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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

    If I have correctly understood your description, it sounds a poorly designed system.

    An isolating transformer for the control circuit is almost certainly a prudent addition. Operation of an emergency stop or similar facility needs to be reliable, even under fault conditions. A relay or contactor that sometimes stays in is not acceptable.

    Isolating transformers are widely used for control circuits and I am not aware of any prohibition.

    If the supply starts out as a 3 phase 4 wire system, but with the neutral not distributed to the point of use, then 230 volts between phases will be 133 volts to earth, not 110 volts.

    If the system is a true floating three phase system without an earthed neutral at the origin, then with 230 volts between phases, the voltage between any phase and earth can vary between the full 230 volts and zero. Variable according to leakage currents and earth faults elsewhere in the system.

    In either case, an isolating transformer will ensure a consistent voltage for the control circuit without any concerns as to voltages between phases and earth.

Reply
  • 0

    If I have correctly understood your description, it sounds a poorly designed system.

    An isolating transformer for the control circuit is almost certainly a prudent addition. Operation of an emergency stop or similar facility needs to be reliable, even under fault conditions. A relay or contactor that sometimes stays in is not acceptable.

    Isolating transformers are widely used for control circuits and I am not aware of any prohibition.

    If the supply starts out as a 3 phase 4 wire system, but with the neutral not distributed to the point of use, then 230 volts between phases will be 133 volts to earth, not 110 volts.

    If the system is a true floating three phase system without an earthed neutral at the origin, then with 230 volts between phases, the voltage between any phase and earth can vary between the full 230 volts and zero. Variable according to leakage currents and earth faults elsewhere in the system.

    In either case, an isolating transformer will ensure a consistent voltage for the control circuit without any concerns as to voltages between phases and earth.

Children
  • +1 in reply to broadgage

    I must thank you for confirming exactly my thoughts. May i ask if you are a Chartered Engineer as i would like to present what you have said to my Engineering team. If you wouldn't mind can i copy/paste your reply obviously without your name to my management?

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