Phase / switching problem.

Discussion in 'Electrics UK' started by D4dog, 1 Mar 2018.

  1. D4dog

    D4dog

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    I have a house which has the strangest electrics ever - converted from 3 phase. Originally, it used a three phase board, but lighting circuit, power circuits etc were random - and highly dangerous, because I could have sockets on one phase and lights on another in the same room.

    I had a sparky do his best to sort it out - we now have one phase for downstairs, and one for upstairs, with the third not being used - and lots of warning labels. However, I have one issue which I cannot sort out.

    I use a thermal store powered from solid fuel, with an oil boiler as a boost, which has an overheat stat. This will turn on the heating if the store gets too hot, and this sits on one phase. I also have a tank stat that governs the temperature of the thermal store - and because the boiler is downstairs, it sits on the other phase - again, lots of warning labels. It will turn on to keep the thermal store at the right temperature.

    The problem is that when the tank "dumps" because the solid fuel Raeburn is being efficient (ie the overheat stat cuts in), the returning cold water then turns the boiler on, which I do not want. I cannot run the stats in series, because they are on different phases. So what I need is to be able to safely switch the boiler out of the circuit from the overheat stat using the NC contact, so it wont fire if the overheat switches to its NO position. This means I need a relay, which I can hide our f harms way, which will take a feed form one phase and turn the switch on or off, but providing a switched output for a separate phase to control the boiler. Clearly, this phase isolation must be foolproof - anyone know of such a thing before I contact my sparks for advice?

    Thanks
     
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  3. ban-all-sheds

    ban-all-sheds

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    In what way would that be "highly dangerous"?


    What sort of warning labels? Where, and why?


    The "phase isolation" you seek is between coil and contacts - I would have thought that any contactor with contacts rated at 400+V would be OK.
     
  4. D4dog

    D4dog

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    Thanks ban-all-sheds. I was advised by my local sparks that having different phases in one room, where potentially someone could connect two phases via their own bodies was not allowed (or at the very least inadvisable) - and I have to say, seems sensible. But he is the expert (or not, as the case may be). The warning labels simply point out that there are two different phases used, with potential high voltages between them...but again, as advised.

    The problem I am having is finding a relay with 240v input to energise the relay, and 240v outputs for switching the other phase - all I can find (because I don't know where to look!) is low voltage switched relays that use either DC or low voltage AC to energise the relay. As you say, provided I have isolation between contacts and coil, I should be fine...are there any building regs issues in such an approach?

    Thanks
     
  5. ban-all-sheds

    ban-all-sheds

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    It is allowed.

    Whether you want to consider that there are degrees of advisability which make it OK enough to connect 230V across your body compared to 400V for the latter to be inadvisable is up to you. I suggest that neither is.


    They haven't been required for years.

    Your electrician is behind the times, and/or prone to old-wives-tales-ism.


    Look for contactors, not relays.


    No more than apply to any electrical work.
     
  6. flameport

    flameport

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    Ask them which regulation prohibits that.

    Hint - there isn't one.

    As for ' someone could connect two phases via their own bodies ' - attempting to do so would result in a severe shock as soon as they contacted the first phase, just as it would in a single phase installation.
    Would also require that live parts were exposed, which is already a huge fail for safety regardless of the number of phases.
     
  7. Taylortwocities

    Taylortwocities

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    As it is just a 'house' unless it is a very large one, or has substantial load requirements, I would have thought that just one phase would cover it. Then your imagined problems disappear.
     
  8. D4dog

    D4dog

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    Thanks both - I think the (however highly unlikely) scenario is person using hoover on one phase with a fault turns a light switch on with the other hand - and two faults happen at the same time, resulting in a novel earth route. Not sure if feasible, but as you say, the first belt would be a good reminder in most cases! Label's do me no harm - so will probably leave one on the contactor box just for clarity!

    Thanks both - I will talk to my sparks...now I know what I am asking for!
     
  9. EFLImpudence

    EFLImpudence

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    If you can do the wiring necessary for your proposal, then surely it must be possible to put all of your heating system on the one phase.
     
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  11. D4dog

    D4dog

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    The house had three phase because it was entirely heated by storage heaters - which all came on at 0100 via a massive 100Amp time switch. Long since gone - but there is also the ability to connect a small generator in the event of power failure, which connects via an isolation switch to one phase for the "critical" services - but only when all incoming supplies are isolated. This was to ensure that the heating pump will always run if the solid fuel is lit. Not used it, and not sure I want to try, although logically it looks fine and nobody who knows about these things (my heating engineer and sparks) seems to think it would be a problem

    Bizarrely, they only ran one phase to the detached garage and workshop, about 50m away - where it would actually have been useful...go figure!
     
  12. ban-all-sheds

    ban-all-sheds

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    Not just two simultaneous faults, but two simultaneous faults and two simultaneous failures of earthing of appliances and accessories.
     
  13. D4dog

    D4dog

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    Yes, I could put the boiler on the upstairs phase...but this goes against the advise I was quoted - the boiler service engineer uses the utility room sockets for his testing tools...which would then be on a different phase! But if you guys don't think that is a problem, it would be my quickest solution.
     
  14. JohnW2

    JohnW2

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    As you have been told, that's not really 'dangerous'.
    I have a similar situation - in broad terms, one phase for each of three floors of my house.
    I have similar potential issues - in relation to 'between-floors' heating control and 'phase failure relays'. There are at least two possible approaches ...

    Firstly, as has been said, any 'mains-rated' relay would probably be OK. The only 400V potential difference that would exist would be between the coil and the contacts, and even a relay/contactor with contacts 'rated at' 230V would almost certainly be OK for that (it's what I use for my 'phase failure' relays).

    Alternatively, but more complicated, for my between-floors heating control I use a 'very low voltage' connection between floors. On one floor I have a 12V ('lighting') power supply driven by the phase on that floor, and on the 'target' floor that feeds a relay with a 12V coil which switches the other phase.

    Kind Regards, John
     
  15. Risteard

    Risteard

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    Having a phase used on each floor and the remaining phase unused is certainly not doing anything to assist balancing the load between all three phases. Really this input sounds like an epic fail.

    Some frown upon multiple phases within a room but frankly it is a nonsense. I would much prefer to concentrate on balancing all three phases as best as possible.
     
  16. flameport

    flameport

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    That advice was wrong.

    It's not a problem at all.
    Most commercial and industrial installations have multiple phases in the same locations, including things like multiple gang light switches which contain all 3 phases.
     
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  17. JohnW2

    JohnW2

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    I'm not sure that there is any real reason to be concerned about balancing the phases within a particular house/building installation. Mine (one on each of three floors) is certainly very unbalanced - as you might imagine, the ground floor load greatly exceeds that on either of the other floors. Over the past 30 days, I have used 484kWh on P1, 247 kWh on P2 and 80 kWh on P3.

    As far as the network is concerned, having one building using only two of the three phases is no different from a three-phase supply to a row of houses, the number of which is not an exact multiple of three.

    Kind Regards, John
     
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