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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

Parents
  • 0

    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?

    It certainly does. In any TN system overcurrent protective devices (MCBs or fuses) normally provide ADS - the primary protection against electric shock (where an earthed part is made like by a fault). The supplier's PE is connected to N (either at the cut-out in the case of PME systems or at the substation (TN-S)) so a L-PE fault should cause a large current to flow (at least a couple of hundred amps, if not several thousands), so the 5A fuse should blow pretty rapidly, making things safe. 30mA RCDs are really only for additional protection.

    On TT systems it's slightly different as the use of a separate electrode for the consumer's earth means there's a considerable extra resistance in the fault path (soil isn't that good a conductor) so earth fault currents are usually too low to blow fused (or trip MCBs), so in that case RCDs are needed for ADS. That could be the same 30mA RCD used for additional protection, or often a much higher rated one (100mA or 300mA) with 30mA ones used downstream for additional protection where needed.

       - Andy.

Reply
  • 0

    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?

    It certainly does. In any TN system overcurrent protective devices (MCBs or fuses) normally provide ADS - the primary protection against electric shock (where an earthed part is made like by a fault). The supplier's PE is connected to N (either at the cut-out in the case of PME systems or at the substation (TN-S)) so a L-PE fault should cause a large current to flow (at least a couple of hundred amps, if not several thousands), so the 5A fuse should blow pretty rapidly, making things safe. 30mA RCDs are really only for additional protection.

    On TT systems it's slightly different as the use of a separate electrode for the consumer's earth means there's a considerable extra resistance in the fault path (soil isn't that good a conductor) so earth fault currents are usually too low to blow fused (or trip MCBs), so in that case RCDs are needed for ADS. That could be the same 30mA RCD used for additional protection, or often a much higher rated one (100mA or 300mA) with 30mA ones used downstream for additional protection where needed.

       - Andy.

Children
  • 0 in reply to AJJewsbury

    Hi Andy, I appreciate the thought out reply and explanation of ADS!

    Reading your reply, it seems as though I may have misunderstood how an earthing arrangement actually works so it looks like I need to hit the books again. My understanding of BS3036 is that, in the event of L-PE fault, the fuse wouldn't break as it's not a short circuit fault. So if I was to come along and touch a light fitting which was live, the fuse wouldn't blow as that's what we need RCDs for.

    So, now that my view has shifted due to your reply and others, a BS3036 fuse will achieve ADS in an alloted time (dependant of Table 41.2) if there was a L-N fault and/or a L-PE fault. Is that correct? Basically I'd potentially get a short burst of, how you said, "a hundred if not several thousands" of amps which could be enough to kill me, hence making it dangerous. Do we look to achieve ADS in any fault condition?

    So if a BS3036 fuse from decades ago can achieve ADS, why are RCDs mandatory as additional protection on new installations?

    As they say, everyday is a school day! Thanks so much, Andy.

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