100mA RCD location TT??

So I believe a 100mA time delay RCD is required for a TT system if a split load board but not an RCBO board, why the difference?? Also any regs reference please?

I have been told that I can just swap out the isolator switch in the board for an RCD however I read that the RCD should be upstream in a plastic enclosure?

Any input appriciated Slight smile

Metal enclosure BTW...

  • Ah, well you have re-opened a proverbial can of worms.

    So step back from what you 'have heard' for a moment. The risk with a TT supply, is that as the earth path back to the substation includes a significant chunk of high-ish resistance soil, so the RCD is the only thing that provides any automatic disconnection for earth faults, as there is no chance of blowing a fuse.

    So if there is any credible mechanism that could allow the pre-RCD live to touch the TT earth, it would be very dangerous, as everything 'earthed'  would become live, but there would be no disconnection.

    So far so good, but how best to guarantee this is where it becomes a matter of opinion.

    An all RCBO board with a switch incomer is only OK if,

    a) the incoming live tail could not possibly come out - perhaps clamping mean that can be assured, in some designs.

    b) the pre-RCBO bus-bar is insulated, so a stray CPC or a paperclip falling into the box,  could not enliven the box.

    c) The total earth leakage from all circuits just below tthe threshold of tripping the RCBOs cannot add to a total that with the earthing impedance would make the CPC rise to a dangerous voltage.

    d) the chance of any one RCBO failing is acceptably low.

    There are fewer conditions to an RCD in a plastic box.

    a) even if the incoming tail isl loose, it touches a plastic box - no risk.

    b) There is no need for any CPC in the plastic box

    c) The trip threshold and earth rod resistnace is set by the RCD.

    d) The chance of one RCD failing has to be compared with that of any one of several RCBOS.

    Some pedantic folk consider a plastic box with an RCD in it to contravene BS7671.

    The rest, as they say, is entirely a matter for you.



  • Thanks Mike,

    I get the risk and I see that an RCD in a plastic box upstream would be the answer.

    I am just confused as to why the likelyhood of a stray tail or bus bar doesnt affect a RCBO board only a split load board?

Reply Children
  • It may come down to how a given consumer unit is constructed/wired.  An insulated bus bar is unlikely to come adrift, while a bunch of live wires looping between the main switch and the RCDs might.

  • well the bus bar is held both by the main switch and several RCBOS, so it is not a single loosened screw to danger but several. Some dual RCD boards have the pre-RCD links to the switch in a similar way but others are just flappy flex and in at least one case, Volex from memory, plain enameled wire, held only by one screw at each end - so a single loose screw would be more serious.

    It is all down to a game of chance and your appetite for relative risks.

    An alternative may be to put a delay RCD in as the main switch and add some positive anchoring to the tails.

    Personally I'm firmly in favour of an RCD in its own plastic box, but I'm not usually working on minimum budget jobs these days - I can full see that simple updates to an existing board are attractive, and with care and perhaps some extra supports and/or sleeving are probably possible.


  • A bus bar supported by a main switch, and several RCBOs is probably not so vulnerable to a single loose screw failure mode. Some of the split load designs have floppy wires to the RCD inputs, and in at least one design Volex I think, just a thick enameled wire. To achieve a similar safely to the bus-bar  with that one, in terms of loose screws at least, some additional cable anchoring and maybe carefully placed sleeving would be needed.

    I must admit I don't like the RCBO bus-bar idea either, personally I prefer the RCD in a box, but I can see that in budget limited situations it is attractive.