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

  • 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?

  • Fair comment! So if there is any doubt about the functioning of an RCD, why not ensure that the distribution circuit is not soft sheathed and buried in plaster?

    From what I can see in my neighbourhood, the norm is to bring the PV output down externally in SWA or similar to somewhere near the DNO's service cable.

  • but perhaps a battery would produce sufficient current?

    Battery systems fault currents into the a.c. side are still limited by the electronics in the inverter - so still only a small factor above Ig.

    From what I can see in my neighbourhood, the norm is to bring the PV output down externally in SWA or similar to somewhere near the DNO's service cable.

    Ah, using SWA on the d.c. side is another point of contention - as there's no ADS from the panel output, it's meant to be protected by  double/reinforced insulation - which SWA doesn't usually provide. OK if the SWA is on the a.c. side of course. There again some inverter manufacturers seem to ask for RCD protection anyway.

       - Andy.

  • 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.


  • My enquiry to the manufacturer of my particular device has yielded an interesting reply - while they confirm it is not bi-directional, they suggest that as it's still working after several years of service (I've just run a full set of RCD tests to confirm that), they don't see any immediate safety issue - so basically no need to worry about it - but if I wanted to upgrade to meet current regs they'd recommend their two-module version.

    Which sort of leaves me wondering what the problem really is?

    (I'm reminded of my enquiries to a combi boiler manufacturer about converting the system to open-vented - they initially said no - when I asked why, the answer wasn't any particular technical limitation, just that they hadn't specifically tested it in that situation so were unwilling to guarantee that it would ... in practice it was of course fine. I might be being unfair though.).

       - Andy.

  • How often has the RCD ever tripped, and in what direction was power flowing when it did ? I'd not expect any damage under any conditions until it tries to trip. Have you tested it trips OK on the tester both ways round,.
    You may be right and it is just an example of stickerism and a lack of understanding by the makers of their own product,


  • As I think that Mike implies, that can perfectly simply be explained by no fault having occurred!

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