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Another change in AMD2 - 551.7.2 - embedded parallel generation

AJJewsbury

551.7.2 is the one that says where you have some kind generating set (e.g. embedded generation) connected directly to a final circuit (i.e. a circuit that also has current-consuming equipment or sockets connected) then the conductors of the final circuit need to be rated for the sum of the protective device and the generated current - Iz ≥ In + Ig

I've long argued both that the laws of physics aren't peculiar to final circuits and that there overload of conductors can be avoided in more subtle ways in many circumstances - e.g. by feeding the mains supply and generator from opposite ends of a conductor.

They've now extended 551.7.2 so that the same logic is now to be applied to 'a low voltage switchgear and controlgear assembly' - which I presume includes CUs and DBs, which seems a bit odd to me.

Firstly it still seems to neglect distribution circuits (which can serve loads as well as the generator - e.g. an PV CU henley'd into the tails with the normal CU).

Secondly it seems to outlaw the normal practice of connecting PV and similar to a way in the main CU unless the DNO's fuse can be shown to be <<100A or the assembly is rated for In+Ig (e.g. 100A + 16A for the PV so 116A) - which as far as I know only the fictitious 'Concept' CU range was. Even if the PV is connected to the opposite end of the bus-bar from the main incomer and so overloading of any part seems impossible, it would still not comply.

Are there going to be lots of PV installations getting C2s on the next EICRs for lack of overload protection?

      - Andy.

  • In the case of a final circuit I can see the logic of requiring that the cable be rated for the total of the embedded generation and the OCPD.

    Depending on the exact layout, which may change, then parts of the circuit could be loaded to a total of the PV production AND the OCPD.

    iIn the case of a distribution circuit then it depends on the location of the embedded generation. If a 63 amp Fuse AND a 16 amp PV inverter feed the origin of the circuit, then it could be loaded to 79 amps and the cable rating should reflect this.

    If however the grid tie inverter is located at the load end of the 63 amp distribution circuit then the TOTAL LOAD could reach 79 amps, but the cable used for the distribution circuit cant be loaded to more than 63 amps.  

  • AJJewsbury said:
    Are there going to be lots of PV installations getting C2s on the next EICRs for lack of overload protection?

    Have a look at 722.551.7.2 ... effectively means the PV has its own circuit so the final circuit provision of 551.7.2 are not applicable.

    AJJewsbury said:
    They've now extended 551.7.2 so that the same logic is now to be applied to 'a low voltage switchgear and controlgear assembly' - which I presume includes CUs and DBs, which seems a bit odd to me

    Interesting ... but it's not the "same logic". It's to do with heat of OCPDs (and possibly, the busbar, as it's fed from two ends now).

    At it's most simple, fuses work by getting hot. MCB's and RCBOs also have an element inside that gets hot.

    So, if the max demand of the installation is, say 80 A, without the generator connected, the DB gets "80 A hot". If I connect a 16 A output generator through an OCPD in the same DB, we have additional heat, now the DB gets "96 A hot" if you will.

  • Have a look at 722.551.7.2 ... effectively means the PV has its own circuit so the final circuit provision of 551.7.2 are not applicable.

    It's not the 'final circuit' provisions of 551.7.2 I'm worried about, so much as the new 'Where the generating set is connected via a low voltage switchgear...' bit - which as I read it applies regardless of whether the generator circuit is deemed to be a final circuit or a distribution circuit.

    At it's most simple, fuses work by getting hot. MCB's and RCBOs also have an element inside that gets hot.

    Now that's a very interesting point. I wonder how we're meant to cope with that in SSEG situations where both the DNO fuse and CU rating are often 100A - can't fuse down the mains side without losing available supply capacity so perhaps have the generator OPDs in a separate enclosure and hard-wire the connection to the main CU?

    I'm curious to how much of a problem it is likely to be in reality - in some areas (e.g. Ireland) it's common to have either a HBC fuse or a DP MCB as the incoming device - which would effectively double the heat generation from thermal elements within the enclosure (e.g. 126A "hot" for a 63A incomer and the same 63A then going through the outgoing devices) - yet the overall construction seems pretty similar to our CUs with a plain switch disconnector as incomer.

       - Andy.

  • in reply to AJJewsbury

    Thinking more about this, if we had true double pole MCBs (so a thermal element in 'N' as well as L), we'd be in the same situation - but I've never heard of having to de-rate a distribution board in such a case (e.g. when used for RLV).

    Further more, considering a 100A TPN DB - whose basic design is pretty similar to a 100A CU - with long rows of modular devices right next to each other (if anything worse thermally as the rows are normally arranged vertically rather than horizontally so less scope for heat to convect away from individual devices). By this theory we'd have "300A hot".

    Or a dual-tariff CU - we could have two 100A supplies into the same enclosure (typically one 24h and one off-peak for storage heating) - potentially from two different DNO fuses - same basic design yet copes with "200A hot".

    Is there really any problem with "116A hot" - except perhaps that the CU manufacturer hasn't thought to write down that it'll be fine?

        - Andy.

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