Do I need to Earth a metalclad back box?

[PaulBaker4]

I am replacing a load of old fittings in the garage with nice new plastic conduit and MK Metalclad boxes/sockets. Do I need to run an earth from the socket to the back box earth terminal. I know this would be needed if I was using metal conduit, but it this required with plastic?

Also, Is there any advantage/disadvantage to me continuing the current mains spur and joining it up into a ring?

Thanks.

Paul

 

[Spark123]

You need to earth the backboxes as they are exposed conductive parts.
How are they currently wired, radial, spurs off ring, fused spur off ring etc? Fuse size?

 

[plugwash]

techically i think you don't have to provided they have at least one fixed lug. but i'd certainly consider it very bad practice not to especially as the boxes are going to be exposed rather than burried in plaster.

 

[securespark]

techically i think you don't have to provided they have at least one fixed lug. but i'd certainly consider it very bad practice not to especially as the boxes are going to be exposed rather than burried in plaster.

No, according to 7671, you don't have to run a lead to the box, providing the accessory is connected to the cpc's, and screwed securely to the box, and as plug says, the box has at least one fixed lug used by the M3.5 screw.

 

[PaulBaker4]

Garage is currently wired as a Spur but I am fitting a couple of extra sockets also, so thought it might be better practice to complete the ring. The garage is fed from a 32A MCB in the main consumer unit via an underground cable into a smaller garage unit.

So even though not a legal requirement, sounds like you would all suggest earthing the metalclad boxes. Is it acceptable to take the earth to the box post first, and from there into the socket, or must I do it the other way round, i.e. into the socket, and then a loop from there to the post?

 

[Spark123]

What size is the circuit supplying the socket circuit in the garage consumer unit?
The metal back box is an exposed conductive part and must be earthed. Wether it is earthed through the screws/lugs or a piece of cable is up to the installer, personally for what it is worth I always put this link in. This way, if someone should unscrew the socket cover whilst it is live it removes the risk of the backbox becoming live should it touch a live terminal. I normally run the supply earths to the socket terminals and loop to the back box.

 

[Qedelec]

What size is the circuit supplying the socket circuit in the garage consumer unit?
The metal back box is an exposed conductive part and must be earthed. Wether it is earthed through the screws/lugs or a piece of cable is up to the installer, personally for what it is worth I always put this link in. This way, if someone should unscrew the socket cover whilst it is live it removes the risk of the backbox becoming live should it touch a live terminal. I normally run the supply earths to the socket terminals and loop to the back box.

With you on that, always carry a bunch of 1.5mm Cond G/Y cut to 6"
Its surprising how many you come across that are not earthed.

Would like to know more about how this cicuit is supplied as a radial on a 32A breaker is not recommended.
From the MCB in the CCU what heve you got? leave nothing out!!

 

[PaulBaker4]

Would like to know more about how this cicuit is supplied as a radial on a 32A breaker is not recommended.
From the MCB in the CCU what heve you got? leave nothing out!!

Consumer unit in house. Garage circuit is on 32A MCB.
6mm T&E cable in conduit underpathway up wall and into Wylex old style fused consumer unit (fuse holders, not MCB's)

From there there is a lighting circuit with a 6A fuse and the mains sockets which are running from a 16A Fuse. There are currently 6 double sockets on this spur. Hope this clarifies the configuration.

 

[ban-all-sheds]

That's not a spur, it's a radial circuit, and 16A is fine. No advantage to making it a ring, unless you find that 16A capacity is not enough.

I hope the 32A MCB in the house CU is on the RCD side.

Ideally the cable between the house and the garage should be SWA, not T/E.

 

[Terran]

That's not a spur, it's a radial circuit, and 16A is fine. No advantage to making it a ring, unless you find that 16A capacity is not enough.

I hope the 32A MCB in the house CU is on the RCD side.

Trouble with this is I know of an accident because the RCD fed both sockets and lighting in a garage. Someone using a disc-cutter was suddenly plunged into total darkness when the RCD tripped of couse the tool was still rotating at near operating speed - you need little imagination about the result - very nasty injuries.

Much better if lighting is not fed from RCD IMHO

 

[ban-all-sheds]

True, but with no RCD CU in the garage, nor RCD socket outlets.....

 

[TeesdaleSpark]

I scanned this in from Guidance Note 1

EARTHING OF FLUSH METAL ACCESSORY BOXES
It has been questioned whether flush metal accessory boxes come within the definition of 'exposedconductive-parts', which require to be connected to the circuit protective conductor.
An exposed-conductive-part is defined as follows:
'A conductive part of equipment which can be touched and which is not a live part but which may become live under fault conditions'.
In normal use, such accessory boxes cannot be touched. Only when an accessory is removed can its metal accessory box be touched.
However, the view of the Wiring Regulations National Committee is that flush metal accessory boxes should be considered to be exposed-conductive-parts and connected to the main earthing terminal by means of a circuit protective conductor.
Where an accessory does not have an earthing terminal incorporated, such as certain light switches, the circuit protective conductor should be terminated at the earthing terminal in the accessory box.
Where an accessory has an earthing terminal incorporated, the protective conductor should be connected at the earthing terminal. Accessories which have an earthing terminal incorporated, such as socket-outlets, normally have an earth strap connecting the earthing terminal to one or both of the fixing holes. Flush metal accessory boxes usually have at least one fixed lug. The Wiring Regulations National Committee has agreed that such boxes may be earthed via the tight metal-to-metal contact of the fixing screw in the fixed lug. Consequently, unless both lugs are of the adjustable type, it is not necessary to connect the earthing terminal of the accessory to the earthing terminal in the associated flush metal accessory box by a separate protective conductor (i.e. an earthing tail).
Where the circuit protective conductor is formed by conduit, trunking, ducting or the metal sheath and/or armour of a cable, an earthing tail must be fitted.
Where there is any doubt about the continued effectiveness, reliability and permanence of an earthing connection formed by the screws between an accessory and a metal box, it is recommended that a suitably sized earthing tail is provided.

 

[Spark123]

He is using metalclad which is normally surface mounted.

 

[PaulBaker4]

Thanks for the info guys. Sorry, my lack of terms calling it a spur, rather than a radial circuit! I knew what I meant!

The T&E is from the non RCD side of the consumer unit, however I thought I would get the current garage unit replaced with a 2 way unit fitted with an RCD. Guess from the previous comments that you would suggest getting a bigger split way unit so that I could run the lights not through the non RCD side to avoid any accidents?
I did get some nice 10mm armoured cable a while ago, and will get an electrician to wire that in when we have some major work done in a year or so.

One of the main reasons for thinking about completing the ring was that i'm putting conduit on 3 and a half sides of the the garage so seemed the neatest option to do the other 3 meters, and would then save any worry about load in the future. If my interpretation is correct. There's no reason not to do this. Just normally cheaper and easier not to make it a ring.

 

[ban-all-sheds]

The T&E is from the non RCD side of the consumer unit, however I thought I would get the current garage unit replaced with a 2 way unit fitted with an RCD. Guess from the previous comments that you would suggest getting a bigger split way unit so that I could run the lights not through the non RCD side to avoid any accidents?

Unless the exising garage CU is battered and folorn, keep it and just replace the socket MCB with an RCBO if you can.