The following is taken from an IET Wiring Matters article:
Can compliance with this regulation be achieved through the use of a Bi-Directional protective device (as introduced in Amd 3) within the main CU or would it require a separate CU sharing the incoming meter tails using Henley blocks?
Interested in all your opinions and advice.
Cheers,
Steve
It probably helps to quote that regulation in its original form ... "The generating set shall be installed on the supply side of all the protective devices for final circuits of a distribution board" (my emphasis). The thing to note here is that the definition of a final circuit is a circuit that directly supplies current using equipment (or sockets for the same) - so a circuit for a generator would normally not be a final circuit (even though it's the last leg of the connection to the generator) - as the generator is a current sourcing, not current using. Note also that the requirements for final circuits, especially disconnection times for ADS, are far more onerous than for other circuits (e.g. 0.4s rather than 5s). If you had a mix of a generator and normal appliances on the same circuit, there is a potential to undermine ADS - the protective device may open, but the generator could still provide power to the loads (even ones that disconnect automatically on grid fail take time to notice the grid fail - often anything up to 5s) . So what I believe 511.7.2.1 is trying to say you're not permitted to share one circuit between a generator and normal loads, but that the generator must have its own dedicated circuit - the result being that all normal final circuits are supplied in such a way that opening of their protective devices ensures immediate disconnection.
So as to your original question, as far as 551.7.2.1, either approach is valid and acceptable.
There is a side consideration though - in that the DB might be obliged to carry not just current from the grid, but current from the generator at the same time, so if the grid side overload protection is rated for the full current carrying capacity of the DB (often the case for 100A CUs and 100A DNO cut-out fuse for example) the DB could lack overload protection when the generator is run in parallel with the grid. Often the solution to that is a separate CU for the generator - but not simply Henley's into the existing tails, but with additional overload protection (say a 100A fuse) between that point and the original CU.
- Andy.
It probably helps to quote that regulation in its original form ... "The generating set shall be installed on the supply side of all the protective devices for final circuits of a distribution board" (my emphasis). The thing to note here is that the definition of a final circuit is a circuit that directly supplies current using equipment (or sockets for the same) - so a circuit for a generator would normally not be a final circuit (even though it's the last leg of the connection to the generator) - as the generator is a current sourcing, not current using. Note also that the requirements for final circuits, especially disconnection times for ADS, are far more onerous than for other circuits (e.g. 0.4s rather than 5s). If you had a mix of a generator and normal appliances on the same circuit, there is a potential to undermine ADS - the protective device may open, but the generator could still provide power to the loads (even ones that disconnect automatically on grid fail take time to notice the grid fail - often anything up to 5s) . So what I believe 511.7.2.1 is trying to say you're not permitted to share one circuit between a generator and normal loads, but that the generator must have its own dedicated circuit - the result being that all normal final circuits are supplied in such a way that opening of their protective devices ensures immediate disconnection.
So as to your original question, as far as 551.7.2.1, either approach is valid and acceptable.
There is a side consideration though - in that the DB might be obliged to carry not just current from the grid, but current from the generator at the same time, so if the grid side overload protection is rated for the full current carrying capacity of the DB (often the case for 100A CUs and 100A DNO cut-out fuse for example) the DB could lack overload protection when the generator is run in parallel with the grid. Often the solution to that is a separate CU for the generator - but not simply Henley's into the existing tails, but with additional overload protection (say a 100A fuse) between that point and the original CU.
- Andy.
It is also worth noting that many of the concerns about plug in solar or batteries also relate to the same question - where normal socket loads share the final circuit with what is in effect a generator, albeit a low power one and not a traditional mechanical alternator , then extra precautions need to be taken to make sure it stops generating when the external supply is removed. In comparison, at least in principle, a hard-wired one may be used more like a stand-by genset, and if interconnected with suitable change over switching, might be used to carry on going as a generation island in the event of supply loss.
Its an area that has recently been in the spotlight, and will probably remain there while the problems and solutions for these newer situations are bottomed out.
So - true as the answer above, but be aware that there may be subtle changes to the wordings of some of the regulations around connected generation and what is a 'final circuit' coming up in future.
The good news is that physics of what works and what is sensible remains unchanged however.
Mike.
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