Metal backboxes complete with earth tail ?

[Ellis Greensitt]

If a metal backbox was supplied with a preinstalled piece of earth wire onto the earth terminal, how much time do you think you could save 2nd fixing a house ?

[Christian McArdle]

I'd guess about 30 seconds per fitting, or about 50 minutes on a big house. However, I reckon you could save 15 seconds just by cutting the earth wires in batch fashion, especially if done from a reel of insulated cable, rather than uninsulated bits found lying about + separate sheath.

Christian.

[RichardS]

More to the point, how much more likely would it be that the backbox would be actually connected to the accessory plate! In the places I've lived in, I haven't come across a single accessory plate that was properly earthed to the backbox, until I've touched 'em that is.

I really like that idea. The incremental purchase cost surely can't be that great, though a screw terminal would still have to be provided for the case of the conductor becoming damaged or breaking off in the future.

[Dave Plowman (News)]

It depends. Were you thinking of offering this? The trouble is whether your time - or the person you pay to do it is worth less than an on site sparks, since it can't easily be automated. Also, would the person making these have to be certificated? ;-)

I quite liked the idea of pre-wired ceiling roses and pendant. But not all houses have the same height ceilings.

[RichardS]

for a manufacturer, it probably could be automated - provide a screw terminal as before, but additionally crimp a length of insulated CPC onto a different part of the back box.

It all comes down to price point & who does the first fit, really. If the price is right & sparks do the first fit of back boxes then I reckon it'd be a good selling point in a commodity marketplace, as long as the price differential isn't great. If, however, general builders do the first fit, then they won't care & will go for the cheapest product.

[Andrew Gabriel]

Sparks never earth back boxes, so it would add time to either connect it or cut the wire off, neither of which they would otherwise do. Also, it's another dangly bit to get in the plasterer's way.

Some conciensous DIY's earth back boxes, but their time is free to a first approximation, so the time saved isn't worth anything.

[Ellis Greensitt]

This has sparked off a bit of a debate on the screwfix forum here

to a vote on here

[Dave Plowman (News)]

They do, you know. Just sorted out a friend's new kitchen where a socket wasn't working. Tail to the back box, but the neutral left so loose the insulation had melted. And the line wasn't properly tightened either. Same in all the other fittings.

[Miketew]

Sparks never earth back boxes, so it would add time to either connect

it or cut the wire off, neither of which they would otherwise do. Also, it's another dangly bit to get in the plasterer's way.

Some conciensous DIY's earth back boxes, but their time is free to a first approximation, so the time saved isn't worth anything.

-- Andrew Gabriel More slagging off electricians and B*****it from Mr Gabriel, you mus have a very shallow life or perhaps you need to get one

-- Miketew

[Ed Sirett]

AIUI the regulations _require_ the extra connection to the back box when i) The earthing is supplied by the protective conduit. I.e. singles in buried steel conduit. ii) _Both_ the lugs on the back are movable (this is usually not the case with most boxes in my experience. iii) The countersunk eye of the fixing hole is only earthed on one side (I've yet to see a socket like this - even the cheapest contract range) and that side goes to the moveable lug. iv) non conductive fixing screws are used.

I guess if the wire were pre-installed I would connect it up rather than chop it off.

[Andy Luckman (AJL Electronics)]

Interesting. How about grommets in the cable entry holes?

[Dave Plowman (News)]

Rare without tails to the box earth. ;-)

[Andrew Gabriel]

It probably happens sometimes, but I don't recall seeing missing grommets much in professional installs.

BTW, I wasn't claiming it was a requirement to always earth a backbox -- it isn't unless it's exposed somehow.

[Andy Luckman (AJL Electronics)]

Well I have been doing it religiously. Can't find either of my two packs of grommets though!

[Andrew Gabriel]

Well, grommets are probably a requirement as Good Workmanship, unless you can achieve the same effect some other way.

I have occasionally used boxes which don't take grommets, and have the holes pre-punched with the metal formed into a smooth rounded lip (you couldn't fit a grommet even if you wanted to). I had a number of these in a box of odds and ends leftovers which have been knocking around for a while, but I haven't seen them for sale anywhere recently, and I can't remember where I originally bought them.

Also, recently I replastered and properly buried a lighting cable into a wall which had been rather badly done in the past. The box had oval entries and no grommets, and I had no matching grommets. So instead, I pushed the oval trunking right through the oval hole (for which it was an excellent fit) and about a mm into the box, thereby protecting the cable from any sharp metal edge.

As I've posted before, you can make grommet strip from the outer sheath of T&E by stripping it off by running a knife down the side of the earth conductor. A length cut this way, and slightly over long so it's forcing itself to stay in the hole, would be better than nothing in an emergency.

[mungoh]

You can also buy grommet strip from places like rapid electronics:

costs a fiver per five metre length.

Mungo

[BigWallop]

A bloody quid per metre? You're being ripped off mate. Local sparks merchants will have grommet strip for around half that price.

[Dave Plowman (News)]

If the box is mounted on brick, etc, and made good with plaster, there ain't going to be any possibility of cable movement and therefore chafing. However, since they only cost a couple of pence when bought by the hundred, a sensible DIYer would use them as a matter of course.