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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.
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    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.
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  • 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.
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