Replacing an RCD with a Mains Switch?

Discussion in 'Electricians' Talk' started by Sploo, May 5, 2018.

  1. Sploo

    Sploo New Member

    Short version: if a circuit is protected by an RCBO, do you also need an RCD?

    Long version: I have a two storey garage with a Consumer Unit; the CU has a 100A Mains Switch, a couple of 32A MCBs (one for the downstairs sockets, one for the upstairs), and a 6A MCB for the lighting.

    The garage itself appears to be fed from the house via a small separate CU with a single MCB and an RCD (what I'd think of as a "Shower Unit"). It looks a lot like this:

    The problem is that the garage has a mains-powered roller shutter door, so if I'm in the garage and trip the RCD (in the house), I'm left in the dark (and have to override the door and lift it to get out).

    I can't change the 100A Mains Switch in the garage CU for a DP RCD, as if the RCD is tripped from a socket on the garage upper storey (accessible through another door) it would kill the power to the roller shutter door (which would need to be manually opened in order to get into the ground floor, in order to reset the RCD).

    To try to solve this, I've replaced the two 32A MCBs in the garage CU with 32A RCBOs. Everything's Type B, but unfortunately it seems the RCD in the house trips before the garage RCBOs; so it doesn't help.

    Because all circuits in the garage (apart from the lighting) are now on RCBOs, would it be acceptable to replace the RCD in the house "shower unit" with an identically rated Mains Switch? I'm aware the RCBOs are Single Pole (breaking the live only), but I assume the DP RCD in the house will break both live and neutral when it trips.
  2. Coloumb

    Coloumb Well-Known Member

    It might be better to try to establish the cause of the fault that is making the RCD trip. In general, an RCD shouldn't trip so much that you should be trying to "design" your way out of it.
  3. spinlondon

    spinlondon Well-Known Member

    It may be that the cable supplying the garage requires RCD protection.
  4. Bazza

    Bazza Well-Known Member

    What he said^^^^
    Find and fix the bloomin problem.
    Get a competent electrician if you don’t have the nouse or equipment to do what are basic tests.
  5. spinlondon

    spinlondon Well-Known Member

    Sorry but having the RCD remote from where the power is being used, is not just poor design, a non-compliance but also appears in this instance to present a danger.
    If I were to find such a situation during a periodic inspection, I would probably apply a code C2.
  6. retiredsparks

    retiredsparks Well-Known Member

    What is the cable type supplying the garage.(if swa you dont need rcd in that small cu)
  7. Bob Rathbone

    Bob Rathbone Well-Known Member

    The garage supply should be on a circuit unprotected by a the house will not affect the garage and visa versa. Fit a split load board in the garage with the sockets on an RCD, the lights on another RCD/RCCBO and the garage door on another RCCBO or just a MCB. Sounds like a mess to me, you should not have RCD's cascaded.
  8. Sploo

    Sploo New Member

    Thanks for the replies; some more context/info (and answering a few points raised above):

    The ground floor of the garage is basically a workshop (full of finger eatin' machinery). I realised there was a problem because the NVR switch on my drill press sometimes trips the house RCD if I "miss" the on button (i.e. don't press it cleanly). I'm looking into getting a replacement NVR.

    Apart from the annoyance of ending up in the dark (no windows on the ground floor), having to lift the garage door manually, and resetting the RCD in the house, it's more of an inconvenience than a danger (as the drill doesn't even start).

    However, the problem is that the sockets in the upper floor are used by the wife for her gardening (including the lawnmower). Obviously I don't fancy a situation where I'm using the table saw in the garage, the wife trips the RCD, and I end up in the dark with a spinning saw blade (and no braking on the saw, as nothing will have power). So the problem is somewhat theoretical (but very real).

    As noted above, I assume the reason the RCD is in the house is that if a main RCD switch were in the garage CU, and it tripped out (due to a socket on the first floor) there would be no way of opening the roller door to get into the ground floor and reset that RCD.

    The cable supplying the garage is an SWA. It's buried underground, and is only visible as it rises out of the floor inside the garage. I'd need to hit it with something pretty substantial to do any damage.

    I was hoping to get away without having to fit a new split load CU in the garage - both due to the amount of work required, and the fact I'd have to move some existing wall mounted cables in order to install a larger CU.

    If I changed the RCD in the house "shower unit" for a Mains Switch, it would mean the SWA supplying the garage, and the lighting circuit in the garage, weren't protected by an RCD (all other circuits off the CU supplied by the SWA are on RCBOs). That doesn't feel like a big danger to me; but would it contravene the regs?
  9. Coloumb

    Coloumb Well-Known Member

    To my mind the most obvious thing would be to fix the drill so it doesn't trip the rcd.
    Spike_hiker likes this.
  10. spinlondon

    spinlondon Well-Known Member

    From what I understand from the OP and subsequent posts, if the RCD trips, it would be difficult to gain entry to the garage.
    If that is the case, I see it as dangerous because if someone inside the garage required medical help, it would be difficult for help to gain entry to the garage with the RCD tripped.
  11. Bob Rathbone

    Bob Rathbone Well-Known Member

    As I said earlier, it needs splitting up with separate RCD protection for the circuits and no RCD on the safety critical circuit, the garage door. Do you have a fire door at the back?
  12. Sploo

    Sploo New Member

    Coloumb: Yep, will do (but that doesn't fix the underlying danger).

    Spinlondon: That would be the case if I installed a DP RCD in the garage CU (which is why I don't want to do that).

    Bob: No; it's just a residential garage, so the only entry/exit is the rolling shutter door. You're right that a full CU replacement would be the ideal solution; it's just the one I don't want to take due to cost/time/and wall space related logistics!
  13. spinlondon

    spinlondon Well-Known Member

    You still haven’t told us the type of cable which supplies the garage.
    It may be that you do not need RCD protection for this cable.
    Any type of RCD protection for the roller shutter is not a good idea wherever it’s situated.
  14. Bob Rathbone

    Bob Rathbone Well-Known Member

    Sploo, you have identified the issues and have proposals for rectification. You are looking at this job from a DIY point of view, with overall cost as the main driver to solution. We look at it from a professional POV with safety as the main driver. Bite the bullet, change the board and find the fault.
  15. Bazza

    Bazza Well-Known Member

    Change the board? Surely the advice here was to find the fault.
  16. spinlondon

    spinlondon Well-Known Member

    Find the fault and make the installation safe.
    Bazza likes this.
  17. Bob Rathbone

    Bob Rathbone Well-Known Member

    Bazza, read my post to the end, find the fault was included. Ta! have a great HB Monday in the sun.
  18. Bazza

    Bazza Well-Known Member

    Same to you. I was a bit concerned that your post indicated that by changing the board you would find the fault.

    Would have been clearer to say

    Bite the bullet, find the fault and then chsnge the board. (If a board change is needed).
  19. JP.

    JP. Screwfix Select

    Not being funny or owt but the chap did say that the cable from house intake position to garage was probably a switch fuse at intake position is indicated..however the chap would have to state what supply type it is before anything definitive can materialise. If all go then no more tripping at house position. Do agree the thing which is causing the tripping must be sorted.
  20. Bob Rathbone

    Bob Rathbone Well-Known Member

    Yes, should have mentioned the find the fault first.

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