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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

 

 

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  • 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 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) 

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  • 0 in reply to robopj
    robopj said:
    it could be that the Kewtech 107 isn't suitable for 1mm cables.

    It's not a simple suitable/not suitable - more that 1mm² cables have a higher resistance per meter than thicker cables, so moderate lengths are more likely to push things over some arbitrary limit. E.g. 1mm² have a resistance of about 36.2 milli-Ohms/metre whereas modern 2.5mm² T&E is 19.51 mΩ/m - so you need about 50m of 2.5mm² cable to add 1Ω to the earth loop impedance (or likely significantly longer lengths if it's part of a ring circuit, so there are two cables in parallel to some extent) whereas less than 28m of 1mm² will have the same effect.

      - Andy.

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