Chapter 52: wiring systemsWhy must cable supports not collapse in a fire?

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Chapter 52: wiring systems

First, what the book means by the term. Look it up in Part 2:

?A wiring system is an assembly of cables or busbars together with the parts that secure and, if necessary, enclose them.

So conduit, trunking, cable tray, ducting, busbar and power track are all wiring systems. A consumer unit is an enclosure, but it isn't a wiring system.

Table 4A1 — what you may install, and how

In Appendix 4, Table 4A1 lists cable types against installation methods and tells you which are permitted.

Cable	Clipped direct	Conduit / trunking	On insulators
Bare conductor	✗	✗	✓
Non-sheathed (single core)	✗	✓	✓
Multicore (e.g. twin & earth)	✓	✓	✓

✋The classic confusion. Twin & earth contains a bare CPC, so people conclude they're clipping a "bare conductor", which the table forbids. They aren't. That CPC is part of a sheathed multicore cable, and the multicore row is the one that applies. The bare-conductor row means a genuinely bare conductor on its own — overhead lines on insulators.

The same chapter gives you the installation reference methods you met in cable design: A, B, C and the numbered ones 100 to 103.

Part 2 · definition of a wiring system 521.4 · types of wiring system Table 4A1 · permitted installation methods Table 4A2 · installation reference methods

Premature collapse in the event of a fire

One of the most quoted regulations in recent editions, and it has a reason behind it.

§521.10.202 — wiring systems shall be supported so that they are not liable to premature collapse in the event of a fire.

Why it exists. In a fire, cables held only by plastic clips can come down when the plastic melts.

So plastic clips alone are not a suitable method. The cables need support that will not give way early in a fire.

Where	What you'll find
BS 7671 · 521.10.202	The requirement: support against premature collapse in a fire (cited in Guidance Note 3, p.47)
On-Site Guide · Appendix D	Actual spacing tables, by cable diameter, horizontal and vertical — in millimetres

A rule of thumb from the trade: clip spacing about elbow-to-wrist, or the width of your hammer. Fine on site, but the table is what's assessed.

521.10.202 · premature collapse in the event of a fire · cited in GN3 p.47 On-Site Guide · Appendix D · cable support spacings

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