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Isolation Transformer Control Circuit

P Trim
Hello, I have been carrying out an EICR on an industrial installation and it was brought to my attention a control circuit at a machine has been supplied from a 230V Isolation transformer.  The transformer has no Neutral to Earth bond on the secondary side, the controls enclosures are metal clad with all exposed metal parts earth bonded back to the main earthing system on the primary side. There are several simultaneous conducting parts adjacent i.e. sockets, FSU, wall heater, structural steel support to building all adjacent to the console and it is conceivable that all conducting parts could easily be reached so a non-conducting location is not possible. 


My understanding of control circuits is that the common 0V connection must be grounded for several reasons, single fault detection and to ensure correct operation of contactors, relays, coils etc in the event of a single fault so as to mitigate maloperation.


Can anyone provide any advice/feedback please.


Cheers.


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  • 0
    Just in my opinion, but I always feel slightly uneasy seeing anything above extra-low voltage for control circuits especially where there are several touch points i.e. controls, actuators etc and the environment requires addtional measures to prevent electric shock.

    From purely a shock perspective an unearthed secondary isn't that bad at all - as the first fault rather than making some exposed-conductive-part live merely degrades the system to a kind of TN arrangement (just as if you'd installed a N-PE link in the first place) - it then takes a second fault, on the other pole, to create a voltage difference between two different exposed-conductive-parts and as long as those parts are both earthed (or just bonded together) you then get a short and the overcurrent protective devices will usually provide ADS. Such separated or IT arrangements are actually preferred for some safety critical systems where not failing on first fault is an advantage - although an insulation monitor to warn of the presence of a first fault is preferable.


      - Andy.
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  • 0
    Just in my opinion, but I always feel slightly uneasy seeing anything above extra-low voltage for control circuits especially where there are several touch points i.e. controls, actuators etc and the environment requires addtional measures to prevent electric shock.

    From purely a shock perspective an unearthed secondary isn't that bad at all - as the first fault rather than making some exposed-conductive-part live merely degrades the system to a kind of TN arrangement (just as if you'd installed a N-PE link in the first place) - it then takes a second fault, on the other pole, to create a voltage difference between two different exposed-conductive-parts and as long as those parts are both earthed (or just bonded together) you then get a short and the overcurrent protective devices will usually provide ADS. Such separated or IT arrangements are actually preferred for some safety critical systems where not failing on first fault is an advantage - although an insulation monitor to warn of the presence of a first fault is preferable.


      - Andy.
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