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Are two-pole breakers required on single-phase 230 V loads when connected to a 3-phase supply with IT earthing system (with distributed neutral) in Germany?

Where in the standards is this requirement specified? What fault would be isolated by a two-pole breaker that wouldn't be isolated by a single-pole breaker?

I'm looking at https://www.se.com/ww/en/download/document/ECT178/ and it says:

Multi-pole breaking, including the neutral conductor when distributed.The reasons for this are:
--breaking only of the faulty phase conductor of a feeder means that three-phase machines are supplied by the two other phases
--breaking of the neutral exposes to phase-to-phase voltage, single-phase loads normally supplied by phase to neutral voltage

The 3-phase load has no neutral, and is protected by a 3-pole breaker, so the first point above is covered

The single-phase loads wouldn't have a breaker on neutral, only on the live phase, so the second point above...doesn't make sense to me.

I'm also about to purchase IEC 60364, but I'm not hopeful that I'll see a cut-and-dry answer in there.

Thanks.

Mike Jones
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  • This question is much better suited for the [DIY Stack](https://diy.stackexchange.com/) IMO. – brhans Mar 23 '22 at 19:15
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    If you have multiple unbalanced single-phase loads on your 3-phase supply, and you lose neutral, you could potentially see the full phase-to-phase voltage appear at some of your single-phase loads (depending on how unbalanced they are). – brhans Mar 23 '22 at 19:21
  • Thanks, It's an industrial application, not home DIY. Looking at other posts here, and other posts there, I would have thought that this was the right place. I'm happy to take advice wherever it comes from though? Anyone else have thoughts in this regard? – Mike Jones Mar 23 '22 at 19:21
  • Yeah even though it's industrial I think the member population over there deals with this kind of thing more often, particularly when looking at it from a "code rules" perspective, but I wouldn't say it's off-topic here. – brhans Mar 23 '22 at 19:23
  • "If you have multiple unbalanced single-phase loads on your 3-phase supply, and you lose neutral, you could potentially see the full phase-to-phase voltage appear at some of your single-phase loads (depending on how unbalanced they are)." - OK, I understand, but I'm not proposing to put a single-pole breaker on neutral. The single-pole breaker would be on whatever phase feeds that branch. – Mike Jones Mar 23 '22 at 19:24
  • So you lose a phase and you've got other equipment on the other two phases. So "neutral" is something like the centre of the Y. The Y has lost one arm, so it's more of a V, but with random stuff on the V tips and something-or-other in the middle, where N is. But this is IT earthing, so N isn't pulled to earth. What voltage is that? Who knows? I'm not gonna touch N in that case! – Dan Sheppard Mar 23 '22 at 19:28
  • "So you lose a phase and you've got other equipment on the other two phases." - No, I wouldn't lose a whole phase, I'd lose the live for a branch circuit. It would throw the phase balance out "a bit", but not a whole lot. – Mike Jones Mar 23 '22 at 19:36
  • Ok, so you've got a little bit less on one phase after the breaker goes, maybe you've still got it all because the load was trivial. Where is your neutral? If you've got unbalanced phases in normal operation, your neutral can still be anywhere. Just do the phasor thing with an unbalanced load on a Y-secondary and plot where N sits during one cycle. – Dan Sheppard Mar 23 '22 at 19:38

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EN 60364-4-43:2010 section 431.2.2 contains the answer:

It clearly specifies that on IT earthing system, with distributed neutral, two pole breakers are required for single phase loads.

Mike Jones
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