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 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.

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.