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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.


Parents
  • 0
    Thank you for your detailed response, this clarifies my initial assumptions based on what I had found which is good to know I was on the correct track as I was not 100% certain. 


    I also had a read through BS EN 60204 this morning and found further guidence on control supplies in particular Section 9.4.3 Protection against maloperation due to earth faults, voltage interruptions and

    loss of circuit continuity     which goes further into detail.  


    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. With todays technology, extra-low voltages i.e. 24VDC can easily be utilised even for larger applications with power contactors etc. 


    This installation is >40 years and the Iso Transformer is only a recent modification, in an ideal world I would rather see they utilise extra low voltage but that will require a substantial financial investment to upgrade.


    Thank you all for your help.

Reply
  • 0
    Thank you for your detailed response, this clarifies my initial assumptions based on what I had found which is good to know I was on the correct track as I was not 100% certain. 


    I also had a read through BS EN 60204 this morning and found further guidence on control supplies in particular Section 9.4.3 Protection against maloperation due to earth faults, voltage interruptions and

    loss of circuit continuity     which goes further into detail.  


    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. With todays technology, extra-low voltages i.e. 24VDC can easily be utilised even for larger applications with power contactors etc. 


    This installation is >40 years and the Iso Transformer is only a recent modification, in an ideal world I would rather see they utilise extra low voltage but that will require a substantial financial investment to upgrade.


    Thank you all for your help.

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