Since neutral is bonded to ground at building distribution point in a TN-S or TN-C-S earthing system, could floating neutral happen under the following conditions:

  1. Loss of N and/or PE at transformer
  2. Loss of N and/or PE at building distribution point
  3. Loss of N and/or PE at transformer and building distribution point

I'm trying to understand how to prevent floating neutral in a 3-phase power supply and thus avoid damage to low voltage equipments connected to a single phase.

  • \$\begingroup\$ Is this in the UK? and you are referring to the UK wiring regs for TN-S etc \$\endgroup\$ – Solar Mike Jun 17 at 21:13
  • \$\begingroup\$ @SolarMike Yes, UK wiring regulations for TN-S and TN-C-S earthing systems. \$\endgroup\$ – Joseph Jun 17 at 21:23
  • \$\begingroup\$ Add that info to the question, add the UK tag. \$\endgroup\$ – Solar Mike Jun 17 at 21:24
  • \$\begingroup\$ @SolarMike The building wiring and earthing follows UK regulations (which I have control over) but the sites are located in various parts of the world where grid does not offer sufficient protection like in various parts of Africa and Asia. \$\endgroup\$ – Joseph Jun 17 at 21:32

Speaking about TN-C-S, the N an PE are the same wire on building input-PEN, floating N also means floating PE:

The PEN wire has to be connected to the building ground and/or earthing rods/strips buried in place.

In such way all PEN conductors make a grounding mesh connection, if one fails the other takes over. The most important is the one in place: if the incoming PEN is broken, then you still have a ground connection through your's ground, otherwise all the metallic parts would get on live potential.

For TN-S the PE and N are separated, a floating N isn't an issue.

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    \$\begingroup\$ The (very) difficult bit is to achieve a ground resistance low enough to achieve anything useful in the event of a lost PEN connection. Most UK installations don't even try. \$\endgroup\$ – Simon B Jun 18 at 8:05
  • \$\begingroup\$ @SimonB Why not? I guess UK is not on sand soil. The problem would be achieving low resistance in Saudi Arabia or Kalahari, Sahara desert, but not in UK. For sure the buildings in UK have a grounding which have very low resistance. \$\endgroup\$ – Marko Buršič Jun 18 at 9:48
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    \$\begingroup\$ A 3 phase supply in the UK is usually at least 100A, 230/400V. A simple earth rod may give you a few hundred ohms resistance. But suppose you bury something more substantial and achieve 10 ohms. A broken PEN won't give a fault current through the earth more than about 230/10 = 23A. That's nowhere near enough to blow the supply fuse. Meanwhile, that earth mat will still be sitting somewhere close to the supply voltage, and getting rather warm. There was a proposal to add a requirement for earth rods on TN-C-S supplies to the IEE Wiring Regulations recently. It was abandoned. \$\endgroup\$ – Simon B Jun 18 at 12:19
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    \$\begingroup\$ @Joseph It's almost both the same, TN-S floating PE, TN-C-S floating PEN. In any case you need the PE/PEN to be connected to building ground, earthing rods, pipes,..as per regulations. \$\endgroup\$ – Marko Buršič Jun 19 at 20:11
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    \$\begingroup\$ @Joseph The RCD won't trip, because it measures the sum of all currents, I1+I2+I3+In=0, so if the device insulation is not damaged, then you won't get any difference. I can confirm you (my experience) that in case of loss of neutral, your laptop charger will blow, as you already found out. \$\endgroup\$ – Marko Buršič Jun 20 at 5:55

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