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In a TN-S system and for a configuration below (single-phase isolation transformer, rectifier and single-phase DC/AC converter) what should be earthed (connected to PE wire)?

Transformer, rectifier and single-phase inverter

I've been going through various sources and there are different views. In Ref1 it says that isolation transformer output is floating and should be referenced (grounded). Same as in Ref4, page 20 where neutral-ground bond is mentioned. However, Ref2 says in case of fault, "current path is contained within the secondary of the transformer, the RCD will not detect an imbalance and will therefore not trip. The operator is now in an unsafe environment with the potential for an electric shock as they may become the lowest point of resistance for the leakage current". Finally, Ref3 says "the inverters generate a neutral wire that is grounded in the auxiliary power supply", i.e. inverter output terminal 4 should be grounded. Not sure if this implies grounding rectifier terminal 4 as well. This reference is for rolling stock applications, so there might be some different general requirements. I could not find any recommendations on what to do when the inverter is supplying load (i.e. off-grid application).

Since the system is TN-S, neutral has been referenced to the ground at the source before entering the main panel. Do I need to re-reference neutral at every output terminal (transformer N, rectifier 4 and inverter 4)?

I guess, if there is a 400 V input, then it would make sense to reference inverter's output and create a TN-C system (Ref4, page 20).

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  • \$\begingroup\$ It might be determined by regulation, and possibly also depend on whether the load is contained with the secondary side or not. Generators have a similar quandary. Have a look here cpower.com/PDF/InfoSheets/44.pdf and here jadelearning.com/blog/… What is your jurisdiction and have you looked at the electrical code? \$\endgroup\$
    – P2000
    Jul 9, 2020 at 5:40
  • \$\begingroup\$ Thanks P2000 for the links. The model I am using is similar to a non-separately derived system. I should connect inverter's neutral terminal (4) with neutral wire from the grid (Diagram 2 from the PDF link) if I don't want system to float? \$\endgroup\$
    – mikebuba
    Jul 9, 2020 at 10:51
  • \$\begingroup\$ I will write it up as an answer, because I am running out of comment space here, ok? \$\endgroup\$
    – P2000
    Jul 9, 2020 at 17:16

2 Answers 2

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Normally inverter outputs float and are not directly grounded. Even though it floats relative to Earth ground, touching pins 3 and 4 at the same time will cause a nasty shock and burn. Hard grounding pin 4 after the diodes means touching just any pin 3 will cause a bad shock.

For this reason grounding is often done using 1 M ohm resistors rated for twice the line voltage, or 2 470 K ohm 1/2 watt metal film resistors in series, which would have a 800 volt rating. Sometimes a 3 KV disc capacitor of 1 nF to 5 nF is in parallel with the resistors to ground RF noise.

The resistors offer a static ground so output has at least a high impedance reference to Earth ground. The resistors limit "shock" current to a safe level. You may feel a mild shock but it has a million times less current to hurt you with. This is about as far as you can safely take this grounding issue.

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  • \$\begingroup\$ Just Earth-ground every node. Then you are safer. ;) That said, I like the way you put this. So +1! (I don't recall the term "rolling stock", but looking it up I think it means "trains" or "trolley cars." That's a whole different subject about which I know far too little.) \$\endgroup\$
    – jonk
    Jul 9, 2020 at 3:49
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It might be determined by regulation, and possibly also depend on whether the entire load circuitry is contained with the secondary side or not.

Generators have a similar quandary.

Have a look here https://cpower.com/PDF/InfoSheets/44.pdf and here https://www.jadelearning.com/blog/grounding-requirements-for-portable-generators/

What is your jurisdiction and have you looked at the electrical code?

From how I read it, I think the neutral should not be earthed, but a ground/shield (casing, armature etc) can be earthed.

And it must be earthed if there is any connected load that is not contained "within" the generator.

Should any load be outside the generator (or -as in your case- an isolated power supply) then having a GFCI or RCD on the 230V side will provide protection of any live-to-casing shorts (and often neutral-to-casing shorts too).

Also, earthing the secondary side's neutral will protect the entire system from any primary-to-secondary shorts, as might be caused by a fire in or around the transformer. This melt-down would trip the primary side breaker or better -if available- the primary side GCFI/RCD.

Once you decide on earthing the ground, and including a secondary-side GFCI/RCD, you can then earth the neutral to also protect a human in the event of a contact with live (or in some cases also with neutral).

As a commenter points out, if all you want is to stabilize (remove) any static charge, you can earth the neutral with a high-ohm resistor.

Note that a high-ohm resistor would not provide sufficient earthing to enable use of a secondary side RCD/GFCI; for these to work you have to earth the neutral with a low-ohm connection.

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