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Three Phase 230V three wire systems

Mark Whittingham

We have a emergency shutdown system on our rig that is wired in such a way the relays always have a minimum of 90v on L2 side of the coil so it is intermittently not opening on ESD activation. I believe this is incorrectly designed as we have a three wire three phase system, no neutral so in theory 110v each phase and 230v across any two phases.

Intermittently when activating the ESD buttons the activation relays will not open because of the constant voltage of anywhere between 90-110v on L2. I am trying to convince my engineering department that this is incorrect and we need to redesign but it is difficult as they don't understand the issue.

What i would like to propose to temporarily fix the issue is to put in a Isolation transformer and tie down the L2 side to earth of the secondary to give us a 0vL2 and 230v L1. This guarantees the relays will work when intended but i am not 100% sure this is allowed?

Can someone please assist me if at all possible.

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  • 0

    You are welcome to quote my reply and/or to forward it, it was made in a PUBLIC forum.

    Do not however take my remarks as being utterly correct until confirmed by at least one other respected member. I believe that I have sufficient qualification and experience to make the suggestion that I have made.

    I do not publish details of full name and qualifications, partly for fear of legal action if it all goes wrong, perhaps as a result of me misunderstanding the original brief description. Or even that description being inadvertently  misleading.

    Returning to the actual control circuit, I would add an isolating transformer, in line with accepted practice. If one side of the control transformer is earthed, then consider carefully the consequences of an earth fault AT ANY POINT in the circuit. It is sometimes preferable for a control circuit to be "floating" that is without either side earthed. 110 volts is popular, but other voltages are used.

  • 0 in reply to broadgage

    Fully understood and thank you for the reply. I look forward to some more input into this subject. 

    I have taken into account the consequences and my risk assessment process was based on all parts in the system are plastic and non conductive at all, primary and secondary of transformer are protected by suitable rated MCB's and all E-Stops are Ex rated Exde so i feel we are ok to proceed.

    I feel the benefits far outweigh the dangers on this occasion. I would prefer to completely re-design the control system but due to operational and internal politics it is far easier to temporarily fit the transformer and work on a more permanent fix.

    At least the system is operational and will give us confidence it will work in a emergency situation, at present we don't have a shutdown facility.

Reply
  • 0 in reply to broadgage

    Fully understood and thank you for the reply. I look forward to some more input into this subject. 

    I have taken into account the consequences and my risk assessment process was based on all parts in the system are plastic and non conductive at all, primary and secondary of transformer are protected by suitable rated MCB's and all E-Stops are Ex rated Exde so i feel we are ok to proceed.

    I feel the benefits far outweigh the dangers on this occasion. I would prefer to completely re-design the control system but due to operational and internal politics it is far easier to temporarily fit the transformer and work on a more permanent fix.

    At least the system is operational and will give us confidence it will work in a emergency situation, at present we don't have a shutdown facility.

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