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3 wire 3 phase ungrounded systems

Former Community Member
Former Community Member
I suspect I already know the answer but are 3 phase, 230 volt, 3 wire ungrounded systems permitted in a UK installation?


I know this is common practice in some parts of the world but it's not something I've come across in the UK.


Just for clarity this would entail a delta connected secondary with 230 V between phases requiring separate earth fault detection.


I wait with abated breath.


Bill
  • What equipment is this circuit intended for, I presume it is a piece of imported equipment that serves a very specific function for which there isn’t a UK alternative?


    Andy Betteridge
  • Former Community Member
    0 Former Community Member
    As far as I can tell nothing too specialist. Control system computer, I'm assuming DCS. It looks like this has come from the US without taking into account "local" standards and practices. Not an uncommon mistake. My concern is that the unconventional (from the UK perspective) in itself introduces additional risk to those working on it in the future. If I'm going to redesign in line with UK standards I need a good reason.
  • You can use electrical seperation for one item of equipment, but I don’t have a clue where you can source a suitable isolating transformer to give the required reduction in voltage and electrical separation.


    A conversation with the technical guys at a transformer manufacturer would probably be a good starting point to see what’s available off the shelf or if you need to get something made.
  • I presume this is what you need.


    If it is combined with the appliance rather than supplying a circuit within the electrical installation to the appliance then surely BS7671 need not really be considered as it would come under the regulations covering the appliance you are modifying.


    When the transformer is installed will it be part of the appliance or the fixed electrical installation?
  • Former Community Member
    0 Former Community Member
    All fixed wiring supplying various DB's some above 100Vac so 7671 doesn't apply, some at 400 other at 200 so firmly in the court of BS7671.
  • I would probably install the transformer at the point of utilisation with a standard circuit probably to a earthed three phase socket outlet, then install the transformer feeding directly and incorporated into the appliance supplied through a plug and flex.


    I would not install the transformer at the distribution board then run a circuit to where the supply is required as that is far more problematic when it comes to specifying circuit protection and the type of cable to be used.


    Is the equipment that is to be supplied actually double insulated or does it require an earth connection?
  • In addition to Andrew's transformer recommendation, I have found this company helpful for supplying three-phase transformers: https://www.bcet.co.uk/
  • Former Community Member
    0 Former Community Member
    Many thanks for the inputs. I suspect it's not so much a case of specialist equipment and more a case of the author not researching the standards where the equipment is to be used. I think I'll follow my instinct and go for a rework inline with local (UK) standards.


    Thanks to all
  • Former Community Member
    0 Former Community Member
    Bill,

    Don't forget that for the unit to be legally used in the UK it must comply with the requirements of the Low Voltage Directive, and it is the responsibility of the importer to ensure this is done, it will require a declaration and technical file etc. and be CE marked, regardless of Brexit, the Low Voltage Directive has been brought into UK law.

    I often see this being ignored, but as I say it is a statute law requirement.

    I would look to see if it complies and look to doing any modifications to meet the legal requirements.
  • A 3-wire 3-phase ungrounded system is the norm for power distribution on merchant ships and I would expect navy ships.  Used both for 440 volt power distribution and 220/240 domestic distribution. Comes with double-pole switching for sockets, although I never saw double-pole fused plugs when UK style 13 amp sockets were used. Whilst earth leakage resistance was monitored, I do not recall ELCBs as such, only an alarm output when a predetermined value of minimum earth leakage resistance was breached. From memory, this was typically 220k ohms.

    Clive