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Switch the circuits, or the busbar ?

Discussion in 'Electrics UK' started by SimonH2, 15 Apr 2019 at 4:59 PM.

  1. SimonH2

    SimonH2

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    So, which way round would you do it ?
    You have a request to be able to switch off everything but a dedicated socket circuit, some lights, and the heating - partly as a "can't leave something on" measure, mostly as a "leave as little as possible powered up when the old, listed, and containing a lot of wood building is empty" measure.

    So, do you route each circuit individually via a contactor ? Pros - the contactor only has to be rated for the highest RCBO rating; it's flexible as to which circuits are switched. Cons - you quickly start needing a lot of poles (multiple contactors ?). That's the way the night storage was switched in the office I used to work in - but then it was only one or two circuits to be switched.

    Or split the live bar and switch the feed to the RCBOs ? Pros - only need one contactor pole (or 2 if you switch the neutral as well), wiring is simpler as you don't need to split the L for each circuit off via the contactor. Cons - need a higher rating for the contactor; less flexible in allocation of circuits.

    Neither option seems to have an overwhelming "that's the right way to do it" argument :unsure:

    For background ... at the moment there's only 7 circuits (3x lights, 2x sockets, 1x heating, 1x organ blower). But I know they are looking to significantly increase this - especially if they get funding for some alterations (including adding a kitchen).
     
  2. SimonH2

    SimonH2

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    Ah, one further pro-con that might just swing it for me.
    Switching the feed to the RCBOs means either one large fuse or MCB, or the contactor being protected only by the supply fuse.
     
  3. EFLImpudence

    EFLImpudence

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    Switch off everything except those, then.

    I wouldn't.

    I don't really understand that.
    The contactor? Only one for everything? Surely it would have to be rated for the total.
    If not, why do they have to be rated higher than the circuit they control?

    I would do that. Have another (main) switch after the circuits that remain on.

    You don't need any contactors.

    One option seems an awful lot of work.
     
  4. JohnW2

    JohnW2

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    Either would obviously work but, for what it's worth, I'd be inclined to the latter. I would think that the reduced need for effort and wiring, and the lack of need of multiple-pole contactor (or multiple contactors) would probably more than compensate for (in cost, as well as anything else) the need for a higher-rated contactor.
    That's presumably another reason for leaning towards the 'single-contactor' approach, since it would not need modifying if/when further circuits were added.
    Would the contactor really need over-current protection beyond that afforded by the cutout fuse? - after all, it wouldn't really be in any different a situation (in this respect) from that of a CU Main Switch or a post-meter Isolator, would it?

    Kind Regards, John
     
  5. bernardgreen

    bernardgreen

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    My choice would be split the bus bar into three section, one for the dedicated socket circuit and two switched with two contactors. Maybe as far as two consumer units.

    That way a failed contactor does not cause loss of all power / lights.
     
  6. ban-all-sheds

    ban-all-sheds

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    Whatever you do, don't forget that this is the 21st century, and therefore it is essential that the switching be internet enabled and that you write apps for mobile devices so that people can do the switching and interrogate the status of it from beach cafés in Thailand...
     
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  7. Adam_151

    Adam_151

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    I think in the situation given I've be looking to have essential and non-essential boards, is a contactor necessary?, can't they just switch off the NE board when the building is empty.

    Have you considered the emergecny lighting?, remember if you have the fittings on dedicated circuits, then they have to be maintained fittings as per BS5266.

    Most of the places that I go to that have been wired like this, it has fallen out of use, stuff has been tagged off whatever is closest, switching off office sockets was all well and good until every desk got a PC on it in the late 90s!, now I understand your installation is a little different, but will the whole lot get forgotten as soon as someone puts a fridge somewhere you didn't antipate a fridge and has a fridge load of rancid food pretty quickly....
     
  8. SimonH2

    SimonH2

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    It would be each switched circuit routing via one pole of a contactor - so 3 or 4 circuits to a contactor.
    So the rating if the contactor would be the highest RCBO on a circuit routed through it.

    As to just using another main switch ...
    Are they rated to make/break on load repeatedly ? Also it means someone going to the DB, opening the cover, and operating the correct switch - which is a slightly different requirement than just using the red & green buttons which can be more conveniently located.
     
  9. EFLImpudence

    EFLImpudence

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    Ok. Fair enough.

    Yes. They are (100A?) switches.
    Anyway, there presumably wouldn't be much laod, would there?
    Are they going to leave with the cooker on?

    You could place it alongside the CU.
     
  10. ban-all-sheds

    ban-all-sheds

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    Are "ordinary persons" (or whatever the phrase-du-jour is now) allowed access to the DB(s), or the room they are in?
     
  11. SimonH2

    SimonH2

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    Yes, but only in the same way that people have access to a domestic CU - so if a breaker trips they can reset it. Any contactor or other stuff will be behind a separate cover needing a tool to open.
     
  12. 333rocky333

    333rocky333

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    seperate board fed via a motorised mccb or even just a normal contacter, wired to a last man out key switch
     
  13. SimonH2

    SimonH2

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    While our modest church is but a broom cupboard by comparison (though we do have a high quality stained window in the east end), today we have a timely but sad reminder of why such precautions are considered good practice.
    Notre Dame fire

    It would seem that the building was largely saved by prompt reporting of the fire - in our case, unless someone happened to be around at the time, it could be very well alight before anyone could see it. Prevention is definitely better than cure :whistle:

    Answering a few of the queries suggestions, in no particular order ...
    Well splitting the bars is much the same thing, only a bit tidier. There are five (yes FIVE) fuseboards at the moment - the plan is to get down to one modern DB. As to switch vs contactor - I'm considering whether a switch that's "off the beaten track" as far as leaving and locking up is concerned would get used, a contactor offers flexibility for remote operation.
    It is something I have raised. At present we have no ELs at all - and one response I got was that most of us carry around a mobile with a torch function :rolleyes: Getting the 'lectrics into the 21st century is step one - after that there are other steps planned. or at least dreamed of.
    At the moment, no kitchen, no fridge. There is a plan to rework some parts of the building (specifically the non-listed, modern addition, porch) so that we do have a kitchen - and if that happens, then (if I have anything to say about it) there will be a dedicated circuit just for a fridge :sneaky:
    I can foresee, at some point, getting (for example) some form of electronic lighting control - whether that's just remote switching of lots of circuits, or multi-channel dimming, or something else, will depend on what we decide is needed. That's not the sort of stuff I'd be happy leaving powered up all the time when it can be easily avoided. And the sound system has been left on a few times. And ...

    Yes, thinking more about it I would agree. Also, it will effectively have downstream protection so unless there was a fault within the DB then the individual outgoing RCBO is going to limit the duration of any fault, and the final circuit is likely to limit the fault current somewhat.

    Some perusal of contactor specs in terms of switching capabilities in the presence of fault conditions is going to be required ...

    But that will be in "phase 2" - it's going to need work to get to the situation of having essential and non-essential socket circuits. However I don't want to get to phase 2 and realise that I should have done something different in phase 1 :rolleyes:
     
  14. JohnW2

    JohnW2

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    The BBC News appears to be suggesting almost the opposite - namely that much/most of the devastation could possibly have been avoided had prompt action been taken ... if I correctly understand what was being said/suggested, a fire alarm was activated about half an hour before the fire showed itself, but that no reason for the alarm was found when it initially went off.

    However, it's an evolving situation - so I suspect that we'll be hearing all sorts of different stories as the days go by!

    Kind Regards, John
     
  15. JohnW2

    JohnW2

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    Indeed.
    Are you not worrying about scenarios beyond the intended purpose of the contactor(s)? I thought that it was only going to be de-acti'vated' by the 'last man out' (at a time when one would imagine/hope that the current to be broken would be minimal, maybe even zero) - under what circumstances would it be called upon to switch a fault current (or even an overload current)??

    Kind Regards, John
     
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