Boiler isolator: Also on that diagram, the boiler has two switched lives (so it can run DHW at a higher flow temp). So I can't provide a normal boiler isolation switch. All I can think of doing is to put the heating FCU by the boiler (I'd rather put it in the airing cupboard, but I suspect Mr Boiler will complain) and make it a lockable one. This is a pain, because it would be much easier for me to simply run a 5 core from the airing cupboard to the boiler, than to additionally run a t+e (or two, if I want to put it on its own circuit rather than spur it off the kitchen). But I can't think of any other possibilities.
Running a submain to an outbuilding. The guy who will be signing off the work for the BCO wants me to TT it because there's a water pipe. I'd rather export the TN-S earth the grand distance of 1 metre, and do it in 3 core 10mm, but if he wants it TT-ed, I suppose he's going to get it. Does let me use 6mm 2-core, at least (the submain has a switch fuse at 60A, and 6mm is good for 62 - it's XLPE before you shout). I need to use a metal CU at the outbuilding, because it's essentially residential. So how do I terminate SWA but not bring the earth in? Looks like I could use a 20mm gland, and then a Wiska 25mm to 20mm reducer, but there must be a more general solution than that? Could use a terminal box outside, and then T&E through the wall - but I'm hoisted on my own petard, because Wiska boxes stop at 57A, and I'm fused at 60A.
Rant. When I've finished all this, I shall write up all the horrible things that were done to the wiring in this house. 70m of external cable done in 2.5mm T&E buried a couple of inches under the grass. That sort of thing.
The isolator switch would be between the fused spur and the wiring panel. So when you turn it off you isolate the boiler, the controls, the programmer and everything else in one hit including both those switched lives (or more correctly, the live from which they are derived)
Remember that XLPE only has a higher max current rating because the maximum insulation temperature is 90 deg C rather than 70. If you plan to make use of that extra "budget" then all the things connected to it need to also be rated for the higher temperature.
Use an insulated gland at the far end of the SWA so that you don't terminate the earth on the SWA to the TT infrastructure. (the source end being connected to the TN-S earth, providing earth fault protection for the SWA as far as the outbuilding).
Are you sure the connections at both ends are rated for 90 degrees? You are using the 90 degree table to get that current carrying capacity, and I think you'll find most domestic units are only 70 degreee type appoved.
The usual white two pole isolator would allow isolation of two lives but no neutral, which probably isn't a great idea.
However there are three pole fan isolator switches:
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if 10A is enough and it was labelled appropriately, you might get away with that. If it was downstream of a FCU you don't need to have a fuse in the isolator.
Two switched lives, plus L+N. It would need to be a 4-pole.
Looks like I will have to run the T+E to the FCU in the airing cupboard, T+E to the isolator, T+E back to the wiring centre, and then 5-core back to the boiler. Well, so be it. At least it is then isolatable from both ends, which has its benefits.
John is correct about the above you need to provide one single point of isolation for the whole system and if I recall correctly that point needs to be adjacent to the boiler. It could be a switched fused spur unit usually with a 3A fuse.
When I replaced a back boiler with a wall mounted in the garage, I tried to leave as much of the system where it was so the wiring centre, valves and pump were in an airing cupboard above the chimney breast where the boiler was. We moved the programmer to the kitchen for convenience and so I had to install wiring for that and feed the boiler from the wiring centre but the main isolator was beside the boiler and the wiring centre was powered from there.
One belt and braces thing I did was to fit a neon to the wiring centre to show if it was powered.
John is correct about the above you need to provide one single point of isolation for the whole system and if I recall correctly that point needs to be adjacent to the boiler. It could be a switched fused spur unit usually with a 3A fuse.
When I replaced a back boiler with a wall mounted in the garage, I tried to leave as much of the system where it was so the wiring centre, valves and pump were in an airing cupboard above the chimney breast where the boiler was. We moved the programmer to the kitchen for convenience and so I had to install wiring for that and feed the boiler from the wiring centre but the main isolator was beside the boiler and the wiring centre was powered from there.
One belt and braces thing I did was to fit a neon to the wiring centre to show if it was powered.
>> so if 10A is enough and it was labelled appropriately, you might
It is traditional to include a switched FCU adjacent to the boiler so you can isolate that in an emergency. However the main isolation switch for the system can be elsewhere - typically wherever the zone valves congregate. So here for example the main switch is in the airing cupboard since that is where the wiring hub is, the zone valves and unvented cylinder.
Make the FCU in the airing cupboard a Switched FCU, and that can be the system isolator. So power to the boiler and the control wiring, programmer etc are all downstream of that.
You could have two switches, assuming there isn't a problem when only one of them is disabled. 3 pole switch for the Lives plus another for N?
Another option would be a 4 pole contactor powered by a single switch like a light switch.
T+E to switched FCU in the airing cupboard, T+E to wiring centre, 7 core over to boiler, isolator switch by the boiler? Power goes SFCU-boiler isolator-wiring centre-controls-boiler.
Use the isolator by the boiler to work on the boiler, use the SFCU in the airing cupboard to work on the wiring downstream of the SFCU.
sounds dubious, someone turns of the neutral switch, boiler goes off, so they assume it's safe to work on, then discovers the hard way that the lives are still live ...
Terminate the SWA into a gland pack inside an adaptable box and take the inner cores with it's protective sheath through the wall into the outbuilding CU.
I haven't used contractors (and reading suggests there would be some quite significant power leakage) but is a contactor a satisfactory isolator? I'm not sure I'd personally want to rely on anything other than a physical switch.
Hmm, it seems some contactors are rated for isolation and some aren't. I suppose you need some kind of physical indication of inactivation, which might make that more complicated than you want...
I've been thinking about this, and when you say 'terminate' do you mean using a conductive metal gland kit where the nut etc is connected to the armour and electrically connected to the "adaptable box"?
Or do you mean do you mean use an insulated cable gland of the correct size on the outer sheath and leave the armour floating? Where the armour is connected to the earth at source.
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