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New Consumer Unit Necessary?

Robert Garrett

We are having our conservatory replaced with a more substantial "garden room". The electricity in the conservatory was a spur off a current ring main, and the new room will be the same. The electrician says we must have the current consumer unit (which is plastic and has no RCDs being ~30 years old) replaced in order for the work to be certified. We had the system checked a few years ago and although advised a new consumer unit would be better, told it was not a legal requirement.

So do regulation require a new consumer unit with RCDs for this ring spur to be re-added, or is he being over cautious?

  • 0 in reply to Robert Garrett

    Is there a fused spur at that point that isolates the compete conservatory  or does the ring loop out and back in again ?
    Even so, any new wiring needs cover from an RCD, but the RCD does not have to be in the main consumer unit - though it usually is.

    Or are retrofit RCDs available for the existing box ?

    M

  • 0 in reply to Phow

    If the existing ring circuit does not have 30mA RCD protection it could not be simply extended. Making a new ring from a single fused spur is pointless you would need an RCD spur for both legs of the ring ( if earth continuity R2 is lost and a fault occurs it's a 50% chance whether it will trip the RCD or not depending on what side of the RCD its on. Unless you plan to run sizeable heaters a 13A  radial power circuit would be plenty for table lights, TV, etc.

  • 0

    It looks like we will have to go with a new "box", the current consumer unit is very old and we did check that RCDs/ELBCs cannot be added.

    Thanks for all the input.

  • 0 in reply to Phow
    Phow said:
    Extent the loop ring from where the fused spur is currently. this assumes the fused spur in on the ring.

    Who said anything about a fused spur? However, SRCDs may still be available. We discussed them 4 years ago: engx.theiet.org/.../omitting-30ma-rcd-protection-for-single-s-o-in-a-domestic-property

  • 0

    you would need an RCD spur for both legs of the ring

    I don't see that working - any tiny difference in L / N resistance around the ring would mean that the L & N currents in each leg wouldn't precisely balance - so quite likely both RCDs would trip even without any leakage to earth.

      - Andy.

  • 0 in reply to Phased

    The only sensible place for an RCD on a ring is at the origin - the behavior of RCDs in parallel is not well defined, and you do not want RCD operation to open the ring and leave part of it alive. However an RCD spur or at most two with fuse 13A or MCB of 16A each with the RCD covering the spur but not the ring part would work. If you have expect more load than this then really it is not suitable for extending the ring anyway. This is only kicking the can along a bit, and probably not worthwhile - the 'proper'  solution at some point will be a new CU with RCBOs or RCDS inside it and take the chance to bring the rest of the installation into the 1990s safety wise..

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