How to Test Lighting Earth Circuit

Discussion in 'Electricians' Talk' started by marc1977, Oct 24, 2016.

  1. marc1977

    marc1977 Member


    I have recently moved into a house. It has a new CU and all wires from this into the house have 3 core earch wire. However when ive inspected the lighting circuit itself most of the lights appear to have an earth but a few of them dont. however most of the switches in the house are not earthed, they just have 2 core wire.

    1) Whats the best way for me to check the Earth on the lights in the house that appear to have it?

    2) Whats the general guidance on the above? Given its mainly switches without an earth should I rip the house apart and get those rewired or would you just use plastic switches and live with them unearthed?

  2. PaulBlackpool

    PaulBlackpool Screwfix Select

    I am just DIY but I have this problem with my electrician who says he cannot fully test my installation as not all my lights have earths even though I have provided them separately to metal light fittings. As I understand it 1.0mm sq. T & E or 1.5 mm T & E should be used for lighting circuits. Where the light fittings are not metal and supplied from the ceiling rose with a twin flex the earth (CPC) should be terminated in the rose. Where the pendant lamp-holder is metal it should have three core flex to include the earth . At wall switches the earth should be terminated in the back-box for plastic switches and on the appropriate terminal as well where the faceplate is metal. So an earth is available for testing on all fittings and all switches
    I now have three lighting circuits. The two new ones comply, The old one does not but as it in PVC singles, appears sound, is protected by an RCD and a 6 amp MCB and just consists of 6 lights I am not losing any sleep over it for the time being.
  3. Comlec

    Comlec Screwfix Select

    Firstly, and most importantly, if you have any concerns about the safety of the electrics in your new house you should ask an electrician to carry out an Electrical Installation Condition Report (EICR).

    It is not uncommon for the CPC (earth) to be missing on lighting circuits. In itself, it does not make the installation dangerous. However care must be taken not to install any fittings that require an earth (Such as metal switches).

    It would help if you could post a picture of one of your switches showing the back box.
  4. marc1977

    marc1977 Member

    Thanks - how can i test the earth is good on the lights that have it though - can a multimeter do this?
  5. Comlec

    Comlec Screwfix Select

    To properly test the continuity of any cable you need a test instrument (see how it is done here). If you do not have a test instrument and know how to use it then leave it alone.

    A multimeter, used correctly, will be able to tell you if there is continuity but not whether the installation is safe.
    FatHands likes this.
  6. fire

    fire Well-Known Member

    Ok this is a test that needs some expertise to carry out.

    I don't say this often but i think in this case it is relevant, a forum is not going to be able to fully provide you with the comprehensive know how to properly test this.
    You need to know how the circuit should be wired and how the circuit could be miswired.

    Light switches only have the earth terminated in the backbox of the light switch. If there is no earth wire at all then someone would have cut the earth back which is in my opinion bad practice as a switch back box has a perfectly good earth terminal to terminate the earth core. Same goes for the ceiling rose fitting, it has a perfectly good earth terminal block so it should be used even if it is in a plastic housing.

    If your circuit is run in singles through metal conduit then that conduit is the earth so it is normal to see no earth wires and some as the conduit is actually the earth.

    When you bought the house surely the solicitors etc would have included an electrical certificate of safe electrics? It is a must inclusion when you buy a house today.
  7. Bazza-spark

    Bazza-spark Screwfix Select

    No it isn't.

    Kind regards
    TP&N likes this.
  8. fire

    fire Well-Known Member

    Maybe i should have said, it is a requirement if you buy your house taking out a mortgage.
    Most Bank or building societies must have the entire condition of the property to grant a mortgage.
  9. Dr Bodgit

    Dr Bodgit Super Member

    I bought a house in January, mortgage is with Santander and they just send a valuation person round to look at the place (often from the outside only), and ensure its worth what you are going to pay for it.
  10. Bazza

    Bazza Screwfix Select

    It is not even a requirement for a mortgage. Normally all the mortgage company needs is a surveyor's report basically saying that the house is worth the money being paid for it.

    In my experience, your average surveyor does not have the electrical competence to make anything other than a weasel-worded comment on the general state of the electrical installation.

    It is up to the house buyer should he/she want to pay to have a separate extensive test and report (EICR) done on the installation. Before or after purchase!
    TP&N likes this.
  11. Bazza

    Bazza Screwfix Select

    Back to your original question. There is a standard test which is called an R2 test. It is used to confirm the continuity of the earth (cpc) of an installation. This comprises of a very long lead being connected to the main earth terminal of the installation.
    You then go to each lighting point, switch etc with a continuity test meter and confirm if there is a low enough resistance when measured from the end of the long lead to the earthing point in the switch etc.

    NOTE: this is what is called a dead test and must be done with the power off!

    I'll try and find a diagram and post it, but the process does involve you getting inside your consumer unit to make temporary connections and (of course) for you to have the necessary test equipment.
  12. Dr Bodgit

    Dr Bodgit Super Member

    Presumably an R2 test could be done using a long length of wire (spare length of T&E) and an AVO. With the power OFF.
  13. senileoldgit

    senileoldgit New Member

  14. senileoldgit

    senileoldgit New Member

    Fire your advice is spot on. I second it!
  15. senileoldgit

    senileoldgit New Member

    An Avo is basically just a voltage / current tester. It is not designed for the test you need to do. The correct meters cost and you ideally need training to use and interoperate properly and see if measurements are within the specification of the current regulations. Another point is that you do not necessarily need to use a long piece of wire. There is another method that can be used that eliminates using the wire.
  16. Bazza

    Bazza Screwfix Select

    Yes. Power OFF is the first and most necessary thing. Check out safe isolation procedures in the WIKI to make sure that everything is isolated properly.
    As you probably need to take off the cover from the CU, then you will obviously turn off the MAIN SWITCH first!

    Any l-o-n-g piece of conductor will do. If you want to note the actual R2 resistnace then you'll need to measure the resistance of your long lead and deduct that from the measurements on your meter.
  17. TP&N

    TP&N Active Member

    I agree with Bazza on this one R2 for a novice is the way to go
  18. Dr Bodgit

    Dr Bodgit Super Member

    Yes when I said AVO I was referring to a multi-meter (amps-volts-ohms). Mine has resistance and continuity (I think with a mV drop readout).
  19. senileoldgit

    senileoldgit New Member

    Multi-meters generally do not have a facility to null the resistance of the test leads and meter, so you probably would not get the accuracy due to this offset.
  20. senileoldgit

    senileoldgit New Member

    You also do not have the facility to do the other test required (Insulation Test) A domestic test meter output 500V to test the insulation. You multi-meter can't do that. If you really need to do your own testing look up Dilog for a good budget meter. Then you need to do some reading and maybe invest in some training then you can also sort your neighbours wiring out too.

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