I recently bought some converta (white plastic) sockets to convert my single plugs to doubles as easy as possible. The instructions clearly state that the earth terminal on the new socket should be connected to the metal wall box, if a metal one is present, as well as obviously being connected to the main earth wires that go into the walls. The boxes are recessed and are flush with the walls.
There are no terminals on my wall boxes, so I just connected a piece of wire to one of the screw holes that are not used on the box from the earth terminal on the converta socket. I did this by wrapping the wire around the unused screw hole. It was covered in dust and debris, so I don't really know if it is making good contact.
I cannot understand why this is needed if the mains earth is connected and the metal box is not exposed? Is there any major issue with what I have done?
No major issues, but one thing does spring to my mind. If the metal box is completely covered by your new boxes, then why fix an earth tag to it at all? It might actually have been safer to leave the boxes disconnected from any earth ground if they are not actually being used to house the working part of the sockets. But I can't see the job, so my comments are only based on speculations.
It isn't completely covered, unless it's both covered and you replace the screws holding the cover on with insulated ones.
To provide a path for a fault current, in the case of a wire coming loose inside the box. It's not just to protect you from a live case fault, it's to signal the occurence of this fault even before you've touched it.
Thanks for the advice. The single sockets were not wired to the boxes from the socket earth terminal, maybe its some new regulation thing. The single sockets have been in since the house was built in the mid 70's.
If you wanted a better way to sequre the arth to the back box, try buying a cheap rivot gun and some biug rivots with loop eye fixings to rivot the earth to the back box just did this in my house
Yes, it's a 'new' recommended practice - but not that new! Thinking changed around the mid80s, AFAIR. Previously, it was considered that the backbox would get reasonably well earthed through the faceplate screws, which rested in little brass cups which were solidly joined to the metalwork of the faceplate, which included the 'wiring accessory's earth terminal. ('Wiring accessory' is electrician-speak for switch, socket, fused connection unit, etc.)
However, there were a few documented cases where corrosion between the screw and the backbox increased the resistance between backbox and earth to the point where a fault current flowed which was large enough to cause notable heating - I think even a fire in a couple of cases - but too low to blow the fuse/MCB. This was more likely where you had the unfortunate combination of only one of the faceplate screws having the brass cup and fixing to faceplate metalwork mentioned above, and that being the side with the adjustable lug of the metal back box. (Most? backboxes have one fixed threaded lug on one side, the other one having some up-and-down play in it to allow the accessory to end up all luvverly horizontal like even if the backbox is marginally skew whiff.)
Since this was publicised by the IEE and in the odd 'How To Do Sensible Wiring' book, practice has changed to prefer a flying-lead connection from the accessory earth terminal to the back-box's earth terminal - and backenboxen aren allen maden withen integralen earthen terminalen. But you'll still find older ones - like the Original Poster's - wot don't have such a terminal. Taking a flylead and poking it hopefully into a spare mounting hole doesn't really make a solid enough contact; it's worth at least using a short self-tapper to secure that lead (you may need a few mm depth of the material behind the box drilling out with a handy masonry drill).
Any reference to when and where "this was publicised by the IEE" and quasi-officially recommended? ICBW but I don't think the NICEIC technical manual mandates it if the mounting box is not touchable.
One situation in which this link /is/ mandated is where the incoming earth (CPC) arrives at the box via a metal conduit, cable armour or sheath. Then the earth connection to the accessory must be made via a separate wire link from an earth terminal on the box to the earth terminal of the accessory. [Reg. 543-02-07]
Of course the 'converta' sockets (WWIANF) mentioned might not have any earth path to the box via the mounting screws, in which case the link is a good idea.
To connect to old boxes with no earth terminal I recommend a short self tapping screw (say No. 8) in conjunction with an insulated crimp ring terminal on the end of the wire.
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