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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
  • 0
    As a point of definition the presence of more than one wire in an MCB does not make it 'two' or more final circuits. Just one final circuit that is not supplied at the end, rather centre-fed  or junction-fed or whatever you want to call it.


    There is no prohibition in BS7671 for circuits that are not simple daisy chains - a branching "Christmas tree" of a layout may be a pain in the wotsit to trace out, but it is not prohibited, nor are rings with spurs at the origin, or rings on the ends of radials (the "Lassoo" or "lollipop"  may not be in the annex or the OSG, but they can be made to work safely and properly and are permitted)


    Or indeed any other permutation,  so long as you can do the design sums to show that cable ratings are not exceeded, and VD and Zs are going to be OK.


    The exam question really  becomes 'is one (centre fed) lighting radial enough for a building with XX rooms?"

    That will depend on load and inrush, and maybe in some huge mansion, voltage drops. I'd argue in many cases it may not even be a C3, and not more unless the cable is clearly cooked or there is history of nuisance tripping.


    As regards the hybrid ring -radial - if it was to one double socket, no fault, as that is a normal spur just from the MCB.  If it is 4mm cable feeding any no of sockets, no fault. If it is 2.5mm cable feeding many sockets, then locally fusing down with a 13A fused spur, or creating a 2.5mm return leg to make a mini-ring or reducing the MCB to perhaps 25 or 20A, depending on cable routing,  may all be seen as fixes that do not involve a CU change.


    Mike.


  • Former Community Member
    0 Former Community Member
    Many thanks for the reply and the clarity about a circuit in effect starting from any point that's very useful.

    My key concern would be therefore in situations where a radial line serving multiple sockets (unfused in it's own right) is added to the 32amp MCB however designed with a 2.5mm T&E cable. 


    The wiring regulations indicate in annex 15 that a radial line can intersect the ring either from a socket or at any point within the ring circuit, however unfused should be limited to a single double socket, as apposed to a fused radial which could permit more depending on the potential load.


    Would that seem a fair assessment?
  • 0
    yes - a string many sockets supplied by 2.5mm cable have the potential to be overloaded - the routing of the cable matters a bit here- clipped direct or in plaster, the 27a rating means it is not such an issue, as even 5A of load somewhere else means the cable will not overload.

    However if the cable was sandwiched by thick insulation, so the rating was more like 18A or something, then reducing the breaker or fusing down that branch would be strong recommendations. To some extent it also depends on the likely use - if we know it is for a few wall lights or a supply to one very small room, where anything creating 3kW of heat would be intolerable after a few minutes, then it the immediate danger is a lot  less than where they really may be all in use, in a kitchen for example.

    And if it looks or smells 'cooked' then it is far more serious.

    Mike.
  • 0
    How did you get on with the insulation testing, did you go as far as determining that the lighting circuits are are two separate circuits?

    There could be a borrowed neutral on the landing light or elsewhere and an electrician deliberately combined the “two lighting circuits” into one, because that is what they really are, so cannot be on separate protective devices.


    The 2.5mm socket radial added into 32 amp circuit protective device needs attention.
  • 0
    What would you do if you saw multiple radials , each feeding one twin socket and connected at the origin to one 32A MCB or RCBO?


    Not elegant, most of us would not do it. But what danger is there? Overload protected (by vertue of 2 x 13A plugtop fiuses max) , Short cvircuit and earth fault addressed.


    2, 3 or 4 rings in one circuit way on the fuseboard with or without a spur of one twin socket att the origin. Again what danger if terminations are electrically and mechanically sound? 


    Again, would we do it - no. could we condemn it?
  • 0
    sort of my point really

    'I would not do it' or

    'feels a bit iffy'


    Are not the correct threshold for condemning the design work of others even if the design appears to be a doodle on a fag packet after a pub lunch by someone with the IQ of a pork pie.


    The correct questions after

    "are there any exposed live bits?", and

    "is anything looking charred and heat damaged ?"

    (in reality a lot of things may well fail one or other of these )


    are 'is any cable able to be overloaded to the point of fire hazard ? '(and a 20% overload is more like an operating life of 20 years instead of 50 years, so not really  an  immediate danger)


    And "does ADS provide the correct safety of life response in the event of a live to earth fault ?" (quite often the RCD means that is OK even if the ring is a bit long or whatever)


    Things like 'the VD is a bit high but only to the point where the lights flicker if the shower is used' are only really C3.


    Mike.

  • Former Community Member
    0 Former Community Member
    Hi and thanks.


    The initial inspection of the switches / pendants indicated some overheating within upstairs and downstairs light points. Evidenced by some nice green ooze at the end of the cables. This was at dimmer switches which do not operate as expected owing to the fact the tenant has non-dimmable bulbs. ? They had also explained that they have issues with appliances not lasting very long. 


    Having then proceeded to open the consumer unit, there are no spare ways, and seeing 3 separate MCB's all with what appeared to be multiple circuits, I wanted to check what would be permissable before undertaking significant further checks at a cost to the client. I will also independently test each radial branch to work out which lights are operated and ensure no borrowed neutrals.


    I'm going to go back to the property next week now and do further checks including the insulation resistance. Will be changing dimmers to standard switches at the clients request too.


    One of the socket radials does feed kitchen appliances Fridge freezer and washer/tumble dryer, so unlikely to overload the circuit. The other runs upstairs to additional sockets in bedrooms supplying TV equipment. Again unlikely to be an excessive load. But a more detailed investigation of the radials will be something I do.


    Thanks for the info and thoughts.

    David
  • 0
    I would not waste too much time investigating the lighting circuit for a landlords EICR.


    Just note that two lighting circuits appear to be combined if you really feel you must and move on.
  • 0
    The initial inspection of the switches / pendants indicated some overheating within upstairs and downstairs light points. Evidenced by some nice green ooze at the end of the cables.


    That is not over-heating, it is the polymer in the plastic breaking down slightly. It, generally, isnt a fault, unless it is excessively bad.


    As for a C3 as upstairs and downstairs lighting on one circuit breaker. I'd say no way is that a C3. What real danger is that? What if the downstairs light circuit had tripped, and the Owner had to go in the dark to the kitchen cupboard to reset it - it  would be no different to all the lights tripping, there is always an area that will be dark. If it is such a proble, we woudl eb fitting emergency lighting in domestic premises. In 30+ years, I think Ive fitted 2 emergency lights in houses. It is very unusual to do so, thus proving that there is very little risk in having no lighting in a domestic setting.
  • 0
    I feel that this again is not the way to carry out an EICR. Why the questions, all the answers are in BS7671? Definitions: a circuit. Green gunk is the opposite of overheating, it is an old cable type with a plasticiser that reacts with the copper in the presence of damp. Fitting a new CU: Part P. Which scheme do you belong to? In my view, you should be nowhere near an EICR, because you do not meet even the most simple definition in the Electricity at Work Regulations. The first step to pass is the 18th edition exam, which will require familiarity with the Regulations. If you are carrying out "EICRs" in order to gain repair work, and the EICR is in any way faulty you are laying yourself open to action in a minor case by Trading Standards, and if more serious to criminal charges. The proper next step is to pass C&G 2391, and gain a great deal of experience. When in that position you will be much less keen to do EICRs, they are a minefield, particularly for rented properties where electricity may well be abused, and the result could be very serious.

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