Mechanical protection/ Safe zones

[sparrrrk]

Hi all,

Just after a bit of advice from all you domestic installers out there.

I've been asked to do a job in a house which involves running a cable in a wall. The cable needs to run vertical the entire height of the wall. However I cannot run it where the walls meet at an angle due to obstructions. This wall is at the base of stairs so quite small.

Can I utilise the thermostate as an acessory and run behind it (obviously taking it off wall and mounting back on after filled) to fullfill the requirementof running to an accessory. Or is this not allowed as it isn't actually supplying the accessory.

If not can I just cap the cable with metal capping at least 3mm thick. I know meeting the requirement to prevent nail penetration is hard to meet.

The circuit is covered by a 30ma rcd. Just a little confused as my domestic regs knowledge is not up to scratch. Don't want to install a cable unsafeley.

Any advice would be appreciated. Thanks

 

[Gavin A]

He made a blanket statement about mechanical protection, nothing specific about it being buried in anything

ah fair enough, I was assuming the conversation was referring to this situation.

 

[davesparks]

It doesn't matter whether it's buried or not, there is no requirement in BS7671 for mechanical protection to be earthed.
You want to earth it carry on, I'll get you some PVC conduit to earth.

Wtf? Steel conduit and trunking is mechanical protection and I'm pretty damned sure it requires earthing.

 

[andyb]

Mechanical protection is not required to be earthed.

That is a sweeping statement Spin, you've just told the op and anyone else reading this, including trainees and diy'ers that you never need to earth mechanical protection regardless of the situation which is not true.

As it happens, in this case the way I read it is that one option being considered is to run the cable in the wall outside of a safe zone with a metal covering.

In this case the mechanical protection does need to be earthed.

 

[davesparks]

That is a sweeping statement Spin, you've just told the op and anyone else reading this, including trainees and diy'ers that you never need to earth mechanical protection regardless of the situation which is not true.

As it happens, in this case the way I read it is that one option being considered is to run the cable in the wall outside of a safe zone with a metal covering.

In this case the mechanical protection does need to be earthed.

I can see where he's going with it, but it didn't come across right.
In your example it will depend on the type of mechanical protection. Steel conduit would need earthing, but a solid steel plate resistant to penetration by nails etc likely wouldn't need it

 

[andyb]

The reg dealing with cables buried in walls mentions various scenarios from >50mm and several 'ors' but the ones mentioning metallic protection all say earthed.

It's the statement 'There's no requirement in bs7671 for mechanical protection to be earthed' that I find misleading.

 

[Darkwood]

Its the term itself which is ambiguous - pvc sheath provides 'mechanical protection' so does SWA but on very different levels.

Mechanical protection does not mean necessarally protection to a high degree of impact or piercing but when it is worded in combination with regulations which clearly mean you are providing a high degree of protection then yes it can be assumed to relate to steel conduit, plate or armour.

Its like saying the pvc jacket of T&E is there to provide mechanical protection albiet a low level - technically it does but it would be better to say the pvc jacket is there to provide environmental protection of which one trait is a low level of mechanical protection, it also provides many other traits of protection from insulation to UV tolerance again in the example of standard T&E it would it would be a low UV tolerance.

Wording is key here and depends on the subject of discussion to what would be assumed it represents.

 

[malcolmsanford]

Can anyone tell me what is the difference between a lump of say 5mm steel plate not conected to an electrical installation but used for protection with a T+E behind it

or

A length of galvanised 20mm conduit not connected to an electrical installation but used for protection with a T+E through it

 

[davesparks]

Can anyone tell me what is the difference between a lump of say 5mm steel plate not conected to an electrical installation but used for protection with a T+E behind it

or

A length of galvanised 20mm conduit not connected to an electrical installation but used for protection with a T+E through it

Presumably both are completely buried in plaster and not available to touch at any point etc?
In which case there is very little difference between the two, other than the thickness of the steel, and a masonry drill will go through the conduit if you push hard enough but the 5mm plate?
I guess it depends a little on how you read the regs but I think it says earthed steel conduit or protected from penetration by nails/screws and the like.

 

[malcolmsanford]

For once I agree Dave but I always found that you have to be really enthusiastic to try and push a masonary drill through a piece of class 4 RMC tube.

But the IET know best

 

[andyb]

Can anyone tell me what is the difference between a lump of say 5mm steel plate not conected to an electrical installation but used for protection with a T+E behind it

or

A length of galvanised 20mm conduit not connected to an electrical installation but used for protection with a T+E through it

I don't think there's any difference if they are being used to comply with the regs re cables buried in a wall.
They will both require earthing.

 

[malcolmsanford]

I don't think there's any difference if they are being used to comply with the regs re cables buried in a wall.
They will both require earthing.

The 5mm plate will almost certainly comply with part (iv) of reg 522.6.204 and will not need to be earthed. As Dave said a conduit has to be earthed as per part (ii)

I gather Andy you were looking at part (i) for the plate to be earthed.

I was just trying to say I still think it would be very difficult to push a masonary drill through a piece of RMC

 

[andyb]

The 5mm plate will almost certainly comply with part (iv) of reg 522.6.204 and will not need to be earthed. As Dave said a conduit has to be earthed as per part (ii)

I gather Andy you were looking at part (i) for the plate to be earthed.

I was just trying to say I still think it would be very difficult to push a masonary drill through a piece of RMC

I see that now Malcolm and you're dead right.

I was posting in regard to the post purporting to say that mechanical protection does not need to be earthed.

I was really looking at part (ii) rather than (i) and pointing out that if the mechanical protection is in the form of metal conduit then this needs to be earthed.

It's one thing to do as you have done and cite an example and reg where possible which advances a debate and another to issue a sweeping statement like 'mechanical protection does not need to be earthed' without being more specific.

 

[Richard Burns]

If the sole purpose of the metallic mechanical protection is to provide mechanical protection and it is not part of the wiring system then it does not need to be earthed.
Where the mechanical protection is part of the wiring system, and so providing another function, then it will need to be earthed.
The example with twin and earth in conduit is that normally conduit is intended for singles and so becomes the "sheathing" for the singles and is an exposed conductive part and requires earthing.
If you have twin and earth in conduit then there is the potential to consider the it may not need earthing, though this is a moot point and I would recommend earthing in this case anyway.

And as already said mechanical protection comes in different levels that are not differentiated by the phrase mechanical protection.
Mechanical protection only means protection against physical movement, pressure etc. effects.
However the force of these effects is not covered.
Mechanical protection sufficient to prevent the penetrations of nails screws etc, is different from mechanical protection against abrasion.

 

[Deleted member 26818]

I will attempt to explain the requirements for concealing cables in walls:
1. Bury the cable at a depth greater than 50mm.
It is assumed that people will not use screws and nails that are longer than 50mm.
2. Use a cable that has an earthed metallic sheath.
The idea being that if a screw or nail penetrates the cable, the screw or nail will be contact with the earthed sheath, and if also contacts a live conductor, will cause the OCPD to operate.
3/4. Use earthed conduit or trunking.
The idea for these methods are the same as for No. 2.
5. Provide mechanical protection sufficient to prevent penetration by screws or nails.
There is no requirement to earth the mechanical protection as in the case with Nos. 2, 3 & 4, because the screw or nail will not penetrate the cable, will not contact a live conductor and will not be required to cause the OCPD to operate.
6. Run the cable in a prescribed zone and provide additional protection by means of a 30mA RCD.
The idea being that if someone is unaware of prescribed zones, then hopefully the RCD will operate.

Remember, not all conduit is 3mm thick, some is very flimsy indeed, and will not provide the required level of mechanical protection. Such flimsy conduit requires earthing.