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Solar PV into load side of RCD

lyledunn

At Section 8.8 in the COP for Grid connected Solar PV systems it clearly indicates that the inverter should not be connected to the load side of any RCD that is shared with other circuits. The note explains the reasons without elaboration.

I am trying to understand how an inverter would continue to supply the fault after the RCD has operated. Is it not a separated circuit at the loss of supply?

 

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    Imagine a split load consumer unit with a 30 mA RCD protecting a couple of circuits including sockets and the PV inverter. Now a fault occurs on the socket circuit such as a dodgy kettle. The RCD trips (within 300 ms) and that should protect the user from a lethal event. However.. the inverter should be shutting down as well but is not necessarily specified to do so as quickly as the RCD - so, briefly, the inverter may continue to feed 230 V into the fault. Making sure that the circuits do not share an RCD would avoid this scenario. This is my reading of what the CofP rule is about. There was a related thread a while ago, “RCD protection of a sub main with solar” (apologies I have not yet worked out how to insert a link to that!)

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    I think I share lyle's thoughts. Presuming the RCD (as required by 551.7.1(ii)) opens all live conductors, including N, then once the RCD has tripped, all the downstream circuits loose their Earth reference - in effect becoming one big separated system. It may continue to have 230V between L & N (or even 400V between L1/L2/L3) but it should remain safe both during 1st L-PE faults and direct contact even without any ADS as there's no path to Earth (other than at the fault itself) - all recognised in 413 or 418.3 (in principle of physics at least).

    I can see that in the case of multiple faults (e.g. a 2nd L-PE fault or direct contact where a latent L-PE fault exists) danger may occur, but that's calling up probabilities that are beyond BS 7671's usual requirements (i.e. to provide protection in the case of 1st faults only).

    I'm wondering about the cases which are less clear cut though - i.e. multiple class I appliances having filters or less than perfect insulation that might mean that the circuit isn't truly separated from Earth once the RCD is open ... whether such paths could have a sufficiently low impedance to allow shocks with the RCD open, but not 'nuisance trip' the RCD in normal conditions, seems less clear.

       - Andy.

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