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Overload protection downstream of inverter on off grid solar pv system

MikeG

I'm looking at 230vac distribution system downstream of 10 KW PV Inverter. System is off grid with storage battery and I'm just involved at present in the ac distribution system for a commercial application

Prospective fault current therefore about 40A but normal circuit breakers or RCBO's cannot provide ADS within 0.4 or 5 Secs. I'm thinking that this situation must arise in every domestic application where a solar pv system integrates with an existing domestic wiring system so there must be a simple solution.

I have no previous experience of Solar and inverter driven supplies so probably missing something obvious so for this of us in this situation any help from the more experienced would be very helpful. 

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  • This also might depend on whether the RCD is voltage-dependent, and also how quickly the inverter output decays on a real fault.

    However, not sure whether you are asking about ADS for protection against electric shock, or 'overload protection' because these are two very different things.

  • in reply to gkenyon
    gkenyon said:
    This also might depend on whether the RCD is voltage-dependent, and also how quickly the inverter output decays on a real fault.

    Indeed - the inverter output may well collapse in the face of a low impedance fault, but (as I understand it) the voltage-dependent RCD behaviour is co-ordinated to shock protection requirements - i.e. if the L-N voltage at the RCD remains above 50V the device should still operate normally. If it drops below that the RCD may well not open, but the voltage available for a shock should also be below 50V - hence not a hazard (at least not in the conventional electric shock sense). Non voltage dependent RCD should trip if the residual current exceeds its threshold regardless of the voltage available (as I found out once when trying to verify the N contact of an RCBO with a multimeter - even on the bench with no mains connections at all the damn thing trips before you can get a reading!)

       - Andy.

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  • in reply to gkenyon
    gkenyon said:
    This also might depend on whether the RCD is voltage-dependent, and also how quickly the inverter output decays on a real fault.

    Indeed - the inverter output may well collapse in the face of a low impedance fault, but (as I understand it) the voltage-dependent RCD behaviour is co-ordinated to shock protection requirements - i.e. if the L-N voltage at the RCD remains above 50V the device should still operate normally. If it drops below that the RCD may well not open, but the voltage available for a shock should also be below 50V - hence not a hazard (at least not in the conventional electric shock sense). Non voltage dependent RCD should trip if the residual current exceeds its threshold regardless of the voltage available (as I found out once when trying to verify the N contact of an RCBO with a multimeter - even on the bench with no mains connections at all the damn thing trips before you can get a reading!)

       - Andy.

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  • in reply to AJJewsbury
    AJJewsbury said:
    Indeed - the inverter output may well collapse in the face of a low impedance fault, but (as I understand it) the voltage-dependent RCD behaviour is co-ordinated to shock protection requirements - i.e. if the L-N voltage at the RCD remains above 50V the device should still operate normally. If it drops below that the RCD may well not open, but the voltage available for a shock should also be below 50V - hence not a hazard (at least not in the conventional electric shock sense).

    Does this not depend on which version of the product standard is being considered, and which classification is accorded by the manufacturer?

  • in reply to gkenyon
    gkenyon said:
    Does this not depend on which version of the product standard is being considered, and which classification is accorded by the manufacturer?

    Ah, Graham (as usual) you know much more of the various standards than I do - please do enlighten us!

       - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    Ah, Graham (as usual) you know much more of the various standards than I do - please do enlighten us!

    In the versions of BS EN 61008 and 61009 amended to 2017, there are different classifications of voltage-dependent RCD and RCBO.

    I understand the technical performance requirements have been revisited in the latest versions of the standards although there will be a cutover period before these are fully applied to all products on sale.

    More information here: https://www.beama.org.uk/static/dda8f294-e43c-484f-8061e92b1f1e64e0/Technical-Bulletin-Guide-to-Residual-Current-Device-RCD-E-Marking.pdf 

  • in reply to gkenyon

    Thanks for that Graham - lots I didn't know there! I'd not come across the "E" markings before.

    So, if I've understood it correctly, we need only worry about devices marked "E2" or "E3" (as the other types are OK for "indirect contact" protection (i.e in the case of L-PE faults)) and those seem to be limited by note b to "Only devices integrated in one unit with a socket-outlet or designed exclusively for being associated locally with a socket outlet in a same mounting box." - so that makes me think of what we know in the UK as BS 7288 socket RCDs (or perhaps those industrial enclosures that have a modular DIN rail enclosure above a BS 4343 (as was) socket outlet).

    If that's the case, it's maybe not a worry for devices intended to be used in CUs/DBs?

       - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    So, if I've understood it correctly, we need only worry about devices marked "E2" or "E3" (as the other types are OK for "indirect contact" protection (i.e in the case of L-PE faults)) and those seem to be limited by note b to "Only devices integrated in one unit with a socket-outlet or designed exclusively for being associated locally with a socket outlet in a same mounting box." - so that makes me think of what we know in the UK as BS 7288 socket RCDs (or perhaps those industrial enclosures that have a modular DIN rail enclosure above a BS 4343 (as was) socket outlet).

    The ambiguous area ... which is also drawn out in the BEAMA Bulletin if I remember correctly, is that some devices intended for use in CUs and DBs are marked E3. The BEAMA Bulletin explains why this might happen.

    So, yes, I guess E1 or no marking are apparently OK for ADS without further enquiry, but E2 or E3 should not be selected to provide ADS, unless further due diligence is undertaken with the manufacturer.

    The Bulletin also tells us what's happening in the future with the newer standards, to avoid this uncertainty.

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