Earthing chrome sockets and switches

[Intothinair]

Hello...

I'm replacing plastic sockets and switches in our lounge with chrome ones. The switches all have earths connected to the metal backing boxes. Is there any reason I shouldn't use 2.5mm conduit cable to connect the earth to the new chrome switches or does it have to be a sleeved 2.5 from a twin and earth? Just checking!

 

[OwainDIYer]

2.5 'conduit cable' would be fine provided it is coloured/sleeved green-yellow.

 

[333rocky333]

The sleeved earth in 2.5 T and E would only be 1.5 anyway, I always use 2.5 pre insulated for the socket links anyway regardless, I dont regard the link as part of the supply cable, and I do recall L N E conducters if not formed within a cable, should be of the same csa, I may be wrong but thats my veiw.
Switch links and lighting id generally use 1.5 pre insulated.

I use a lot of single core cable so have it anway, but I understand for some its not worth buying 100m so sleeved earth is usually adequate

 

[Risteard]

I do recall L N E conducters if not formed within a cable, should be of the same csa

Not correct. The neutral conductor should be of at least the same csa as the line conductor(s). The cpc can either be selected or calculated. If we couldn't calculate then T&E of every size except 1mm^2 would not be permitted.

 

[333rocky333]

L N E conducters if not formed within a cable,

 

[JohnW2]

...and I do recall L N E conducters if not formed within a cable, should be of the same csa, I may be wrong but thats my veiw.

As Risteard has suggested, I think that is only true for the 'lazy' .....

IF the CPC does not form part of a cable, and is not in conduit/ducting or an enclosure, then it has to have a minimum CSA of 2.5mm² if mechanically protected, otherwise 4mm². However, if (as in the example we're discussing), that does not apply (in this case because the CPC is in an enclosure), then there is no majorl distinction between the minimum CSA of CPCs which form part of a cable and those which don't.

In all such cases, the minimum CSA is as defined in 543.1.3 (adiabatic calculation) or 543.1.4 (lazy man's, very conservative, deemed-to-satisfy tabulation, Table 54.7!). The latter, 'lazy' approach does, indeed indicate that for line conductors up to 16mm², the CPC must have a CSA at least as great as that of the line conductor. However, if one determines the minimum CSA 'properly' (adiabatic calculation, per 543.1.3), the only difference is that the 'k' value is higher (143, rather than 115, for 70° thermoplastic copper) if the CPC does not form part of a cable. This means that the minimum CSA is actually lower for a CPC which does not form part of a cable than for one which does form part of a cable.

Kind Regards, John

 

[333rocky333]

I did say i may be wrong
Although in all my career ive only ever ONCE witnessed a smaller earth in singles and that was a dado trunking/ plastic conduit install, so ive always opted for the "lazy way".

Thinking back I think it was only a NICEIC article that i read it in the PE free magazine a few years back, it was regarding cables in conduit/ trunking
It also went on to say that once you put T+E in trunking/ conduit, say down to a board or accesory it no longer conforms, due to L N E wiring in pipe/trunking needing to be equal size., which would be ridiculous

Some of there articles do seem to get there wires crossed

 

[sparkwright]

Very common to use 1.5 mm² earth for sockets on conduit systems, especially old narrower imperial conduit.

 

[JohnW2]

Thinking back I think it was only a NICEIC article that i read it in the PE free magazine a few years back, it was regarding cables in conduit/ trunking ... It also went on to say that once you put T+E in trunking/ conduit, say down to a board or accesory it no longer conforms, due to L N E wiring in pipe/trunking needing to be equal size., which would be ridiculous ... Some of there articles do seem to get there wires crossed

That NICIEC for you, I guess - as you say, that suggestion was just plain ridiculous. One needs to be a bit choosy about what sources one decides to trust!

Kind Regards, John

 

[333rocky333]

Very common to use 1.5 mm² earth for sockets on conduit systems, especially old narrower imperial conduit.

Not too sure it is really

Convention is to use the same size in conduit/trunk (in smaller csa circuits), but reducing the size is fine as long as it complies with table 54.7 or the adiabatic equation.

 

[sparkwright]

It's very common practice nowadays to use the same size CPC as the L and N, should have made that clearer.

Incidently, anyone remember the 2.5 mm² T+E with 1.0 mm² earth? Early 1970s I suppose?

 

[sparkwright]

Deleted.

 

[JohnW2]

Convention is to use the same size in conduit/trunk (in smaller csa circuits), but reducing the size is fine as long as it complies with table 54.7 or the adiabatic equation.

As I wrote earlier, Table 54.7 requires CPC to be the same CSA as L up to 16mm² but, yes, adiabatic calculation will usually allow a smaller CSA (similar to 2.5/1.5mm² T+E). I haven't done the sums, but it's just possible ( actually doubt it) that (because of the higher 'k' value for singles), a 1.0mm² single CPC might just be adequate for a 2.5mm² L.

Kind Regards, John

 

[JohnW2]

Incidently, anyone remember the 2.5 mm² T+E with 1.0 mm² earth? Early 1970s I suppose?

Yep, I inherited a fair bit of it in my house!

Kind Regards, John

 

[PrenticeBoyofDerry]

Hello...

I'm replacing plastic sockets and switches in our lounge with chrome ones. The switches all have earths connected to the metal backing boxes. Is there any reason I shouldn't use 2.5mm conduit cable to connect the earth to the new chrome switches or does it have to be a sleeved 2.5 from a twin and earth? Just checking!

I would connect the CPC(earth) of the circuit directly to the chrome plates, then link between metal plate and metal backbox. Even though there is no requirement to actually link between plate and backbox, providing the plate is connected to earth and a rigid lug or lugs are formed on backbox then they are used to secure/screw the plate to the backbox.