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?

  • 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.

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