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Topic Title: Cable Routing and safe zones
Topic Summary: Mechanical Protection and Accessory safe zones
Created On: 08 October 2017 09:37 PM
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 08 October 2017 09:37 PM
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sdltn

Posts: 7
Joined: 12 September 2017

Hi,

I'm re-routing my cables to where my consumer unit will be installed as the previous wiring when I moved in was a mess and downright unsafe for so many reasons I won't go into now.

Due to a floor joist butting up against the wall it's not as simple as I'd like to make it all flush and keep it inside the walls. To run everything in the 150mm safe zones in the corners I'd have to build something proud of the wall which I'd rather avoid so I have a few queries about mechanical protection and accessory safe zones if you could kindly assist.

Option 1 - Mechanical Protection
Am I correct in thinking galvanised metal conduit (See ScrewFix item 7442J for example, as can't post links it seems) classes as sufficient mechanical protection? From what I've read of the regs it should meet the criteria. Obviously this would be earthed.

Additionally, if this is okay for protection am I right to think I can do a diagonal run as this would be preferable for me to ease routing rather than having to have a change of direction which would likely require an access point should I need to re-pull cable in the future.

Or alternatively.

Option 2 - Accessory safe zones
If I run regular conduit and put an accessory box or larger inspection panel in line with it to provide an accessory safe zone is that still okay even if the accessory box/inspection panel is behind a kitchen unit;

Basically does the accessory box have to readily visible to negate the need for mechanical protection?

Hope I've made sense,

Thanks,

Sam
 09 October 2017 10:00 AM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

"Mechanical" protection is a difficult one - although some manufacturers suggest that 3mm of steel is sufficient, official guidance is surprisingly sparse (i.e. non-existent) and in these days of nail guns it's hard to be sure you're complied.

Earthed steel conduit is usually used under a different heading - concentric c.p.c. - where the live conductors are surrounded by earthed metal. The point of this approach is that it doesn't depend on physically stopping damage to the cable, but if it does happen the screw, nail, drill bit or whatever, will cause a short to earth and trigger an automatic disconnection of supply (in just the same way as a class 1 appliance). You can use earthed steel conduit for that approach, or earthed steel trunking, or even cables with an earthed armour or even aluminium screen as long as, in all cases, it's suitable for use as a protective conductor (which unfortunately rules out flexible or pliable steel conduit).

Earthing flush steel conduit can be tricky, especially if you don't have suitable tools for threading it and so on. You can't just put on a typical earthing clamp and bury it under plaster for example (as the clamp would need to remain accessible for inspection etc).

A simple alternative is BS 8436 cable (e.g. "Earth Shield") - specifically designed for situations where it might be pierced by nails and the like, it has a built in foil shield that's in contact with the c.p.c. - and it's as easy to work with as T&E (at most you might want a new tool to strip the sheath).

- Andy.
 09 October 2017 11:42 AM
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Alcomax

Posts: 231
Joined: 12 November 2009

A simple alternative is BS 8436 cable (e.g. "Earth Shield") - specifically designed for situations where it might be pierced by nails and the like, it has a built in foil shield that's in contact with the c.p.c. - and it's as easy to work with as T&E (at most you might want a new tool to strip the sheath).


I have never been happy with wording or inference of 522.6.202 [ii], in particular where it can lead in respect of 522.6.204 [ i ].

It seems to be at odds with 522.6.1, in particular installing a cable at a depth <50mm where it would not expected to be, that is, not in traditional "safe zones" or whatever you want to call them.

If we go back to earlier editions of BS7671 [ 2001 amd 2004 ], taking such a route as possibly suggested by 522.6.204 [ in later versions 17th etc], this decision was qualified by asking if the alternative to using such a method of install was "impracticable".

There is a logic.

Cables <50mm deep in places in the wall where they maybe expected to be [ "safe zones " or whatever ] can be soft skinned with no concentric earthed outer covering just laid direct in plaster. Of course for the additional protection there is 30mA RCD or you could use a cable with fail-safe earth fault protection such as BS8483 [ with a B type MCB ]. Whichever option, the wiring system selected and erected in such a way minimizes the likely hood of damage due to impact , satisfying 522.06.01, by virtue of not being installed in an unusual or unexpected route.

Somewhere between the 16th and 17th the judgment of "impracticable" [ as stated in BS7671: 2001 reg 522.06.07 ] has gone. Was this deliberate or an error?
 09 October 2017 11:47 AM
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ArduinoXR

Posts: 34
Joined: 16 August 2017

Originally posted by: AJJewsbury

"Mechanical" protection is a difficult one - although some manufacturers suggest that 3mm of steel is sufficient, official guidance is surprisingly sparse (i.e. non-existent) and in these days of nail guns it's hard to be sure you're complied.



Earthed steel conduit is usually used under a different heading - concentric c.p.c. - where the live conductors are surrounded by earthed metal. The point of this approach is that it doesn't depend on physically stopping damage to the cable, but if it does happen the screw, nail, drill bit or whatever, will cause a short to earth and trigger an automatic disconnection of supply (in just the same way as a class 1 appliance). You can use earthed steel conduit for that approach, or earthed steel trunking, or even cables with an earthed armour or even aluminium screen as long as, in all cases, it's suitable for use as a protective conductor (which unfortunately rules out flexible or pliable steel conduit).



Earthing flush steel conduit can be tricky, especially if you don't have suitable tools for threading it and so on. You can't just put on a typical earthing clamp and bury it under plaster for example (as the clamp would need to remain accessible for inspection etc).



A simple alternative is BS 8436 cable (e.g. "Earth Shield") - specifically designed for situations where it might be pierced by nails and the like, it has a built in foil shield that's in contact with the c.p.c. - and it's as easy to work with as T&E (at most you might want a new tool to strip the sheath).



- Andy.


Just to note, I would be wary of these cables for anything larger than say a 4mm^2, as I have only seen annealed copper versions, and I would not recommend annealed copper for the likes of a shower or cooker.
 09 October 2017 02:15 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

and I would not recommend annealed copper for the likes of a shower or cooker.

What's wrong with annealed copper? Is that what's in most cables?

You do touch on a problem though that BS 8436 only specifies sizes up to 4mm2 - larger sizes are made and comply with the equivalent Irish standard (IS 273) so technically can still be used in the UK, but it is a bit messy paperwork wise for larger circuits.

- Andy.
 09 October 2017 02:44 PM
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ArduinoXR

Posts: 34
Joined: 16 August 2017

Haha sorry, I'm using Monday as an excuse, I meant stranded.
 09 October 2017 04:26 PM
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AJJewsbury

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Haha sorry, I'm using Monday as an excuse, I meant stranded.

OK, so what's wrong with stranded? Most mains cable have stranded conductors in almost all sizes (from singles for conduit to SWA) - of the common types only T&E and similar has solid conductors and that's only for 2.5mm2 and below.
- Andy.
 09 October 2017 05:17 PM
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ArduinoXR

Posts: 34
Joined: 16 August 2017

Originally posted by: AJJewsbury

Haha sorry, I'm using Monday as an excuse, I meant stranded.


OK, so what's wrong with stranded? Most mains cable have stranded conductors in almost all sizes (from singles for conduit to SWA) - of the common types only T&E and similar has solid conductors and that's only for 2.5mm2 and below.

- Andy.


Stranded does not suit applications where corrosive elements are present and where there is the possibility of vibration as well as solid. Higher probability of poorer contact and thus CSA. In my specs I always outline anything above 4sq must be solid copper. In fariness I'm mainly concerned with industrial settings.
 09 October 2017 11:50 PM
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mapj1

Posts: 9371
Joined: 22 July 2004

I'd not really agree with that last bit. Stranded wire is more flexible, and less likely to fatigue through vibration. As one who has nursed a design through Def Stan 35 tracked vehicle requirements, which are certainly, shall we say, testing in terms of shake rattle and roll, I'd be in favour of stranded cables every time. Screw terminals become impossible though, and we end up using [s=norlocks] very liberally to get the design to pass.

-------------------------
regards Mike
 10 October 2017 06:26 AM
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leckie

Posts: 4323
Joined: 21 November 2008

Solid core cables! Well good luck finding that in 6943, 6491X or 6242Y for sizes above 2.5. In my data sheets I can only find solid listed in 6491X up to 4mm, and that would be a special order.

The OP is talking about a shower - I think stranded soft annealed copper will be fine!

I cannot actually think of a situation where solid core for a copper cable would be preferable to stranded? We don't install cable core connection in damp situations do we? I think we would enclose and protect them against damp, etc. I have certainly never seen a requirement for solid core conductors for a copper cable in preference to stranded; but I have seen many the other way around.

However, every day is a learning day and I am sure there will be some reason/situation where solid cores may be better. If this information is forthcoming, can we also have a link to where this cable is listed?
 11 October 2017 09:54 PM
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NE1

Posts: 24
Joined: 07 March 2017

Deck trying to bend 6mm or 10mm solid copper cable!!!! Bad enough stranded
 11 October 2017 09:55 PM
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NE1

Posts: 24
Joined: 07 March 2017

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