Good workmanship: maintaining bend radius for cable

[neverwas]

Numpty question: are there standard ways to maintain the minimum bend radius when cable turns from the vertical through a right angle to run along a flat surface (and vice versa) please?

I ask as I am tidying up after running fresh cable for some lights in lofts. There are various places where the cable runs vertically and then along rafters; and others where cable to a fitting runs to a junction box which is not screwed down[1]. In all these it seems to me that a badly placed box, foot, knee or whatever could bend the cable more tightly than allowed by Table 4E.

It occurred to me that I could guard against this by passing the cable round short lengths of 40mm[2] uPVC waste pipe. But I'm bothered 'cos (i) I've never seen that done; (ii) I can't find anything about how it should be done in practice; and (iii) none of the usual suspects seem to sell cable guides for mains cable like the ones I have seen/used to maintain the bend radius for fibre optic/network cables. So I suspect I'm missing something

[1] These are Ashley J501 junction boxes with strain relief designed so as to allow access from below which (I very much hope) don't need to be screwed down. [2] It's mostly 1.5mm with some older 1.0mm so this would do for most. For the 1.5mm T&3 for smoke/heat alarms I have some old 60mm downpipe.

[Graham Jones]

Nice to see someone who takes the time and effort to do things properly. The wiring for the upstairs lights in my loft is like a spiders web with wires and junction boxes lying on top of the insulation!

[Graham Jones]

and also all the cables in the junction boxes have their earth's taken outside the box and twisted together! So all earth's exposed and twisted! (There is a joke there somewhere).

[Tim W]

neverwas wibbled on Monday 14 December 2009 13:57

I don't think anyone worries too much, but it's nice that you care. The bending radius guides are there for a reason, namely that over bending a cable can cause the wires to cut through the insulation over time (cheesewire effect).

Looking at it a different way, the T+E is very good at maintaining its shape if supported with a modicum of clipping. Perhaps the issue to address is how to prevent a foot or a knee clouting it - as a well misplaced boot could do damage to the cable even on the flat. (Neighbour recently had this very problem - booted woodworm contractor trod on a cable and caused a short internally.)

JB's would be best screwed down, but there are plenty that aren't - reason being most of the older style don't have cable clamps, so strain on the conductors can be prevented by fixing the box and clipping the cable near the entry points.

That's one thing I like about the Ashley JBs - clamps on all the entries.

[Steve Walker]

Yeah, that guy did our loft too.

[Nutkey]

When I was doing the kitchen at my previous flat, I discovered a light had no earth. So I popped up to the loft, and found that the cable actually terminated in a chocbox, with a short extension to the actual light.

Yes, the chocbox was completely uninsulated. And submerged in insulation.

[js.b1]

For PVC FTE (grey 6242YH)...

- Do not kink it

- Do not twist it (unroll from the reel)

- Any twist-n-pull (CK) tool will strip it ok

For BS7211 (white) 6242BH 90oC XLPE LSOH...

- Do not pull a cable physically across another

- The sheath whilst hard will abrade themselves rapidly

- Sheath stripping requires a slice tool (very tough)

For FP200 & BS8436...

- Bend radius is critical here to avoid collapse

- General FP200 cables use silicone insulation

- Silicone insulation is a pain as it is easily nicked

- Prysmian FP200 Gold uses insudite insulation

- BS8436 & FP200 have quite high bend radius

For SWA...

- More tolerant of abuse

- Pipe cutter works well (there is a special tool)

- Leave bedding on insulated cores just beyond the gland

The "special tool" like a pipe cutter is about =A37 from Discount Electrical (no connection, just recall stumbling across it - beware postage is about =A37.50 whereas TLC is much cheaper for small orders).

For HiTuff or NYY-J...

- Bend radius is 12x Diameter (IIRC)

- Warm the cable before use

- Do not leave outside in cold as it goes completely rigid

All cables have a min install temperature (which also affects stripping too).

Most people will not buy BS7211 as is hard to work with and more expensive, but it is useful where insulation is concerned - for example in a loft or internally insulated walls and so on. The very high CCC is quoted at 90oC, you can only rely on that for running through insulation - not at wiring accessories (so no you can not do

2.5mm radials with lots of sockets protected by a 32A MCB/RCBO).

[neverwas]

thanks to all those who humoured me

I can do that! gizza' job!

(I am sadly very much from the Yosser Hughes school of DIY.)