Mechanical protection

[benstoneham]

I've got to run a section of 2.5mm final ring chased into a wall that (as a result of a window and expensive coving) can't be run in safe zones (also not possible to run under floor).

The run is between 2 sockets on the 2 adjacent walls.

2 obvious choices are:

1. Add an additoinal socket to the intermediary wall which means I can then run horizontally between the 3 sockets.. but this may not be asthetically OK... or

2. mechanically protect and earth the intermediary run.

...but the only problem is that even 20mm steel tube would exceed the 1/6 rule that BC mandate for horizontal runs (even with plaster)..

The alternative is to use something like http://uk.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=0615602

but this is expensive...and although it looks like it would easily meet the mech protection requirement, I'm unsure what BC woudl say.... opinions, advice (tell me that I've got it all wrong.. again!) welcome.[/url]

 

[Norv]

Safe zones are also 150mm from top & ends (corners) of walls. Cant you use these areas somehow ? Is the cable horizontal or vertical to the 1st socket ? You could come back along this route & around to the other socket

 

[benstoneham]

I wracked my brains.. but no I can't see how.. the first run is vertically down to the socket..

fromt the 1st socket to the next would have to run horizontally via the wall in question to the second socket on the adjacent wall (which is opposite the 1st.. if that makes sense!).

The windows in the minddle of the middle wall prevents use of the horizontal safe zone... while the coving prevents use of the 150mm area between ceiling and wall....

Under the floor isn't possible because I can't lift the 'floating' wooden floor to get to the joists.. the floor void is substantial beneath the joists but with no way of gaining access to clip, this isn't an option either..

 

[JohnD]

1. Add an additoinal socket to the intermediary wall which means I can then run horizontally between the 3 sockets.

do that then

 

[benstoneham]

Occam's razor rules again!

 

[skenk]

2. mechanically protect and earth the intermediary run.

Still needs to be in a safe zone if it's hidden in the wall afaik

 

[benstoneham]

Still needs to be in a safe zone if it's hidden in the wall afaik

But if you think about it, that doesn't make sense.. there would be no reason to mechanically protect in that case..

 

[JohnD]

it's not quite "hidden" if it runs directly horizontally between two sockets.

You should see my kitchen wall...

 

[Spark123]

Earthed metallic conduit which is suitable for use as a CPC is suitable for use outside of the safe zones, adaptflex is not suitable for use for this.

 

[benstoneham]

Why is that spark123? because it is not categorised as metalic conduit? or because if can't be earthed suitably? or something else?

 

[skenk]

it's not quite "hidden" if it runs directly horizontally between two sockets.

horizontally from a socket is a safe zone . . .

didn't know that about earthed metallic conduit, thanks 123.

 

[Spark123]

Why is that spark123? because it is not categorised as metalic conduit? or because if can't be earthed suitably? or something else?

That type of conduit isn't suitable for use as a Circuit Protective Conductor.

 

[GaryMo]

Why is that spark123? because it is not categorised as metalic conduit? or because if can't be earthed suitably? or something else?

That type of conduit isn't suitable for use as a Circuit Protective Conductor.

Reference 543.2.1

 

[NotHimAgain]

Flexible or pliable conduit is ruled out for use as a protective conductor by 543.2.1.

Now is that the end - well yes as far as BS 7671 is concerned, but this ruling is based on the normal use of this conduit - that is as a link between fixed and moving (vibrating etc) parts such as the connection between an isolator and a motor. The reason is because the joint between the flexible conduit and its gland often fails when subjected to movement. So for years we have augmented the connection by including a separate cpc within the conduit.

Now returning to the thorny problem of mechanical protection and 522.6 (impact). We are basically given 4 methods of protection in 522.6.6, that don't need additional protection by RCDs.

Theses are:
(i) something that meets the requirements as a protective conductor - this includes various cables (some with limited application) and excludes flexible or pliable conduit.
(ii) standard conduit
(iii) trunking
(iv) mechanical protection which can withstand my Hilti gun with a black cartridge (12.5 mm steel plate - easy)

Now I was asked this question recently by a large London based Electrical Contractor. They had a situation where they had to run cables through a stud wall because, due to the nature of the (listed) building, no other route was available.

Question can we use flexible or pliable conduit - short answer no, BUT - why, the sheath is robust enough to stay intact and support fault current flow until the protective device operates (a basic requirement not always met by BS 8436 cables) AND, the major objection to the use of this conduit as a protective conductor is that it may fail due to movement BUT, it is in a wall and will not move.

What advice would you give? This is where you earn your money .

 

[Spark123]

Use a conduit which is to the correct BS or use another cable type such as pyro.
Personally, on a domestic I'd be looking for an alternative route which doesn't involve running concealled cables in a non obvious place, or depending on asthetics, place an accessable through box as a marker.