DP RCD for Solar PV?

We are currently installing solar pv systems, and have had a third party at one of our installs carrying out an EICR. They have flagged a C2 for the RCBO we have used only being single pole.
In section 712 of BS7671 ‘Special Locations – Solar PV’ we cannot see a regulation that states that a double pole RCD is required. Any advice on this matter would be greatly appreciated.

  • www.beama.org.uk/.../BEAMA-Technical-Bulletin-Connection-of-Unidirectional-and-Bidirectional-Protective-Devices.pdf

  • As opposed to:

  • I think you should start worrying again. Contact the makers - it may just be a labelism or it may actually matter.

    is it the Garo one ?

    If so you are out of luck I am afraid.

    should have used  the two module width one


  • I am getting a bit confused here. The BEAMA guidance refers to both MCBs and RCDs. I can see how an RCD in a split load CU could be run backwards (for want of a better description) but if the fault is before the RCD, it will not trip; and if it is after it, a fault in the consumer/meter/DNO tails seems pretty unlikely.

    Why would you connect a PV array to a final circuit which is protected by an RCBO? Is it not a distribution circuit, which accordingly, does not need RCD protection?

    What you might do is to put an RCBO downstream of the PV array, but the array would be connected to the supply terminals and the distribution circuit (to the CU) to the load side, so no problem there.

    Now think of night time. The supply side of the RCD is not energised. The load side is energised by the grid. So what are we worried about now? Is a fault on the wrong side of the RCD a realistic proposition?

    My apologies if I am missing something.

  • Well if the inverter makers instruction require it, it needs to be protected by an RCD (or double pole RCBO equivalent)...   But part of the problem  of inverters is that they do not generally provide enough over current relative to normal load current to operate conventional ADS in any sensible time, if at all. So for faults to earth, when running on an inverter derived supply, the RCD is needed to meet disconnection times. However, for systems that cannot run as an island, i.e, without grid power, it is not so important, as the grid always provides enough fuse-blowing current.

    However, one of the increasingly popular features of inverter PV, especially systems that include a storage battery, is the ability to operate as a generation island, and power essential functions in a power cut. In such a case, the RCD performs an ADS function, perhaps the only ADS function, depending on the strength of the inverter.


  • Indeed. I still think there's room for confusion with my unit though.

    RCBO front view

    From the front you could believe it's bi-directional (no arrows or in/out or line/load markings) - although the 1 and 2 notation (which I think comes from a German convention for terminal markings might suggest line and load - on your bi-directional example both L terminals are marked 1/2). But then the label on the side clearly says line and load:

    RCBO label

    So the marking on the device itself seems contradictory. Personally, noting the 3rd contact interrupting the test circuit, which would seem to make the device immune to the problem Mike described, I'm inclined to believe it is bi-directional, but I'd be hard pressed to argue the case if someone pointed to the label and said otherwise.

       - Andy.

  • more importantly - the data sheet for it says in bold that the single module one isn't bi-directional but the double width one is - see my screen grabs from two posts above ;-)

    taken from this PDF.... www.garo.co.uk/.../uk pricelist 2022 april .pdf

    I suspect this is a subtelty of the electronic mechanisms that is only just becoming common knowledge, as text book, passive RCDs and  of course MCBs have no sense of supply and load-side.


  • Is it not a distribution circuit, which accordingly, does not need RCD protection?

    Would be needed for soft sheathed cabled concealed in walls (in my case I've used BS 8436 cable so I could get away without a 30mA RCD strictly speaking, but preferred to have it anyway).

       - Andy.

  • see my screen grabs from two posts above

    apologies - for some reason I didn't see that before I posted (not sure if due to my poor eye sight or the forum software).

    I'm pretty sure none of the manufacturer's instructions said that at the time - so might be a change of design or the later recognition of an older problem. I think I've got a 2-module one of a similar vintage so I'll see if I can dig a bit deeper (but probably not the right rating, so probably can't do a simple swap).

       - Andy.

  • The forum threading is a right pain actually, but that is a side track to the main discussion. I was not trying to tell you off, just to indicate that for GARO there is (in the current datasheets now at least, maybe not earlier ones) very clear advice .

    The earlier datasheet has the same terminal drawing you have on the side of your unit but is nothing like as clear - there is not the explicit statement about the line load allocation actually mattering that appears in the later document.

    It may be fun to power it from the wrong end and see how much current it draws when open and when shut, and  it may even be fun to trip it but I'd have a hand hovering ready to kill the power if it fails to trip in a second or so.

  • Mike, thank you - I see what you mean. If the grid has gone off, a dead short across a 4 kVA array will not bother a 16 A MCB (or 15 A fuse) but perhaps a battery would produce sufficient current?

Reply Children
  • Quite. Actually I'd not hold my breath for that 4KW array to do a fat lot with a 13A fuse either unless it is very sunny or there is a DC battery to help it along a bit.
    Now for an L-N fault or overload, the inverter output collapses until the load is reduced to a level it can sustain - and no harm will come to the wiring, but everything will 'brown out' or 'black out' until the fault or extra load is removed.
    However if this extra current is due to the sort of fault to earth that might actually also shock or even electrocute someone, we would appreciate a rather more prompt disconnection. Enter the RCD stage left to save the day.