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

    ADS provides for a nice simple fault between a line conductor and an exposed-conductive-part (or c.p.c.) - on TN systems that normally works perfectly well with either fuses or MCB and has been the norm where there's exposed accessible metalwork on appliances for the best part of a century now (although it's been gradually tightened up over the decades). ADS probably covers maybe 99% shock risks where failure of basic insulation produces a shock hazard.

    Additional protection is over and above ADS - it provides some extra safety where the route of the shock current isn't via exposed metalwork. In theory that shouldn't happen as hazardous live conductors should be surrounded by either earthed metal or double/reinforced insulation tough enough for the environment, but in practice there are all sorts of ways where ADS can be side-stepped - e.g. a screw or nail driven into a plastic sheathed cable, running a lawn mower over its flex or catastrophic damage to an appliance, or simply a flex pulled out of a badly terminated connector. For these sorts of situations 30mA RCDs are invaluable - but in these situations they only come into action after someone starts to get a shock - unlike ADS where disconnection happens as soon as the fault occurs whether anyone's touching it or not. RCDs also aren't the most reliable beasts in the world (some reckon around 7% of installed ones wouldn't trip correctly) - perhaps why they have Test buttons, when MCBs don't.. So 30mA RCDs are an added extra, a bit of defence in depth - the first line is ADS (or double/reinforced insulation).

    You don't necessarily need 30mA RCDs on every circuit either - for domestics it's only for sockets, lighting circuits, soft sheathed cables concealed in walls and bathrooms -  so there's plenty of other things - from distribution circuits and circuits supplying immersion heater to cookers to EV points where the 30mA RCD protection may well not be required (especially if you can avoid cables hidden in walls).

       - Andy.

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  • 0

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

    ADS provides for a nice simple fault between a line conductor and an exposed-conductive-part (or c.p.c.) - on TN systems that normally works perfectly well with either fuses or MCB and has been the norm where there's exposed accessible metalwork on appliances for the best part of a century now (although it's been gradually tightened up over the decades). ADS probably covers maybe 99% shock risks where failure of basic insulation produces a shock hazard.

    Additional protection is over and above ADS - it provides some extra safety where the route of the shock current isn't via exposed metalwork. In theory that shouldn't happen as hazardous live conductors should be surrounded by either earthed metal or double/reinforced insulation tough enough for the environment, but in practice there are all sorts of ways where ADS can be side-stepped - e.g. a screw or nail driven into a plastic sheathed cable, running a lawn mower over its flex or catastrophic damage to an appliance, or simply a flex pulled out of a badly terminated connector. For these sorts of situations 30mA RCDs are invaluable - but in these situations they only come into action after someone starts to get a shock - unlike ADS where disconnection happens as soon as the fault occurs whether anyone's touching it or not. RCDs also aren't the most reliable beasts in the world (some reckon around 7% of installed ones wouldn't trip correctly) - perhaps why they have Test buttons, when MCBs don't.. So 30mA RCDs are an added extra, a bit of defence in depth - the first line is ADS (or double/reinforced insulation).

    You don't necessarily need 30mA RCDs on every circuit either - for domestics it's only for sockets, lighting circuits, soft sheathed cables concealed in walls and bathrooms -  so there's plenty of other things - from distribution circuits and circuits supplying immersion heater to cookers to EV points where the 30mA RCD protection may well not be required (especially if you can avoid cables hidden in walls).

       - Andy.

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