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.

  • It's a general requirement for parallel generators where additional protection is required - 551.7.1 (ii).

      - Andy.

  • Thanks Andy, I'm aware of 551.7.1, although Part 7, Section 712 does not refer back to 551.7.1. The advice from NICEIC was to use Section 712 as this is specifically for Solar PV. Just looking for other opinions on this.

  • Section 712 does not refer back to 551.7.1.

    You can't read any part 7 section on it's own - as the introduction (section 700) says, the sections 'supplement or modify the general requirements'.  If a part 7 section doesn't say something on a particular subject then the main parts of the regs apply directly. There is no 712.551.7.1 so 551.7.1 applies unchanged.

    At least that's my take on it!

       - Andy.

  • What is the purpose of the RCBO in this application? Has it been installed due to the nature of the circuit? Or is it a type B RCD because the inverter lacks simple separation ? 

  • The regulation NICEIC have directed me to is 712.531.3.5.1.The only reference within this is to 531.3.3 for types of RCD. Although i do understand where your coming from, I have had the same advice from two separate NICEIC technical advisors " 551.7.1 is not applicable in this instance". I don't think there is a black and white answer to this.

  • The RCBO is a SP 16A 30mA Type A to provide additional protection.

  • My only other thought was to check whether additional protection by 30mA RCD is actually required - presumably the circuit concerned doesn't directly supply sockets, mobile equipment outdoors or domestic luminaires; so if it doesn't run through a bathroom, isn't  concealed in walls (without a concentric c.p.c.) and the inverter manufacturer doesn't demand and RCD for additional protection you might be OK. In that situation you could have had a simple MCB - but an RCBO would do no harm and wouldn't need to be DP switching.

    If additional protection is needed, I think you're on a sticky wicket. The idea of using an RCD for additional protection is that it disconnects the supply very quickly (<40ms at higher shock currents) - where there's any kind of generator 'downstream' then the RCD opening doesn't disconnect the supply as voltage is still being supplied from the other end by the generator (in this case the PV inverter). Granted the inverter should eventually notice the grid supply has gone and shutdown itself - but that can take a significant length of time - several seconds - which is useless for protection against shock. The idea of the RCD opening the N as well as L is that it effectively disconnects the inverter from Earth (normally the inverter's N is earthed via the normal supply's N-PE link) - with the DP RCD open the inverter becomes a separated circuit (isolated from Earth - like a bathroom shaver socket) so eliminating the risk of shock to Earth even without disconnection.

       - Andy.

  • Thanks Andy, I know the Inverter we are installing has AC Short Circuit, Overcurrent & Overvoltage protection. However still awaiting confirmation on how quickly this disconnects in fault conditions.

  • Andy Jewsbury is correct, 551.7.1.

    Part 7 does not have to "refer back" - if there were a Regulation 712.551.7.1, it would modify 551.7.1 - but there is NOT a 712.551.7.1, so that Regulation from the General Rules applies wholesale.

    However, most inverter manufacturers recommend RCDs are avoided ... sometimes necessary for TT systems (or other applications) though ... so unless this installation is TT, perhaps question why an RCD is used ? (Solar PV CoP has this advice).

  • Is the RCBO bidirectional, can it be back fed?