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Multiple circuits in a single MCB/RCBO

Former Community Member
Former Community Member
Having undertaken some inspections recently I'm finding a lot of older installations with multiple circuits within a single RCBO. Example being 2 light circuits (Upstairs/downstairs) being added to a single B6 RCBO.

The circuit design covers single pendants in each room and some downlights in the kitchen. The majority of bulbs are LED so from a risk point of view would this be unlikely to exceed 6amps. From the EICR however would this be considered C3 in peoples opinions due to risk level with a potential of splitting to separate RCBO's, or should this be C2 as there are technically 2 circuits fed from a single MCB?


Likewise the same installation has a 32amp MCB (RCD protected) supplying a ring final circuit downstairs. When work has been undertaken in the past, an additional radial circuit has been added to the same MCB in 2.5mm T&E. My view on this is that this instantly requires a C2 coding, as the radial circuit feeds multiple sockets (and there is no Fused spur within the radial line) it is not rated for the potential draw and the 32amp breaker will not trip with the overload of the radial line.


There are no spare ways in the board, so am I correct in claiming this would require a new replacement board to accommodate and separate the circuits? Would this be different if the radial only served a single double socket for ease of the wiring?


There appears to be a lot of confusion on when/if its acceptable to add radials to an existing breaker and I wondered if there was any clarity, given that these circuit amendments are historic and retrospective changing to meet current regulations is not essential providing it met previous regulations.


Many thanks

David
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    Not uncommon I`m afraid, I fact I once found a lighting circuit like that. I originally assumed it was a messer after the original but years later i met that original electrician of that installation and his methodology on other works lead me to strongly suspect he`d very easily do the same again inadvertently.


    Similarly (different electrician) an old factory with a local consumer unit for small offices. PH & N both fed from different DP single phase switch fuses
  • 0
    You wouldn't get this sort of thing if you did your initial verification properly; but you could if all you do is a functional check. ?
  • 0
    Meanwhile, this morning's exercise in safe isolation.

    Presumably not the connection to a PV inverter either?

      - Andy.
  • 0
    AJJewsbury:
    Meanwhile, this morning's exercise in safe isolation.

    Presumably not the connection to a PV inverter either?

      - Andy.


    Any standards compliant inverter will switch itself off if it loses connection with the mains.


  • 0
    Simon Barker:
    AJJewsbury:
    Meanwhile, this morning's exercise in safe isolation.

    Presumably not the connection to a PV inverter either?

      - Andy.


    Any standards compliant inverter will switch itself off if it loses connection with the mains.




    Indeed - but it takes time - supposedly 5s for older ones but seems it can take longer if the conditions are just right (wrong). I recall someone inspecting my installation throwing off the main switch and having time to say "er, why's that light still on?" and me to reply "I'll be the PV inverter - it should shut down soon" before it actually went dead.


      - Andy.


  • 0
    Sparkingchip:

    Meanwhile, this morning's exercise in safe isolation. 

    d1482d8d726b9e4b94e07a2be339d662-original-20210326_120926.jpg




    Adding a fused connection unit to an existing socket circuit.


    So I plugged the socket tester in and could hear it buzzing, I knocked off the first MCB and it stayed on, so I turned it back on and turned the second MCB off and it was still buzzing. So then I turned them both off and the socket tester stopped buzzing, so then I tried alternating between the two MCBs and decided it was time to alter the wiring in the consumer unit, as the ring circuit had a MCB both ends.


    It really is not a good idea to believe what it says on the consumer unit labels.


  • 0
    Well, somebody must have had a very odd idea of a ring final. Were the neutrals and CPCs together in the neutral and earth bars, or had two terminals been used in each case.


    The "electrician" seems to have known that each leg of a ring can carry 20 A, so for my money, confused rather than clumsy.
  • 0
    Chris Pearson:

    Well, somebody must have had a very odd idea of a ring final. Were the neutrals and CPCs together in the neutral and earth bars, or had two terminals been used in each case.


    The "electrician" seems to have known that each leg of a ring can carry 20 A, so for my money, confused rather than clumsy.




    Ah! You spotted the 20 amp MCBs!


    It’s all good fun.


  • 0
    Perhaps the "electrician" thought they had a cunning design for a ring FC capable of handling a 40A total load using only 2.5mm cable? And "easily" isolatable as long as you're of aware of the cunning plan?
  • 0
    I think someone changed the CU who didn't understand because it is newer than the wiring. The ring ends are 7/0.029, probably to the 14th edition, the CU is 15th if I remember correctly.

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