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Earth Loop Test

robopj

Our ground floor extension has 7x 12v Halogen with the necessary 240v to 12v transformers on a circuit from the wall switch. I would like to upgrade to integrated LED lamps. I understand that if I am competent, I can exchange the fittings and still be within the Building Regs rules. The new LED lamps (Collingwood H2 Lite CSP) need an earth connection.

The existing round 5A junction boxes have an in and out for the circuit (1mm twin and earth) and a feed to the transformer for each light (the transformer doesn't require an earth connection).

To verify the earth connection, I have removed the junction box and transformer for the lamp at the end of the circuit, and temporarily connected a 13A 3 pin socket and I have plugged in a KewTech Loopcheck 07. The KewTech reports all the connections are present, (so earth is connected for the circuit), but it is saying "Check loop test" because it has detected a safety earth path between 1.80 ohms and 92 ohms. 

If I plug the Kewtech into a proper 13A power circuit, it reports everything is OK.

My questions

a) Could the 240v / 12v transformers that are before the Kewtech cause an impedance issue?

b) Could one of the junction boxes have a non perfect connection and therefore cause an impedance issue?

c) Is the use of the Kewtech on a lighting circuit a valid test?

I could move the temporary 13A socket to the start of the circuit, not connect the outgoing circuit and use the Kewtech at that point. If it said everything was OK, I could work around each lamp until the Kewtech complains. 

Alternatively, I could look at a solution that doesn't need an earth connection but since the Kewtech has found an issue, it would sit in the back of my mind. 

Thank you

 

 

  • 0

    What's your Earthing system? If it's TT (i.e. relies on your installation's own electrode, rather than a DNO supplied earth terminal) then earth loop figures up to say 200 Ohms would be quite normal.

    If it's TN then a bit more investigation would be required, but ask the obvious questions first is my motto!

       - Andy.

  • 0

    That is an innovative way of checking for earth continuity.

    1.8 Ω seems an odd threshold. Most socket circuits would be on a 32 A Type B circuit breaker, for which the maximum Zs (i.e. earth fault loop impedance) is 1.37 Ω (Table 41.3 of BS 7671).

    However, you may be aware that the resistance of a cable is inversely proportional to its cross-sectional area, so the 1 mm² T&E may well have an impedance > 1.8 Ω. Moreover, lights are generally on a 6 A circuit breaker, for which maximum Zs is 7.28 Ω.

    In answer to question (b): yes, that is a possibility. Naturally, you can check for that when you change the lamp fittings.

    Question (c): it appears to have verified earth continuity, but may give a misleading result as explained above.

  • 0 in reply to Chris Pearson

    1.8 Ω seems an odd threshold.

    Humm, closer to 80% of a B20's limit ... I wonder if it was originally designed with continental style radials in mind?

      - Andy.

  • 0 in reply to AJJewsbury

    Thanks   

    It looks like a TN-S to me. The house was built around 1904, and the supply looks like lead casing near the main supply fuse. There is a copper coil that connects from the main supply cable to green earth cables. 

  • 0 in reply to Chris Pearson

    Thanks   It could be a non perfect connection or it could be that the Kewtech 107 isn't suitable for 1mm cables. The downstairs light circuit has a 5A push on/push off RCD (Could probably do with a new consumer board) 

  • 0 in reply to Chris Pearson
    Chris Pearson said:
    That is an innovative way of checking for earth continuity.

    It is, but not the safest of approaches, and not one that conforms to BS 7671 which requires the test method to be in accordance with BS EN 61557-series (Regulation 643.1).

    The earth continuity test should be tested as a 'dead' test before re-energisation of the circuit, for safety (again, Regulation 643.1).

    robopj said:
    a) Could the 240v / 12v transformers that are before the Kewtech cause an impedance issue?

    If you are connecting the kewtech tester on the 12 V side of the transformer, it will not work at all. Otherwise,. if it's being connected on the 230 V side, no.

    robopj said:
    b) Could one of the junction boxes have a non perfect connection and therefore cause an impedance issue?

    Yes, that is a possibility.

    robopj said:
    c) Is the use of the Kewtech on a lighting circuit a valid test?

    Not really accepted by industry for this purpose, as discussed above. At one time, BS 7671 ('Wiring Regulations') didn't care how you tested continuity (as long as it was done 'dead' and with a safe test method), but now it requires specific instruments to a specified series of product standards.

    It's OK for a quick check of a socket-outlet, and is recommended for appliance repairers going to people's houses, for example, to check a socket-outlet is OK before using a socket-outlet that's already in service, but not really parts of the electrical installation itself, or for putting a new or replacement socket-outlet into service.

    Having said that, any testing is better than no testing ...

  • 0 in reply to robopj

    It looks like a TN-S to me.

    It's not unknown for older lead sheathed cables to have "rotted" a bit in the ground (especially at connections) and problems with the sheath (i.e. Earth) often go unnoticed for quite a while.

    It might be worth plugging in your little tester in a few different sockets around the house ... if most/all, especially those close to the origin, show the same problem it might be an issue with the supplier's Earth. Maybe not the most likely cause, but one to keep in mind.

       - Andy.

  • 0
    AJJewsbury said:
    It might be worth plugging in your little tester in a few different sockets around the house ...

    From the sound of it, the sockets are OK.

    robopj said:
    If I plug the Kewtech into a proper 13A power circuit, it reports everything is OK.

    However, that does not rule out a slightly high Ze.

    If you mention a high EFLI to the DNO (not the supplier), they might turn up fairly pronto.

  • 0 in reply to gkenyon

    Thanks  

    I appreciate I'm not following industry standards. My thoughts were that the Kewtech 107 gave a good indication of the socket condition on a ring main, then perhaps it would test a lighting circuit to earth continuity. There is continuity but the impedance is triggering a warning on the 107. I'll contact Kewtech to see if they have any guidance.

  • 0 in reply to Chris Pearson

     

    Yes, the Kewtech 107 is OK on all my ring main sockets. It could be a dodgy connection, or it is not suitable for use on 1mm2 T&E or something else. It would be good to know the actual impedance reading. 

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