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Panel Isolators - Different Voltages?

Les Spiteri
Where different voltages are fed into a panel is it permitted to isolate them both through the same isolator?
  • For example where you have a Main Fused Isolator for an incoming 3ph supply and a 110Vdc incoming from a UPS?

  • My experience says that as you quite often get auxiliary isolation contacts then it is possible.

  • However should these auxiliary contacts only be used for signalling and therefore cannot be used as a mean of isolation?

  • Also I frequently see 3 or 4 pole isolator used for single phase supply so can you use the remaining contacts to isolate another voltage?


Any thoughts would be most welcome.
  • 0
    I suspect this is going to be the devil's in the detail sort of one.


    As a general principle, it's best to isolate everything simultaneously by a single device - so sharing the same isolator sounds like a good idea ... but there is a lot to get right. Where you have a mix of LV and ELV shock protection considerations might mean that the two systems have to be kept properly separated - which might be tricky with adjacent terminals on the same device meaning the wiring systems will have to be close to each other and/or the internal arrangement of the device might not provide complete separation. Similar consideration might apply if a fault between the two systems could do a excessive amount of equipment damage even if it wasn't a shock hazard. Then there's the mix of d.c. and a.c. to consider - d.c. is actually harder to switch, especially where larger currents & voltages are concerned - as there's no automatic extinguishing of arcs every half cycle - so an isolator that's fine for your a.c. supply might not be suitable for the d.c. side - so that might limit your choice. There might be functional considerations too - is the UPS fed supply actually meant to keep things running when things on the normal side are shutdown? - so there might be times when you might need to be able to isolate the two systems independently of each other.


    So really a shared isolator is just one option in your tool box - there are many others varying from signage to internal barriers to even work procedures - so you can choose whichever (or combination) best suits each individual situation.


       - Andy.
  • 0
    Possible in many situations,  but In cases where you may want synchronous isolation of many supplies or devices, but cannot bring the cabling together for reasons of EMC or ratings,  then one switch operating multiple contactors may be an alternative.
  • 0
    mapj1:

    Possible in may situations but In cases where you may want synchronous isolation of many supplies or devices, but cannot bring the cabling together for reasons of EMC or ratings,  then one switch operating multiple contactors may be an alternative.


    Only where there's no risk of accidental contactor energization when testing.


  • 0
    Great comments guys, gkenyon could you expand this statement? 

  • 0
    You also need to think through the unusual operating requirements. Maybe work to upgrade one supply would require that you isolate one to prevent back feed while continuing to use the other.
  • 0
    Les Spiteri:

    Great comments guys, gkenyon could you expand this statement? 

     


    Means of isolation should be able to be secured to prevent accidental re-energization, for safety in testing, maintenance, repair, etc, activities.


    If you use contactors to provide isolation of a supply (whether that supply is LV or ELV doesn't matter ... and I'll expand on this later), you need to be able to either:

     - mechanically secure ("lock off") the contactors in the isolated state

     - secure the switch operating the contactor in the disconnected (open) state, AND ensure that no test equipment that might ever be used during "dead operations" during repair, maintenance and testing, can energize the contactor - for example, accidental voltage or current injection from a piece of test equipment or a test circuit.


    Why not necessarily a difference between ELV and LV? Well, with many ELV supplies the danger is not necessarily electric shock, but fire, arcing, heat and burns ... and potentially arc flash ... for example with supplies that have secondary batteries. Isolation and safe working practices can therefore be equally important with ELV circuits, as with LV, and all of these types of danger are covered by Electricity at Work Regulations.


  • 0
    In many circumstances, one might wish to isolate the mains voltage supply for safe working in a control panel, whilst leaving the lower voltage and probably low fault current control supply live to assist in fault location.


    I would with reasonable care, work on a live 110 volt control circuit. A fatal shock is unlikely, and arc flash risk minute as the fault current will be limited by the impedance of the control circuit transformer, and the load current be limited by small fuses.

    If the control circuit remains live with the main power circuit isolated, I would consider it good practice to fit duplicated warning lamps INSIDE the panel that show this. Good LED warning lamps should remain serviceable for the life of the equipment.
  • 0
    Agreed @broadgage - this is permitted in BS EN 60204-1, and a reason I also stated "many ELV supplies", not "all ELV supplies". BS EN 60204-1 requires different coloured conductor identification for circuits not under control of the panel isolator as another indication.


    I just thought it was worth highlighting that danger (and hence requirement for isolation) is not just an LV issue.
  • 0
    All good stuff thank you, I also had an occasion recently where
    • We had a 3ph supply incoming going through a main fused isolator,

    • These were heavy cables at 120mm csa for large motors.

    • Strangely in my experience there was also an incoming neutral using for control (240Vac t a single phase)

    • The incoming neutral was only 2.5mm csa, this was put through an auxiliary contact on the MFI.

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