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2nd Year Apprentice - Question on changing a light fitting & BS3036

Joseph Day

Hi all, new to the forum and in my second year as an electrical install sparks.

So I've been asked by my girlfriend's mum to change out a light fitting they've bought which I'm led to believe is a Class 1 fitting. This is the following setup:

  • The existing CU is a Wylex BS3036 rewireable fuseboard with no RCD protection.
  • I've yet to confirm whether an MET is installed and what the earthing arrangement actually is.

I'm naturally safety conscious, but I'm not sure if I'm overthinking it. Because the fitting is a Class 1, and because it's next to a bunk bed, I'm worried if it gets knocked repeatedly the terminations will work their way loose and the fitting develops a L-E fault, and due to no RCD protection, wouldn't trip leading to the fitting becoming live.

If I can confirm the earthing arrangement is suitable, would I be OK swapping out the light? As far as I know, all cables are the red-black type with CPC, but is the CPC redundant if there is no form of main earthing conductor? What would happen if a fault was to develop with no MET present?

Because there is no RCD protection present in the CU, and if a fault was to develop L-E, it would just carry on putting out current. The installation was installed well before 18th, so is this a grey area in relation to RCD protection? I appreciate I'm altering a circuit (swapping out the fitting), which I'm aware requires RCD protection of no more than 30mA.

Additional question: If in this exact case a L-E fault would occur, would anything happen at the DNO transformer? Would the circuit remain energised and still put out 230V (or the potential for 6A)? I've got 7671 and the Student's Guide to 7671 sat next to me as I type this, and page 127 of the latter shows the prospective fault path leading back to the transformer, but in the case of the 3036 CU, does this actually serve any purpose?

Thanks in advance,
Joe

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  • 0

    I would test the earth loop impedance at the light fitting, and if found satisfactory go ahead and fit the light.

    If the earth loop impedance is too high I would be reluctant to replace an all insulated pendant with a class one light fitting. If the whole installation has a too high earth loop impedance, rather than just the lighting circuit, then continued use of the installation is rather a risk, and as a MINIMUM an up front RCD should be fitted urgently.

    Simply testing the earth loop impedance and getting a good reading does NOT mean that all is well, it wont for example detect an installation that is only earthed via a water heater CPC and and a metallic water pipe. such an installation is non compliant, but arguably safe-ish in the short term.

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  • 0

    I would test the earth loop impedance at the light fitting, and if found satisfactory go ahead and fit the light.

    If the earth loop impedance is too high I would be reluctant to replace an all insulated pendant with a class one light fitting. If the whole installation has a too high earth loop impedance, rather than just the lighting circuit, then continued use of the installation is rather a risk, and as a MINIMUM an up front RCD should be fitted urgently.

    Simply testing the earth loop impedance and getting a good reading does NOT mean that all is well, it wont for example detect an installation that is only earthed via a water heater CPC and and a metallic water pipe. such an installation is non compliant, but arguably safe-ish in the short term.

Children
  • 0 in reply to broadgage

    Hi broadgage, thanks for the reply.

    broadgage said:
    I would test the earth loop impedance at the light fitting, and if found satisfactory go ahead and fit the light.

    Sound advice and this will be my second priority after confirming the MET.

    broadgage said:
    and as a MINIMUM an up front RCD should be fitted urgently.

    Would this be an RCD protecting the full fuseboard? I.e. an external Whiska box containing said RCD? Whenever I come across 3036, we (as a company) recommend upgrading to minimum SP RCBOs and I've never personally fitted RCDs into a new CU. What's your take on this?

    broadgage said:
    but arguably safe-ish in the short term.

    Could you elaborate on this a little more, please? Safe-ish how and how short term are we talking? Is it just a case of money being an obstacle or is there something a bit more sinister? I'd like to give sound advice, but I don't have the necessary experience and more importantly, I'm not deemed competent nor qualified! At the end of the day, it's about safety in my eyes.

  • 0 in reply to Joseph Day

    Yes, I suggest an up front RCD as a MINIMUM if the earth loop impedance is too high to promptly operate fuses. these are relatively cheap and are fitted in an insulated enclosure between the meter and the existing consumer unit.

    A better approach would be a new consumer unit that incorporates RCBOs for each circuit. That however is a considerable expense, and a lot of labour.

    If the installation is only earthed via say the CPC to a water heater and a metallic water pipe, then I consider that to be safe-ish in the short term if it passes an earth loop test.

    Contrary to regulations though, and in particular note that the CPC to the water heater is probably only 1.5mm and is very undersized for a main earthing conductor which purpose it is serving. What happens if someone disconnects the water heater to fit a new one ? That could leave the installation without ANY earth connection.

    And of course earthing to a water pipe has been prohibited for decades, it still works fine in practice until someone replaces the metal water pipe with a plastic one.

    Therefore "fortuitous" earthing as described is a lot better than no earthing and in MY VIEW is just about acceptable in the short term. Proper earthing, earthing should be installed

  • 0 in reply to broadgage

    safe-ish i n the context of 'earthed via the water pipes only' means it's not actually dangerous at the moment, but is at the mercy of the plumber or kitchen fitter changing a metal fitting to plastic when called out to do something totally unrelated and then  leaving the electrical system un-earthed without anyone  realising it.

    A dedicated earth connection should look like one, and be labelled as such.

    "earthed and seen to be earthed" if you like.


    M.

  • 0 in reply to broadgage
    broadgage said:
    A better approach would be a new consumer unit that incorporates RCBOs for each circuit. That however is a considerable expense, and a lot of labour.

    Not at the budget end of the market!

    Given that the devices at opposite ends comply with the relevant BS, it does bring into question why one would pay more than the basic cost.

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