I want to replace single ceramic sockets in my bedroom with double
chrome-plated one. I have chiseled out bigger hole and replaced
installation box with the double one. So far so good.
Now I need to connect chrome plated socket to the wires and here is
the problem. I only have live and neutral but no earth in the box.
I've read it could be dangerous because if live cable becomes loose
and touches metal someone could get hurt.
1) Can I use chrome plated sockets/switches or only plastic ones?
2) How can I provide earth to sockets/switches other than direct cable
from CU? (i.e. can I use CH copper pipes or gas pipe for that)
3) Can I use metal conduit (which I have throughout) to transport
earth to sockets/switches ?
any help appreciated
On Wed, 5 Dec 2007 10:24:42 -0800 (PST) someone who may be olo
The metal conduit provides the protective conductor (earth)
connection. There should be a short lead from the terminal in the
box to the terminal on the socket.
However, such systems are prone to a variety of failures, including
rust breaking the protective conductor. The integrity of the
conduit should be verified by someone who knows what they are doing
using the appropriate equipment.
This applies whether the face of the socket is metal or plastic.
By ceramic are we talking about an old 5A unearthed socket?
No, and no. You can't have a socket without an earth at all (with
exception of a transformer isolated shaver socket).
You need to replace the existing cable with a appropriate T&E cable.
Either back to the CU or as a spur to an existing power circuit with
Only if it is the type with threaded joins (rather than the thinner
steel folded type with a join all the way along the side), and they are
in good order. You would need to test the earth integrity at each socket
On Wed, 5 Dec 2007 10:24:42 -0800 (PST) someone who may be olo
I missed this in my earlier reply.
So far probably not good. The old box was connected to the conduit
by a screwed fitting to maintain protective conductor continuity. I
suspect you have not restored this, let alone restored it properly
and then tested it properly.
In the above I assumed that the old socket had been wired properly
using the conduit as a protective conductor. There are other
I think you are out of your depth and should get a proper
electrician, one who has worked with conduit, to deal with it.
thanks for your valuable input
no, normal single 13A socket, I say ceramic because it's not plastic
nor metal, when chipped it looks like ceramic, it's MK brand
right, I see - I thought it was without earth because I didn't know
it's in metal conduit, yes I have threaded one
I take it I can't use CH pipes for earthing then =(
you are right I didn't realise earth was in the conduit, I will
restore it now
So anyone has any DIY method for testing earth integrity in sockets ?
I have multimeter but I guess that's not enough ... Which specialised
equipment do I need ? What are the alternatives ?
On Thu, 6 Dec 2007 02:08:31 -0800 (PST) someone who may be olo
There are very many different types of plastic, each with slightly
different properties. For example some are soft and some are hard.
Your socket is probably made out of some form of hard plastic. I
have seen ceramic sockets, but not switched 13A ones.
It must be tested at a high current, to be fairly sure it will not
break under fault conditions. Testing with equipment that only puts
a low current through the protective conductor may mean that when
there is a fault a joint fails.
There are, obviously, meters which do this. However, from your
postings I do not think you are able to cope with conduit wiring (it
is reasonably specialised) and so I'm not going to tell you what
Jobs on conduit wiring can be done by DIYers, if they know what they
are doing. The first step would be to get a book out of the library
(it would need to be a technical library) which explains the basics
of the system. You could then ask questions here.
An alternative is to measure and do calculations on the fault
currents that would be present in the system. It is sometimes then
possible to extend the conduit system using twin and earth.
Connections can then be made in junction boxes and this would be a
way to connect this new socket. However, the measurements and
calculations can't be skimped, as under some circumstances the
protective conductor in the twin & earth is unable to cope with the
fault currents. In that case another form of cable is needed.
OK, if it is the fairly solid stuff, then there is a fair chance it was
being used as a Circuit Protective Conductor (CPC aka "earth"). The clue
would be how the conduit was terminated at the socket. Normally the
socket would have a metal back box, and the conduit would have
terminated at the box with a screwed gland of some description. There
would also usually be a link wire from the back box to the earth
terminal of the socket.
If you have a proper earthed conduit - then you can and should use that.
But without special tooling to bend and thread the stuff you are far
more limited in what you can do. Obviously you can terminate it
correctly at the existing back box position and wire a spur from there
in T&E rather than twin and conduit should you need to.
You need to make sure the metal to metal contact surfaces and shiny and
clean, and done up tightly.
The ideal bit of equipment is an "earth fault loop tester" - this
creates a brief temporary fault to earth at the socket position and
measures the resistance of the whole round trip connection from line to
earth. It gives a very good indication of how it would perform during an
You can however do some tests with a multimeter. Exactly what you can do
will depend on your circuit topology. Do you know if this is a ring
circuit or a radial? There are more elaborate tests you can do on a
ring, however I will describe one that should work on any circuit...
One simple test would be with a long fly lead (a bit of spare flex etc).
Set your meter to its lowest resistance range, and measure (or null out
if it lets you) the resistance of the test leads and the fly lead. You
could then connect one end to the main earth terminal - may be in the
CU, or may be outside - and the other end to the earth terminal of a
plug. Connect the plug at your socket position and you should read a low
resistance (probably under 1 ohm allowing for the resistance of the test
leads). To be on the safe side - do these tests with all the power
switched off at the main switch.
Before doing the above I would suggest getting a simple plug in socket
tester. These are usually less than £20:
can use this to get a quick go/nogo indication for all the sockets.
It will tell you if you have any gross errors before you start looking
at the fine detail.
You may find the following article on circuit faults and their testing
handy for some background information:
it could be plastic, but it (the old socket that I'm replacing)
doesn't have any connecting point for earth, that's why I reckoned
it'd be ceramic, also it's not switched
true, but low current testing is still better than nothing,
You see I am only replacing single socket for double one, I'm not
touching anything else, surely I only need to test my new connection
between double back box and threaded metal conduit, the rest *should
I am aware that whoever touches the installation last is responsible
for the whole thing, but let's be reasonable - it's been OK for 30-40
years, if I wire my replacement socket right I won't make the whole
thing any worse
it is very solid stuff, and the conduit is terminated with screwed
gland in the box - it is my CPC then
wire a spur from there
yes, I will need a spur from this socket one day (will use T&E for
until I posted here I didn't know that the screwed gland plays such an
important role - thanks
many thanks for the test - both cables end in earth (same potential),
by knowing impedance of the "CU one" I can calculate impedance of the
"socket one" as
R(measured) = R(CU) + R(socket)
If R(socket) < 1 ohm then it's a good earthing.
I understand this is low voltage only test and things can break under
load, but I just want to minimise the risk for now.
Then I plan to learn how to test the earthing in the whole
installation (not only this one socket)
this is a ring
it's a ring circuit, installation hasn't been modified at all, there
are 2 pairs of cables in the back box, and this is true for the other
socket in this bedroom, the ring then goes to the kitchen and back to
CU (I know cause I had some floorboards up during the summer)
when I was buing the house surveyor mentioned in his report that there
is limited number of sockets in the property (indeed only two single
ones in bedroom).
what are the other interesting tests I can do on ring circuit then ?
btw, ta - for the wiki link, very good reading
There ought to be a connection point for earth somewhere - it is after
all one of the pins on every plug that will go into it! ;-)
It will prove you have continuity of earth (which is a good thing). It
does not stress the connection at all - but given the circumstance it is
unlikely to be the bit you have just remade that is likely to fail under
You could argue that. Although you are supposed to attend other matters
like ensuing that main equipotential bonding is up to modern standards
whenever you carry out any change.
Yup with conduit systems and some cable types like MICC or SWA, glands
are very important.
?title=Terminating_SWA>> You need to make sure the metal to metal contact surfaces and shiny and
Not quite sure I follow that... but basically if you measure the
resistance between the earth terminal at the CU and the earth at the
socket you should see a nice low value.
Normally using an earth loop tester at the furthest socket from the CU
will tell you most of what you need to know. You can also carry out
round trip tests at the CU.
this is a ring
Fair enough (might be worth mentioning that there would probably be two
cables to each socket (except the last one) on a radial as well.
Yup, common problem with houses built in the 50s - 70s Often not old
enough to have been fully rewired, but not equipped for modern usage.
Round trip tests - with a ring you have two neutrals, two lives, two
earths. So you can test the resistance of the complete ring and prove
the integrity of it - the wire resistance table at the end of the wiki
article I posted last time gives you some expected results.
For a fuller description of testing a copy of the On Site Guide is worth
(Note that a new version (for the 17th edition of the wiring regs) will
be out next month)
One tries ;-)