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Does a supply from solar inverters impact on downstream RCD's

Alan B

Just been thinking through RCD selection for domestic electrical systems with solar installed.

Standard practice seems to be in most cases to connect the solar system to a separate consumer unit with type A or type B RCD  depending on the inverters specification. Then not worry too much about the RCD's in the existing installation.

Just thinking this through. This seems to assume the risk is that the solar system creates DC leakage as a load and pulls some DC from the grid and has negligible impact on the main CU.

But does the solar system actually generate some DC current in the supply that could then feed in to the main consumer unit, even if the loads are only resistive and therefore create risk of blinding the downstream RCD's, especially if running in island mode?

Is this a negligible risk or something industry just hasn't thought about?

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    Interesting question...

    In normal events any d.c. leakage or fault current should only flow in a loop between the fault and the N-PE link the system - so live (L+N) conductors on another branch should be unaffected. The d.c.may well flow around all sorts of parallel paths in the c.p.c.s of other circuits (as well as through exposed/extraneous parts - but that shouldn't affect the live currents that RCDs look at.

    There is however an "interesting" problem where the "other" circuit has say a N-PE fault - problems have arisen where there's a lot of d.c. currents flowing through odd metalwork - e.g. in places that use d.c. traction currents on railways - so the result can be d.c. flowing through the N side of RCDs even on circuits that have normally no d.c. risk at all. So in that kind of situation I could see a "normal" circuit being exposed to d.c. currents. That said, the d.c. element from inverters etc is usually relatively small, and would be diluted by parallel paths, so the effect on individual other circuits would be much smaller and quite possibly negligible? Higher current d.c. faults - e.g. from battery systems might be another matter. We are then possibly looking at multiple faults existing at the same time ... which traditionally we don't consider (e.g. broken c.p.c. simultaneously with a L-PE fault) ... but still it might be something worth throwing a few numbers at to see what the risks might be,

       - Andy.

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    Interesting question...

    In normal events any d.c. leakage or fault current should only flow in a loop between the fault and the N-PE link the system - so live (L+N) conductors on another branch should be unaffected. The d.c.may well flow around all sorts of parallel paths in the c.p.c.s of other circuits (as well as through exposed/extraneous parts - but that shouldn't affect the live currents that RCDs look at.

    There is however an "interesting" problem where the "other" circuit has say a N-PE fault - problems have arisen where there's a lot of d.c. currents flowing through odd metalwork - e.g. in places that use d.c. traction currents on railways - so the result can be d.c. flowing through the N side of RCDs even on circuits that have normally no d.c. risk at all. So in that kind of situation I could see a "normal" circuit being exposed to d.c. currents. That said, the d.c. element from inverters etc is usually relatively small, and would be diluted by parallel paths, so the effect on individual other circuits would be much smaller and quite possibly negligible? Higher current d.c. faults - e.g. from battery systems might be another matter. We are then possibly looking at multiple faults existing at the same time ... which traditionally we don't consider (e.g. broken c.p.c. simultaneously with a L-PE fault) ... but still it might be something worth throwing a few numbers at to see what the risks might be,

       - Andy.

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