Twin & Earth in a concrete wall

[LondonChap]

A neighbour is running direct burial cable from the house to his outside lights. The lights will be inset in a concrete wall. The direct burial is to the first light in the wall. The rest of the lights in the wall are to be daisy chained from that. The issue is using 1.5mm twin and earth from light to light buried about 10mm below the surface not in conduit, buried directly. Because of re-bar it is not possible to go deeper. This will be on an RCD. In internal installations a cable can be less than 25mm from a wall surface as long as it is on an RCD. I can't see any difference here.

What do others think of this? Sensible replies only please.

 

[sparkwright]

The main issue here is whether the buried cable in the wall is routed in safe zones - ie in line with each light so the chances of drilling or nailing through the cable are unlikely.

What's the other side of the wall? Can the cables being placed the other side, and pass the cable through the wall?

 

[bernardgreen]

Your neighbour needs to be aware of the problems that he will face if any of the re-bar becomes exposed to air as a result of the channelling into the concrete.

There will be no immediate effect but over years the re-bar can rust and seriously weaken the concrete.

 

[LondonChap]

The wall is over a foot thick and 4 foot high and is the boundary retaining wall with earth behind it. The land is inclined, so it keeps next door from falling onto his land.

The cables are to be in the wall and the wall rendered, and each link to the lights is to be run vertically either up or down from the light and then run horizontally near ground level or near the top coping stones. I would think running near the ground would be better.

There is little chance of the wall being drilled. It is all on an RCD.

Should the cable be in conduit? There is no space for a conduit as the rebar in the wall will obstruct. It is external wiring, the lights obviously weather-proof but I see no difference between internal and external in this case when running cable in a wall.

The rebar is deep enough not to be exposed. But not deep enough to accommodate conduit.

 

[Chri5]

You say the lights (and the wire) will be inset. If 20mm conduit is an issue, won't inset lights?

The issue with not having conduit will cause problems if there's ever an issue with the wiring, how would you replace it?

At the very least go for a cable that is more suitable such as NYY.


http://www.tlc-direct.co.uk/Products/NY1dot5slash3.html


Rcd is mandatory, better still an rcbo on the direct side of the cu, since external lights do tend to be a very big cause of nuisance trip, damn water...

 

[LondonChap]

You say the lights (and the wire) will be inset. If 20mm conduit is an issue, won't inset lights?

The issue with not having conduit will cause problems if there's ever an issue with the wiring, how would you replace it?

At the very least go for a cable that is more suitable such as NYY.


http://www.tlc-direct.co.uk/Products/NY1dot5slash3.html


Rcd is mandatory, better still an rcbo on the direct side of the cu, since external lights do tend to be a very big cause of nuisance trip, damn water...

Chris5, thanks. I just checked and the light are to be surface mounted or semi inserted but only by a few mm. Well if the cable needs replacing in the future then it needs digging out. Conduit is is not an option.

So this NYY cable can drop from each light vertically down under the ground, to avoid channelling the concrete wall and up to the next light? At 13mm diameter it may just do to inset into the wall.

http://www.tlc-direct.co.uk/Products/NY1dot5slash3.html

There are 13 lights at 50w each so 1.5mm to the first and daisy chaining the rest in 1.0mm in concrete would be fine (this is if the NYY cable is too thick).

 

[LondonChap]

Rcd is mandatory, better still an rcbo on the direct side of the cu, since external lights do tend to be a very big cause of nuisance trip, damn water...

The CU is split with two RCDs. Is it possible to fit an RCBO after an main RCD? Will this create problems?

 

[securespark]

This is a really bad design.

The external electrics should be on their own RCD, completely independent of any other. This way, it cannot take anything else out if a fault occurs.

The best way would be to put in a second CU with RCD just for the outside gear.

Or, if poss., the existing board may have space for a non RCD circuit before the two RCD's, then put the outside lights on RCBO.

 

[LondonChap]

The external electrics should be on their own RCD, completely independent of any other. This way, it cannot take anything else out if a fault occurs.

The best way would be to put in a second CU with RCD just for the outside gear.

It would only need 2.5mm cable jumped off the input to the main switch in the CU, then to a small CU with its own RCD? It is only supplying lights so no heavy cable is needed.

Or, if poss., the existing board may have space for a non RCD circuit before the two RCD's, then put the outside lights on RCBO.

This maybe possible by sliding over one of the RCDs in the CU.

Or using one side of the CU and using the RCD as just a switch disconnecting the trip circuitry (I think that is called slugging an RCD?), then inserting an RCBO into the non-protected side of the CU for outside? Then L & N are knocked out if the RCBO trips.

 

[SimonH2]

Or using one side of the CU and using the RCD as just a switch disconnecting the trip circuitry (I think that is called slugging an RCD?)

Eh ? You propose to leave a safety protective device apparently present but disabled - so anyone looking at the Cu will assume a level of protection that isn't there ? Besides which, to do what you suggest would mean opening up the case (not normally designed to be dismantled), somehow b***er about with the internals, and then persuade it to go back together again. Good luck - cost more in time than just fitting a DP switch in place of the RCD.

Then L & N are knocked out if the RCBO trips.

Most RCBOs for CUs are single pole switching.

 

[sparkwright]

If you really have 10 mm to play with you may be able to use the smallest oval conduit, which may prove useful.

Is surface wiring an option?

 

[mattylad]

This wall may actually be drilled into, have you never heard of trellis?

Wooden criss cross stuff that wimmin like to stick onto walls so they can grow stuff up.

Maybe not you but maybe the next owner of the house (or the next wife lol).

 

[RF Lighting]

Why are you bothered? It's your neighbours wall and your neighbours installation. Has he asked you if you think it's unsafe?

 

[securespark]

It would only need 2.5mm cable jumped off the input to the main switch in the CU, then to a small CU with its own RCD? It is only supplying lights so no heavy cable is needed.

Or using one side of the CU and using the RCD as just a switch disconnecting the trip circuitry (I think that is called slugging an RCD?), then inserting an RCBO into the non-protected side of the CU for outside? Then L & N are knocked out if the RCBO trips.

Go away.

If you think either of these "solutions" is acceptable for your neighbour, then you should not be let loose near a cable.

 

[bernardgreen]

Why are you bothered? It's your neighbours wall and your neighbours installation.

The wall is over a foot thick and 4 foot high and is the boundary retaining wall with earth behind it. The land is inclined, so it keeps next door from falling onto his land.

If I was the neighbour whose house was on land retained by the wall then I would be very concerned about any work on the retaining wall no matter how trivial it might seem.