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?
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!)
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.
That is a good point and something I had not picked up on in the original post.
Inverters do tend to give rise to some earth leakage and the Code of Practice highlights nuisance tripping of RCDs as another reason for not sharing the RCD with other circuits.
For systems in island mode, a system reference relay (earth-neutral bond) is required. I did come across an inverter that lacked such a relay so it would allow island mode operation with a "floating" earth. Under no-load conditions I measured a L-E potential > 160 V in island mode. Quite how quickly such a potential would collapse where a moderate impedance (i.e. a person) is put across the terminals is not clear to me either.
Regarding the COP requirement is it more to do with if the other downstream circuit experiences a fault and RCD operates then PV is disconnected from upstream network?
Just visited a home where the solar system is sharing an RCD with 5 or 6 other circuits. Wondering if others would consider this a C2 or C3.
In this case I don't think the solar system supported island mode. Wondering if the potential delay in supply disconnection when there is a fault and also potential blinding of the RCD by excess leakage should be considered as a C3 or C2. It comes down to how big is the risk of this happening in real life and personally I just don't know.
I am thinking along the lines that the most significant risk is that the inverter will take time to shut down once the RCD trips for a fault, therefore if the fault is someone touching a live wire it will take longer than the allowed 300mS for the supply to be disconnected.
I thought solar systems shared a common earth with the main supply, including a TT rod when island mode supported, with the main earth disconnected when in island mode. Therefore don't quite understand the discussion about loss of reference earth.
Just visited a home where the solar system is sharing an RCD with 5 or 6 other circuits. Wondering if others would consider this a C2 or C3
BS7671 regulation 134.1.1
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).
The assumption here is that there's no path to Earth on the disconnected side of the RCD ... well, there might be, for example if there is bathroom bonding, connection to bonded pipework say at a boiler, or other fortuitous earthing ... and if at the same time there's already a fault to Earth.
Is it not a separated circuit at the loss of supply?
There's no intentional separation ... I suppose it could be IT, and there might already be a fault on the "disconnected" side of the RCD, making it the equivalent of TN one way or another.
With modern inverters, RoCoF (Rate of Change of Frequency) should cause almost instantaneous disconnection of the inverter, but inverters to older versions of G83/G59 might continue to provide power for some seconds.
There's another "BUT" to consider. Where RCDs are specified for additional protection they don't just protect against faults of negligible impedance to earth, BUT also against accidental contact with live parts. If the RCD operated because of that, and the power is still being supplied (say with an older inverter) then we still have a potential "additional protection" issue that we didn't disconnect. If someone is in contact with only one live conductor, and there's no fortuitous or impedance earthing (which could come from capacitances in the inverter itself) then perhaps this isn't so much an issue, provided neutral conductors are disconnected from the supply as well as line conductors ... BUT if the user of the installation is in contact with two live conductors, additional protection is certainly not provided for.
The earth reference is mentioned as far as fault currents goes relating to the location of a neutral earth bond - in effect if the inverter keeps going, but the NE path to earth is broken, how do you get a shock ? Its a bit like the transformer isolated shaver socket, in the sense you can toucvh either pin and be earthed yourself and not get a shock.
If it cannot run as an island, then it is OK to rely solely on the DNO earth, as when the DNO supply has gone the inverter stops. The trip time thing is a more tricky one, and I dont have an answer to that without knowing a lot more about the behaviour of the specific kit. clearly its not ideal, but if it is potentially as dangerous as no RCD or not ?
Mike
if the user of the installation is in contact with two live conductors,
Not sure you can do a lot for someone who presents as a valid L-N or L-L load ;-)
Thinking even how to detect, let alone any kind of ADS.
Mike.
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