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Something keeps bugging me.

Let's assume we have a machine electric cabinet connected to 3x400v with no neutral wire. Only ground is supplied. 

How do you generate 230V?

Almost every machine I come across has a voltage transformer (400V/230V) with  grounded secondary for supplying control circuits and single phase fans, cabinet air conditioning unit ..., thereby creating a psuedo TN-C-S grounding system.  Why is it not allowed to omit the transformer and connect the neutral directly to the ground busbar?

And then use a RCB or RCBO downstream for protection? Besides the unevan phase loading, I don't see any drawbacks to using the second approach. 

Am I in the wrong?

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  • \$\begingroup\$ Assuming you own the house negotiate with the company who owns the mains AC distribution network that your house is connected to. They tell what they allow. They may offer something that you see useful. All actual works should be done by a qualified contractor - one which is trusted by the owner of the mains AC distribution network. If you are a third party electrician called by the frustrated owner the house, do not try your solution. The consequences may spoil the rest of your life, no matter it might work just when assembled. \$\endgroup\$
    – user136077
    Commented Jun 11, 2022 at 8:30
  • \$\begingroup\$ "How do you generate 230V? Almost every machine I come across has a voltage transformer (400V/230V)" That is how you safely obtain 230 V from a 400 V delta supply. That's why you keep seeing it. \$\endgroup\$
    – Graham Nye
    Commented Jun 11, 2022 at 10:34
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    \$\begingroup\$ It's not creating a pseudo TN-C-S system, it is creating a TN-S. The transformer secondary is the source with neutral and earth connected at that point. There should also be a fuse or circuit breaker in the active of the secondary. \$\endgroup\$
    – Kartman
    Commented Jun 11, 2022 at 12:17

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Even if neutral and earth/ground are connected somewhere upstream in the electrical system, under normal conditions, the neutral wire is the conductor intended for carrying return currents and the ground/earth wire is for safety and carries current only during fault conditions.

So what you propose would technically work but it would break electrical code/regulations for safety. If the earth/ground wire upstream ever breaks then effectively every part in the whole machine that should be earthed/grounded becomes live with mains voltage and the 230V control parts would stop working while the machine still has working 3 phase 400V.

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  • \$\begingroup\$ Thank you for your answer. The RCB (30mA) would still activate in case someone touched a live metal part, disconnecting power. \$\endgroup\$
    – SonetKP
    Commented Jun 11, 2022 at 12:37
  • \$\begingroup\$ Yes, and it would still be illegal to use earth/ground wire as return path if neutral is missing. \$\endgroup\$
    – Justme
    Commented Jun 11, 2022 at 12:56
  • \$\begingroup\$ @SonetKP If you use the earth as a return for the control circuitry any RCD will trip as soon as the control circuit current approaches 30 mA. \$\endgroup\$
    – Graham Nye
    Commented Jun 11, 2022 at 13:31

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