There will be electric heating in each room, seven of them. A new CU is being installed for the whole flat. The idea is to have a sub CU off the main CU holding only the mcbs for the electric heating, so the heating is all in one unit. On the main CU, is an 63A RCD feeding the sub CU OK?
Sub CU
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The obvious and usual thing would be to have that 'heating CU' fed separately from the meter tails (using Henley blocks/whatever), via a switch-fuse if the distances are long, not from the main CU. Apart from anything else, what you propose would provide no over-current protection to the cable between CUs.
Kind Regards, John
Good point. They would be 16mm, but taking the sub CU off Henley blocks is no different, it will still be relying on the main fuse. An advantage is that the 16mm cables from the main CU to the sub CU will be RCD protected. Another advantage is that when the main switch is throw on the main CU all the power is off. One switch, one point.
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Fair enough - but unless those cables are buried, there is probably no real need for them to be RCD protected (any more than the cables from meter to CU are).
True - in fact, a 'single point of isolation' is actually required. However, although this requirement is frequently ignored, you could install an isolator (even plus a Type S RCD if you really wanted) upstream of the splitting of the meter tails in the Henley if you wanted.
Kind Regards, John
... but that surely would be true no matter how the 'sub-CU' was supplied, wouldn't it?
Kind Regards, John
It is neither required nor ignored. The IET have made it abundantly clear in guidance on this matter that what we think of as the installation may in fact be considered to be a number of installations.
It it is certainly true that the definition of an (electrical) "installation" in BS7671 (hence a definition to which the IET, as co-authors of BS7671, presumably subscribe) is so vague that it could be taken to describe virtually anything that involved electrical components.
However, I would personally say that the spirit of the regulations is that 'everything electrical' within a domestic dwelling should have a 'single point of isolation' - and that would seem to make total sense.
Again. For the safety of fire fighters.
No. The user knows that the smaller CU is the heating only which has only one switch switching off all the electric heating, while all the rest of the installation is on. Divide and rule. A timer could be fitted to switch off and on all the CU, the electric heating. It would ned to be a hefty timer, or a smaller timer switching in and out a contactor. Then all the heating is on a master timer.
... and the occupants. In a state of panic, a 'single point of isolation' that people know about is much more likely to be used that multiple ones.
Kind Regards, John
I don't understand your 'No'. What you say is true, but, as I said, that would obviously be true whether the small CU was fed from the 'large CU' or direct from the meter via a Henley.
Kind Regards, John
With this setup all of the heating can be isolated simply and easily to the uninitiated - simple to understand. The system also has a main switch as well, so all is off in the house. Could have both CUs off Henley blocks and a separate mainswitch for both, which produces messiness with three units on the wall.
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