earthing back boxes
Fred
Does the OSG (17th ed.) cover something as basic as wiring a socket?
Or do they think that people who read the OSG already know something
as simple as that? I wanted to look-up the requirements for earthing a
metal back box. I had a quick look through the OSG but could not find
it mentioned.
I seem to remember someone else asking a similar question on this
group. If I remember correctly, the reply was that if you were wiring
a socket, which included an earthed screw terminal, you did not have
to run a separate earth wire from the box's earth terminal; provided
that the earthed screw hole was screwed onto a fixed tab in the box,
not the moveable one. If you are wiring something without an earth,
such as a light switch, then you have to connect to the box. Have I
remembered that correctly?
I have bought some round pin sockets to use with a table lamp and some
back boxes from Toolstation. The sockets only earth one screw hole and
guess what, it's the one above the moveable tab! The problem is that I
put the boxes in, plastered, and papered around them so I do not want
to try removing and rotating the boxes. Am I right to think that in
this case I need to run the extra earth wire from the box to the
socket?
I want to have two boxes with a switch in the top one, running to a
socket in the one below. I was hoping to fit a 2-gang light switch to
use a system like they have in hotels: switch one for the main ceiling
light and the adjacent switch for a table lamp. That means quite a few
wires:
L, switched L, and E from the ceiling rose
3-core and E for two way switching of ceiling light
twin and earth to table lamp switch
twin and earth from table lamp switch to socket
Can I comfortably fit all four earth wires into the box's earth
terminal? Or would it be advisable to crimp or otherwise connect some
of them together separately?
TIA
Toby
Personally, I would always run a flying lead from the socket to the back
box, so that if the socket or switch plate was unscrewed, the back box
will still be earthed - so in the event the plate if off, and the live
finds it's way to the back box (loose or damaged cable), it is still
protected, when it is fully exposed and most vulnerable (in my opinion
anyway)
Toby...
Andy Wade
> [...] if you were wiring a socket, which included an earthed screw
> terminal, you did not have to run a separate earth wire from the
> box's earth terminal; provided that the earthed screw hole was
> screwed onto a fixed tab in the box, not the moveable one. If you are
> wiring something without an earth, such as a light switch, then you
> have to connect to the box. Have I remembered that correctly?
There's no absolute right answer here. The requirement is that
exposed-conductive-parts must be reliably earthed. Some authorities
(e.g. the NICEIC/ESC, IIRC) are happy with the above interpretation,
others say good practice is always to include the link wire.
There's a further debate: is a flush box (sunk in plaster or drywall
construction and fitted with a plastic-fronted accessory) an
exposed-conductive-part at all? Arguably not since it's not accessible
to be touched. If you're using wiring accessories with metal front
plates though, there's definitely E-C-Ps to be earthed.
So, FWIW, my view is that it's only OK to rely on earthing via a fixing
screw to a fixed tab if the box ends up inaccessible and a plastic
accessory is fitted. With metal-faced accessories it's best to earth to
a terminal on the accessory, if there is one, and link to the back
box if necessary.
> I have bought some round pin sockets to use with a table lamp and some
> back boxes from Toolstation. The sockets only earth one screw hole and
> guess what, it's the one above the moveable tab! The problem is that I
> put the boxes in, plastered, and papered around them so I do not want
> to try removing and rotating the boxes. Am I right to think that in
> this case I need to run the extra earth wire from the box to the
> socket?
Clearly you do.
> I want to have two boxes with a switch in the top one, running to a
> socket in the one below. [...]
>
> L, switched L, and E from the ceiling rose
> 3-core and E for two way switching of ceiling light
> twin and earth to table lamp switch
> twin and earth from table lamp switch to socket
It sounds as if the first-fix wiring is all done now, but you could have
saved a cable there, assuming the ceiling and table lamps are on the
same final circuit. A 3C+E cable from the ceiling light could have been
used to bring down a neutral (grey core, blue sleeve) as well as the L &
SL wires in the 'switch drop. Then a separate feed cable for the table
lamp would have been unnecessary.
> Can I comfortably fit all four earth wires into the box's earth
> terminal? Or would it be advisable to crimp or otherwise connect some
> of them together separately?
With 1.0 or 1.5 mm^2 T&E (1.0 mm^2 CPCs) That's unlikely to be a problem.
--
Andy
js.b1
> There's a further debate: is a flush box (sunk in plaster or drywall
> construction and fitted with a plastic-fronted accessory) an
> exposed-conductive-part at all? Arguably not since it's not accessible
> to be touched.
With a metal backbox the screws are, but usually at least 1 screw
enters a fixed lug so is earthed via the wiring accessory.
Then again, you can argue a rusty screw doesn't conduct well enough to
disconnect a fault should one arise.
--
Jason.
Dave Plowman (News)
Fred <fr...@no-email.here.invalid> wrote:
> Does the OSG (17th ed.) cover something as basic as wiring a socket?
> Or do they think that people who read the OSG already know something
> as simple as that? I wanted to look-up the requirements for earthing a
> metal back box. I had a quick look through the OSG but could not find
> it mentioned.
> I seem to remember someone else asking a similar question on this
> group. If I remember correctly, the reply was that if you were wiring
> a socket, which included an earthed screw terminal, you did not have
> to run a separate earth wire from the box's earth terminal; provided
> that the earthed screw hole was screwed onto a fixed tab in the box,
> not the moveable one. If you are wiring something without an earth,
> such as a light switch, then you have to connect to the box. Have I
> remembered that correctly?
Just how much extra work is it to run an earth from the socket to the box
terminal? There's usually enough ECC left after stripping back so the only
real cost is a piece of sleeving...
--
*If you must choose between two evils, pick the one you've never tried before
Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.
ARWadsworth
"Dave Plowman (News)" <da...@davenoise.co.uk> wrote in message
news:5076d73...@davenoise.co.uk...
More work than an electrician can be bothered to do:-)
I have however, seen one DIY garage rewire using trunking and plastic
surface mount boxes where the DIYer took an earth lead from the socket to
the earth lug in the plastic box.
Adam
Dave Plowman (News)
ARWadsworth <adamwa...@blueyonder.co.uk> wrote:
> > Just how much extra work is it to run an earth from the socket to the
> > box terminal? There's usually enough ECC left after stripping back so
> > the only real cost is a piece of sleeving...
> More work than an electrician can be bothered to do:-)
Well, imagine the scenario where a metal box sits below the plaster - or
has tiles over it. Would you really rely on the thread of a screw to make
a decent contact throughout its likely life?
> I have however, seen one DIY garage rewire using trunking and plastic
> surface mount boxes where the DIYer took an earth lead from the socket
> to the earth lug in the plastic box.
To be sure to be sure. ;-)
--
*Why is the man who invests all your money called a broker? *
Dave Plowman da...@davenoise.co.uk London SW
ARWadsworth
> In article <xGK4m.52737$OO7....@text.news.virginmedia.com>,
> ARWadsworth <adamwa...@blueyonder.co.uk> wrote:
>> > Just how much extra work is it to run an earth from the socket to the
>> > box terminal? There's usually enough ECC left after stripping back so
>> > the only real cost is a piece of sleeving...
>
>> More work than an electrician can be bothered to do:-)
>
> Well, imagine the scenario where a metal box sits below the plaster - or
> has tiles over it. Would you really rely on the thread of a screw to make
> a decent contact throughout its likely life?
I actually would not worry. The backbox is not touchable in day to day use
and the screws sit on the sockets "earth bar".
However, as you say, it is not that much extra work for someone just fitting
a couple of sockets. I carry a roll of 1.5mm green/yellow cable for jobs
that specify the earth link.
Adam
Dave Plowman (News)
ARWadsworth <adamwa...@blueyonder.co.uk> wrote:
> However, as you say, it is not that much extra work for someone just
> fitting a couple of sockets. I carry a roll of 1.5mm green/yellow cable
> for jobs that specify the earth link.
I put it in the same class as using grommets in knockouts.
--
*It is easier to get older than it is to get wiser.