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.
Questions:
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 ?
On Wed, 5 Dec 2007 10:24:42 -0800 (PST) someone who may be olo wrote this:-
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 appropriate earth.
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 position.
On Wed, 5 Dec 2007 10:24:42 -0800 (PST) someone who may be olo wrote this:-
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 possibilities.
I think you are out of your depth and should get a proper electrician, one who has worked with conduit, to deal with it.
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 wrote this:-
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 they are.
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.
Nooooo!
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 actual fault.
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:
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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 be intact* 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 that) 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 ?
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 load.
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.
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?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 buying:
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that a new version (for the 17th edition of the wiring regs) will be out next month)
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