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.
