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Three Phase RCD & MCB Wiring?

Nano Brewery
Please could somebody advise on the most cost effective procedure of wiring up RCDs & MCBs on a DIN Rail for independent circuit protection.  Tried Googling to no joy.  Plenty of Single Phase advice, images, videos.  If using 4 Pole MCBs & RCDs, would I be correct in assuming that similar to single phase, it's just a case of Busbar-ing the MCB Lives to the RCD Lives and taking the RCD to Neutral Bar?


Have seen diagrams (like below) where the Incoming L1, L2, L3 get split to separate DP MCBs, RCDs and SP MCBs for each phase.  Seems like a bit of a faff and a pricey option (3x DP MCBs + 3x RCDs + 3x SP MCBs) per circuit.


I have 3x Three Phase Circuits (13A, 40A, 48A) that I wish to protect independently with RCDs.


Thanks in advance!  Dan.

ddbd56232f43509726e961c7a00902b6-huge-image-20210208123140-1.png
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    no -it isn't you need a box fitted with a proper 3 phase busbar, which is really  3 or 4 isolated busses in one block

    Each maker has (slightly incompatible) variations on the theme.

     


           

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    mapj1:

    What you draw is OK for  a system where 3 phase comes in, but all loads are single phase.   In effect you have drawn 3 single phase consumer units - and that is one  legitimate approach, if your loads are like that.

    3 phase boards for 3 phase loads have a 3 pattern L1 L2 L3  bus bar so every tooth is only connected to the ones multiples of 3 places along.


    . And just to catch you, L and N  (pitch of two) and 3 phase and neutral (pitch of 4 ) busbar is also manufactured but rarer.


    an RCD for a 3 phase load must carry all the phases, and neutral if there is one, so that in normal use, it sees no net current.


    I suggest you contact the helplines of  one of the makers of commercial consumer units (Hagar, Eaton etc) to advise you about what they make, as there are a lot of options, and it is very easy to specify the wrong thing, the advice is free, so long as you do not string them along..

    Are the loads in your drawing correct, or just nice numbers ? Thinking diversity.

    M.

     




    Thanks M,


    Ah understood.  It isn't my drawing by the way, just something I found on the web.


    You've lost me a little with the Busbar info???...


    Was just trying to understand if it is possible to connect/wire up 4P RCCBs and 4P MCBs on a DIN Rail in the same manner as a 1P RCCBs and numerous 1P MCBs can be?  i.e. By connecting the L1 L2 L3 of the 4P RCCB to the L1 L2 L3 of the 4P MCB via a busbar or individual wiring.


    Only 3 Circuits of the 3-Phase Distribution Board will be feeding 3-Phase gear.  Rest of the gear will be single phase. 


    Circuit 1 - 48A - feeding a Control Panel which in turn will be switching 4x individual 3-phase Immersion Elements.


    Circuit 2 - 40A - feeding a 3-phase Instant Hot Water Heater.


    Circuit 3 - 13A - feeding a 3-phase Pump.


    Trying to avoid RCBOs or RCCB Sockets as these are pricey options.  Will have to go this route if need be though!


    Cheers.


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    Apologies for any confusion. I do intend to use brand new components from the same manufacturer. Probable Wylex in this situation.


    Was just trying to understand if it is possible to connect/wire up a 4P RCCB and a 4P MCB in the same manner as a 1P RCCB and numerous MCBs? i.e. By connecting the L1 L2 L3 of the 4P RCCB to the L1 L2 L3 of the 4P MCB via a Busbar or Individual wiring of L1 L2 L3?

    It's not just a matter of using components from a single manufacturer if you want to rely on their type approval - you have to assemble it according to their instructions ... which they likely don't have for anything not based on their normal layouts (but feel free to ask them of course).


    In terms of wiring, in theory it is that simple  - in practice a 3-phase MCB will usually have 3 pairs of terminals and a 3-phase RCCB will have 4 pairs (to include the N) - so simple bus-bars won't usually fit... you could then go for simple bits of wire between, but then you've got the problems of fault handling etc. that the bus-bars were probably designed to avoid (especially if you're relying on the type-test for a 16kA or better fault rating or the assembly). 3-phase MCBs that also include a N would be simpler, but they're rarer beasts (especially when it comes to actually buying one rather than just perusing catalogues) and then you'd need 4-P bus-bars which can be rarer still in the UK. Not impossible, but I'd advise a fair amount of caution. Be especially careful of examples of 'continental' practice from the internet - the UK supply system has a few characteristics (in particular a relatively high protective fault current - due to our approach of supplying relatively large loads single phase) that demands a bit more consideration that is needed elsewhere where the higher impedance of lower rated 3-phase supplies is the norm.


       - Andy.
  • 0
    mapj1:

    no -it isn't you need a box fitted with a proper 3 phase busbar, which is really  3 or 4 isolated busses in one block

    Each maker has (slightly incompatible) variations on the theme.

     


           

     




    Ah I see! ?  Are these adjustable length wise like normal Busbar's?  Thanks for sharing.


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    AJJewsbury:
    Apologies for any confusion. I do intend to use brand new components from the same manufacturer. Probable Wylex in this situation.


    Was just trying to understand if it is possible to connect/wire up a 4P RCCB and a 4P MCB in the same manner as a 1P RCCB and numerous MCBs? i.e. By connecting the L1 L2 L3 of the 4P RCCB to the L1 L2 L3 of the 4P MCB via a Busbar or Individual wiring of L1 L2 L3?

    It's not just a matter of using components from a single manufacturer if you want to rely on their type approval - you have to assemble it according to their instructions ... which they likely don't have for anything not based on their normal layouts (but feel free to ask them of course).


    In terms of wiring, in theory it is that simple  - in practice a 3-phase MCB will usually have 3 pairs of terminals and a 3-phase RCCB will have 4 pairs (to include the N) - so simple bus-bars won't usually fit... you could then go for simple bits of wire between, but then you've got the problems of fault handling etc. that the bus-bars were probably designed to avoid (especially if you're relying on the type-test for a 16kA or better fault rating or the assembly). 3-phase MCBs that also include a N would be simpler, but they're rarer beasts (especially when it comes to actually buying one rather than just perusing catalogues) and then you'd need 4-P bus-bars which can be rarer still in the UK. Not impossible, but I'd advise a fair amount of caution. Be especially careful of examples of 'continental' practice from the internet - the UK supply system has a few characteristics (in particular a relatively high protective fault current - due to our approach of supplying relatively large loads single phase) that demands a bit more consideration that is needed elsewhere where the higher impedance of lower rated 3-phase supplies is the norm.


       - Andy.




    Cheers Andy.  So it's possible but not normal practice or layout?  Have you ever seen it done?  Or are RCCBs nearly always downstream?  Or a RCBO used?


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    Cheers Andy.  So it's possible but not normal practice or layout?  Have you ever seen it done?  Or are RCCBs nearly always downstream?  Or a RCBO used?

    Traditionally RCDs have been a bit of a rarity on 3-phase DBs - since they tend to be more industrial so tend to be associated with wiring methods that don't fall foul of the 30mA RCD requirement (for soft sheathed cables embedded in walls etc), don't feed bathrooms and where industrial sockets need an RCD it's often placed at the socket for convenience. There's not really a 3-phase DB equivalent of the "split" CU we see for domestics. The manufacturers will no doubt try to sell you an RCBO, but an MCB in the board with a downstream RCCB will be just as effective and likely cheaper.  At least by using a normal 3-phase DB you get all their type testing for free as it were.


       - Andy.
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    Where is your Brewery? This quote is probably a bit high, someone may offer you a better one, perhaps with some beer attached!
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    At the risk of once again making it more complex than it really is.


    In a 3 phase board will be a 3 phase bus bar. If you are directed by competent product support people to chose the right ones, you can fit single module RCBOs into it, or single module  MCBs for that matter and have single phase circuits one per phase. Or if you prefer to have say all your lights on one phase, then blanking mdules are available so you go fit one miss two up the board, to create a single phase board, Normally you don't do that except for the odd circuit, as if you really wanted a single phase board you should have bought one.


    But a 3 phase RCBO with neutral  is harder, as really you need a 4 pole bus bar, and that is not so common, though I suppose it will come, after all we've only had RCDs in common use for about 40 years now.

    So commonly  then , a 3 phase MCB goes in the box for the 3 phase load and  a 4 pole RCD goes somewhere else in another  box nearer the load.

    Along side it go  a sprinkling of single phase MCBs or RCBOs to feed the single phase stuff.

    Here is a pic of a typical "8 way 3 phase"  box from t'interet, to illustrate the key points  (Thanks to Coppice Electrical). Note that this has rather more covers removed than you'd expect if it was to be energised, but shows clearly the sort of thing you want to be asking about.

    Here the 3 phase MCB mounts on the right with the Brown black grey cores going out, and most of the  single phase stuff is on the left, but like the cable dressing, that is more about house style than any rules,  (some folk leave it so when you open the cover screws  it is fired off by the coiled up energy of the spaghetti inside, that  is not really the effect  to aim for .. ).

    Note that here the single phases are not RCBOs but MCB. If they were then both cores of the T and E would go to the breaker, and a blue neutral tail would go south from the RCBO to the N-Bar.

    This one is yet to be fitted with module blanks to cover  the unused  positions of the bus bar, and the cover over the incomer at the bottom.

    While the tape flags on the earth lugs are a tad unusual and  I'd have started filling from the bottom not the top, and I like pirahna nuts, but that is just  'house style' again  It is always possible to find something 'odd' with the work of others.

    Mike.
    1a85377f01456c7610276b0b8f65e985-original-3phase-board.jpg



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    mapj1:

    But a 3 phase RCBO with neutral  is harder, as really you need a 4 pole bus bar, and that is not so common, though I suppose it will come, after all we've only had RCDs in common use for about 40 years now.


    Eh? TPN RCBO takes up 4 (SP) ways with N sitting over one of the phases and a pigtail to the N bar.


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    Eh? TPN RCBO takes up 4 (SP) ways with N sitting over one of the phases and a pigtail to the N bar.

    Well yes I've seen that too but I'm not convinced that is the maker's approved method. I have seen bus bar with a repeat period of 4 as well, but it is rare, and I cannot recall which make.


    EDIT  errata, you are quite right it is official,  Eaton do RCBOs with  fly lead and blocking one way in the next triplet 

    Mike.

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