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RCD protection of a sub main with solar

Alan B

EICR with a house connected to a detached garage via a submain, part of which is PVC T+E and due to the route it takes through the house requires RCD protection.

I need to change the RCBO at the house end to a bidirectional one, luckily MK have just started shipping a suitable device. This gives me protection when power is flowing from house to garage.

But now looking at when solar is generating power, the supply from the solar is just protected by an MCB. Therefore when surplus solar is feeding the garage and submain to the house, from what I can see the garage circuits and submain do not benefit from RCD protection.

Section 826 covers overload protection well but doesn't cover RCD protection. I know that good practice is to install so that RCD protection is not required, but I am dealing with a legacy system.

My initial thought is to add a bi directional RCBO where the solar feed is connected to the garage board.

Will this then provide suitable RCD protection?
Any risk of interactions / imbalances causing nuisance tripping

I  believe that once the house side RCBO trips the inverter would stop seeing a supply and shut down, I assume this won't be quick enough to provide shock protection and is unacceptable for other reasons.

I believe the ideal solution is to have solar connected to the house DB direct by a means that does not need RCD protection, this isn't financially viable, would make more sense to decommission the solar system.

There is a possibility that the sub main from house to garage is converted to a buried SWA cable and the problem goes away, but this may not happen and looking for cost effective acceptable solution with existing cable run.

As some background the solar system is 10 to 15 years old, actually mounted on some stables which the house owner doesn't have have easy access to.

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    Alan B said:
    My initial thought is to add a bi directional RCBO where the solar feed is connected to the garage board.

    Is this needed if the inverter is not used as a grid-forming inverter in island mode? If the RCD at the "DNO supply" end of the cable operates, a grid-connected inverter conforming to G98/G99 requirements will automatically stop generating, usually rather rapidly due to rate of change of frequency (ROCOF) trip requirements.

    I agree it may be required if the inverter operates as a grid-forming inverter in island mode.

    Alan B said:
    EICR with a house connected to a detached garage via a submain, part of which is PVC T+E and due to the route it takes through the house requires RCD protection.

    In any case, more importantly, you need to select the correct Residual Current Type of RCD ... BUT FIRST check whether the inverter manufacturer recommends that RCDs are NOT used for the connecting circuit. If you don't, you might find that the RCD experiences lots of unwanted operation.

    Basically, if RCDs are recommended not to be used by the inverter manufacturer, it may require another solution, such as replacing the wiring with SWA, or use of earthed metal containment, which perhaps should have been done in the first place under those conditions.

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  • 0
    Alan B said:
    My initial thought is to add a bi directional RCBO where the solar feed is connected to the garage board.

    Is this needed if the inverter is not used as a grid-forming inverter in island mode? If the RCD at the "DNO supply" end of the cable operates, a grid-connected inverter conforming to G98/G99 requirements will automatically stop generating, usually rather rapidly due to rate of change of frequency (ROCOF) trip requirements.

    I agree it may be required if the inverter operates as a grid-forming inverter in island mode.

    Alan B said:
    EICR with a house connected to a detached garage via a submain, part of which is PVC T+E and due to the route it takes through the house requires RCD protection.

    In any case, more importantly, you need to select the correct Residual Current Type of RCD ... BUT FIRST check whether the inverter manufacturer recommends that RCDs are NOT used for the connecting circuit. If you don't, you might find that the RCD experiences lots of unwanted operation.

    Basically, if RCDs are recommended not to be used by the inverter manufacturer, it may require another solution, such as replacing the wiring with SWA, or use of earthed metal containment, which perhaps should have been done in the first place under those conditions.

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  • 0 in reply to gkenyon
    gkenyon said:
    Is this needed if the inverter is not used as a grid-forming inverter in island mode? If the RCD at the "DNO supply" end of the cable operates, a grid-connected inverter conforming to G98/G99 requirements will automatically stop generating, usually rather rapidly due to rate of change of frequency (ROCOF) trip requ

    I was considering this as a thought process to follow but unsure if it's acceptable as I couldn't find anything in BS7671 discussing it. Sounds like it is acceptable.

  • 0 in reply to gkenyon
    gkenyon said:
    In any case, more importantly, you need to select the correct Residual Current Type of RCD ... BUT FIRST check whether the inverter manufacturer recommends that RCDs are NOT used for the connecting circuit. If you don't, you might find that the RCD experiences lots of unwanted operation.

    Agreed I should check for information. But it's been connected to a standard unidirectional 30mA RCBO for more than 10 years without any problem, suspect it will be fine. It has to be worth a go, £20 for an RCBO against at least £1000 for alternative approaches.

    AJJewsbury said:
    b) When the grid-side RCD disconnects, it opens N as well as L, so disconnecting the circuit from earth - and so although the circuit still has power from the PV inverter, it's in effect a separated circuit, so safe from the point of view of electric shock (to Earth) even though a L-N voltage persists.

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