Mythbusting: The use of flexible cables in fixed installations

Mythbusting: The use of flexible cables in fixed installations

In this article, originally written for the IET Wiring Matters publication, James Eade looks to dispel some of the myths surrounding the use of flexible cables in fixed installations.

The content for this piece was inspired by a comment that was posted on the IET EngX Forum which stated: ‘Flexible cables are not permitted in fixed installations’. This myth is not uncommon.

The origins of it are unclear but seem to date back to the 15th Edition of the IEE Wiring Regulations (if not before) and are believed to have stemmed from the types of insulation materials used for flexible cables in years gone by.

In the 16th Edition, there were definitions for flexible cables (for example, a cable designed to be flexed in use) and flexible cords (a flexible cable with conductor cross-sectional areas (csa) of 4 mm² or less), but no reference to them being specifically prohibited.

Like phase separation, the memories of some old requirements linger on. In the 16th Edition (including AM2, 2004), Regulation 521-01-04 did state that:

“A flexible cable or flexible cord shall be used for fixed wiring only where the relevant provisions of the regulations are met.”

This requirement does not come across in a negative fashion, giving the impression that flexible cables may be used as something of a last resort. Interestingly, this requirement still exists today in BS 7671:2018+A2:2022 in Regulation 521.9 as follows:

521.9 Use of flexible cables

521.9.1 A flexible cable shall be used for fixed wiring only where the relevant provisions of BS 7671 are met. Flexible cables used for fixed wiring shall be of the heavy duty type unless the risk of damage during installation and service, due to impact or other mechanical stresses, is low or has been minimized or protection against mechanical damage is provided.

NOTE: Descriptions of light, ordinary and heavy duty types are given in BS EN 50565-1.

Continuing with the current edition, Regulations 521.9.2 and 521.9.3 go on to require flexes to be used for connecting equipment that may be moved during use. The definition of flexible cords has also gone, leaving just flexible cables listed in Part 2. However, there is still no prohibition on their use.

The main differences between cables used for installations and flexible cables are the use of Class 5 finely stranded conductors rather than Class 1 (solid) or Class 2 (semi-stranded), coupled with differences in insulation materials to allow for the flexibility.

Such cables are in widespread use from construction sites to events (where they are used almost exclusively), as well, of course, as final connections in installations for lighting fixtures or from fused connection units to equipment such as heaters, for example.

Having decided to wire your installation in flexible cable, what are the “relevant provisions of BS 7671” alluded to in 521.9.1? As well as complying with the general rules, there are a couple of key points to note with this:

• The flexible cable should have appropriate mechanical robustness for the application as described in the note to the Regulation which refers to BS EN 50565-1. This Standard describes various categories, summarised thus:

1. “Heavy duty” describes the stresses that might be expected in, for example, industrial and agricultural premises.

2. “Ordinary duty” describes the stresses that might be expected in, for example, domestic, commercial and light industrial applications.

3. “Light duty” applications might include flexible cables for small appliances.

4. “Extra light duty” applications are, for example, very small appliances such as electric shavers, chargers for mobile phones, etc. Where this protection cannot be achieved using the cable alone, suitable additional mechanical protection should be provided.

● The temperature rating of the flexible cable should be suitable for the connected wiring accessories (Regulation 526.4).

● Wiring terminations may need to be finished in an appropriate termination as required of Regulation Group 526.9. IET Guidance Note 3 has details on appropriate wiring accessory terminals and their markings in Table 2.1.

● Generally, all the current-carrying capacity and voltage drop tables in Appendix 4 are for Class 1 and Class 2 stranding, with the exception of Tables 4F1 to 4F3. The current carrying capacity and voltage drop may need to be corrected if using tables for other cables, such as PVC cables complying with the insulation and temperature requirements of Table 4D2. Information is given in Section 2.4 of Appendix 4.

Far from being prohibited, flexible cables are required in some parts of BS 7671:2018+A2:2022 and examples include:

● Regulation 422.3.201 permits their use for locations where a particular risk of fire exists, although has some requirements for their construction or protection as detailed in Regulation Group 521.9.

● Regulation 413.3.4 has a particular requirement concerning their use in parts of the installation with protective measure double or reinforced insulation.

● Regulation 418.3.6 for electrical separation to more than one item of equipment requires flexible cables to incorporate a protective conductor.

● They are required to be used for suspended current-using equipment (Regulation 522.7.2).

● They are required in installations in flexible structures, or where structures are intended to be moved (Regulation 522.15.2).

● There are minimum cross-sectional areas for flexible cables “for any other application” in Table 52.3.

● Flexible cables are required for the connection of equipment that may be regularly or occasionally moved in use (Regulations 521.9.2 and 521.9.3).

● Flexible cables are required as a method of connection of equipment having a high protective conductor current as detailed in Regulation 543.7.1.202.

● Some Part 7 special locations specify certain wiring systems for which flexible cables are options, or mandated as noted in Regulation 522.15.2: (Regulation Group 704.522.8, Regulation 711.52, Regulation 717.411.3.1.2, Regulation Group 717.52, Regulation 721.521.2, Regulation 740.521.1, Regulation 740.55.1.1).

As ever, it is worth reviewing the fundamental principles in Part 1. Of relevance here are Regulations 132.6 and 132.7 for the cross-sectional area of conductors and types of wiring and installation methods, plus Regulation Group 133 for the selection of equipment.

Flexible cables can tick all the boxes and, if the extra cost over more rigid types is not a concern, there is no reason why they can’t be used.

With thanks to Graham Kenyon for his contributions to this article, originally published in May 2023 for IET Wiring Matters.

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