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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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    The following applies to this discussion as far as I can see:

     

    The Code of Practice for Grid-connected Solar PV Systems (second edition) includes a requirement (section 8.8) “Solar PV systems shall not be installed to the load side of any RCD that is shared with other circuits (for example, where the RCD is feeding a number of circuit breakers / circuits).” with an explanatory note “this is necessary to ensure the continued safe operation of the RCD, prevent the possibility of a fault on a circuit continuing to be fed by the inverter for up to five seconds after an RCD has tripped, and prevent nuisance tripping”.

     

    So having an RCD only at the house side is problematic.

     

    A more general requirement for generating sets not sharing RCD protection is also included in the Draft Amendment 4 to BS7671, 551.7.1 (d): “A source of supply shall not be connected to the load side of any RCD providing additional protection in accordance with Regulation 415.1 that is shared with other circuits...”

     

    ENA EREC G99 as well as the preceding G59 stipulate a delay time of 0.5 s before the Loss-of-Mains protection is triggered. For small inverters up to 16A, G98 does not mention a delay time so response may be swift – but the preceding G83 could allow a trip time up to 1 s. It should be known / visible which of these protection settings apply to the existing inverter. I do not know why the CofP mentions a more pessimistic five-second timescale.

     

    Altogether, if there is a way to bypass the problematic section of cable in the house (SWA around the house for instance), and thus avoid the need for an RCD at the house end, that would be ideal – but may be a lot of hassle. Failing that, I would vote for RCD protection at both the house and the garage end – with the RCD for the PV not shared with other circuits in the garage. This will not allow for selectivity between the upstream and downstream RCDs but that is probably the lesser evil.

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  • 0

    The following applies to this discussion as far as I can see:

     

    The Code of Practice for Grid-connected Solar PV Systems (second edition) includes a requirement (section 8.8) “Solar PV systems shall not be installed to the load side of any RCD that is shared with other circuits (for example, where the RCD is feeding a number of circuit breakers / circuits).” with an explanatory note “this is necessary to ensure the continued safe operation of the RCD, prevent the possibility of a fault on a circuit continuing to be fed by the inverter for up to five seconds after an RCD has tripped, and prevent nuisance tripping”.

     

    So having an RCD only at the house side is problematic.

     

    A more general requirement for generating sets not sharing RCD protection is also included in the Draft Amendment 4 to BS7671, 551.7.1 (d): “A source of supply shall not be connected to the load side of any RCD providing additional protection in accordance with Regulation 415.1 that is shared with other circuits...”

     

    ENA EREC G99 as well as the preceding G59 stipulate a delay time of 0.5 s before the Loss-of-Mains protection is triggered. For small inverters up to 16A, G98 does not mention a delay time so response may be swift – but the preceding G83 could allow a trip time up to 1 s. It should be known / visible which of these protection settings apply to the existing inverter. I do not know why the CofP mentions a more pessimistic five-second timescale.

     

    Altogether, if there is a way to bypass the problematic section of cable in the house (SWA around the house for instance), and thus avoid the need for an RCD at the house end, that would be ideal – but may be a lot of hassle. Failing that, I would vote for RCD protection at both the house and the garage end – with the RCD for the PV not shared with other circuits in the garage. This will not allow for selectivity between the upstream and downstream RCDs but that is probably the lesser evil.

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