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.



  • It depends. It may be that an upfront time-delay rcd is needed anyway. If you look at the standard, the rcd element of an rcbo must trip at 30mA (assuming a 30mA device), so its worst case leakage is 30mA. In theory, the worst case total leakage of 10 rcbos is 300mA. Is your earth rod good enough for this? Something to think about, especially with the larger rcbo boards. 



  • 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?

  • Thanks Alan,

    So its a metal enclosure split load board with 2x 30mA RCDs, I have a Ze of 23.8 ohms and pefc of 10A. The board has a 2 pole isolator incomer with no RCBO upstream.

  • so, look at the 2 RCD board and run exactly the same questions about insulated terminals and pre RCD wiring, and ask could a CPC or a non insulated  live credibly come loose in a dangerous way .

    What you want to avoid is a single fault to danger condition where one loose screw or wire off-cut could be fatal.

    It may be just as OK as soem designs of all-RCBO board, or not, depending on the  design.

    Despite what you seem to think, there is no reg that says that a dual RCD board can never be OK on TT, nor is there one that says RCBO boards never need an up-front RCD - some might and others might not depending on the internal details.


  • Also, with a TT supply, my employer (a DNO) requires “. . . on a TT System the tails between the M.U.A. meter and the metal clad customer distribution board must pass via a Time Delay 100mA RCD which is to be housed within a class two insulated enclosure before entering the metal clad distribution board via an insulated Gland . . . “. If this is not done, then we will not connect the supply. 



  • 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?

    One of the problems with a split board is the connections between the incomer and the RCCBs - usually just bits single insulated wires which are usually touching the metal case (or c.p.c.s/earth bar) - without ADS that doesn't give us the usual minimum of a double fault to danger, as we're relying on just one layer of insulation.

       - Andy.

  • 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.