High kA & RCD's

Hi All,

Looking for a bit of advice on the following.

We have a DB with a calculated PSCC of 13kA so will install MCB's at 15kA.

From the MCB though, we then will immediately go through RCD's but most of these on the market are rated at 6kA only with some at 10kA. As it is the MCB that provides the short circuit protection and not the RCD, will the 10kA or even 6kA rating of the RCD then be sufficient. 

Also,  if there was a dead short between line & earth, would the MCB not operate before the RCD??

Thank you in advance. 


  • if there was a dead short between line & earth, would the MCB not operate before the RCD??

    Generally yes - but you'd normally have to refer to manufacturer's data to verify that - see Reg 536.4.2.4.

       - Andy.

  • If you are getting a bit stuck, then sometimes, and it may not apply here, but the nicest thing can be to have a death or glory fuse in the supply that cuts off if there is a real silver spanner incident that gives a high current, and means you are not relying on the MCBs at all for that corner case.

    For all normal overloads, and faults that are not really zero resistance - which is most of them, the MCBs/ RCBOs etc will still all work in the normal way.

    Folk often describe this as using  the fuse for 'limiting the current', but in practice it is more a case of it limiting the total energy let-through by cutting off the fault current before it has time to do too much damage. ( The current of course  is really limited by the circuit and supply impedances in the normal way until the fuse opens ) The curves are expressed in a way that makes it look like an equivalent reduced fault current, but realise that this hides the fact that when things get faster than one cycle, the fuse blowing  waveforms are far more complex than that.

    (it is exactly this thinking that allows domestic consumer units full of '6kA' MCBs behind any suitable  house service fuse of 100A or less, on any supply with a PSSC up to 16kA.)

    typical curves for this equivalent fault current as below.

    (PS Here for small faults the peak current is 2.8 times the prospective RMS fault - that being the p-p value you see if the fault comes on in the wrong point of the waveform. This is the limit line to the left,  and applies for small faults, while the PSSC is perhaps less than 10-20 times the fuse rating. Then, for any given fuse rating, there is a much less steep line, that applies once you get into the 'blows in less than a full cycle' regime - so you look up your metered PSSC reading of 15kA or whatever on the bottom axis, run your finger up until it intersects the line for say the 63A fuse, and find 9kA peak - which you can read across and see it does comparable damage to a fault of more like 3kA RMS without the 'top cover' of that fuse.
    There are various limiting fuse designs and it critically relies on the internal arc quenching so some care is needed to select one with the right performance - and it must also have an adequate breaking capacity of course..)