire-tested?

In relation to the 18th edition, there are no specific demands for fire-testing products. However, regulations do tend to raise more questions than answers. Fire-testing is beyond what the regulations call for, however, while the sole fact that a fixing is made of metal means it’s technically compliant, the question of ‘will it work in action?’ still stands. As a result, it makes sense to undergo fire-testing from time to time. In terms of testing guidance, there is advice with regards to safety-critical cables, such as for fire alarms, however, this testing guidance doesn’t consider application-specific testing. An example of the latter would be fire-safe tie mounts tested in conjunction with the substrate i.e. concrete, the fixing that anchors it to the substrate, and the cable tie.

6. Conclusion

Nothing is always as simple as it seems, and implementing regulations in your installations to become more compliant can get quite confusing – especially when different sources of information tell you different things. Nevertheless, I hope you’ve now gained a better, more comprehensive understanding of the BS 7671:2018 18th edition wire regulations, how to apply it and compliantly select the correct fixings for cable-related applications. In most cases, it’s usually safer to use metal over plastic, but as we established, sometimes it’s just redundant and you could save yourself costs and time just by thinking of these things prior to execution.