Max Zs for 400V circuits

If your 400V system is a normal 3-phase with 400V between lines (normally notated as U), then each line is still 230V from Earth (or N) - usually notated as Uo.

For disconnection for shock protection we only worry about faults to exposed-conductive-parts (bits that would pose a shock hazard if they became live), so the voltage involved is just that between any line and Earth - so it's still 230V. Notice that the tables mention Uo rather than U.

Rarely you might find systems that are 400V Uo (e.g. 690/400V 3-phase) - for those you'd have to calculate things (working backwards from the current that would open the protective device in the required time, the supply voltage and a fudge factor or two).

  - Andy.

And even more rarely you find 1k2/690 with 690v to earth on any one phase, but as that is out of the top of the voltage scope of 7671, in the UK you would be  very much on your own - although in practice it really is not so different, though the breaker arrangements are different,  the maximum of 1000V is a nice round number but an arbitrary boundary. I have only seen 1k2/690 used in a factory Germany of the kind where you could drive from one end to the other in the building.

Like the tables of installation methods are just the common situations, the Zs and voltage drops and so on in BS7671 are all based on what its by far the most common system in the UK, which is 400/230  and all the Zs are calculated for the LN case, as the most onerous they will also  pass higher.

BUT, as demand for things like EV chargers. very large giga-factories and  maybe supplies to out of the way places are required, it may be that we will come across or require to consider  a non 230V systems  more frequently in the future, certainly the power and distances that that it can be sent over thinner cables make for a very neat and efficient, if initially slightly surprising system,

Mike.

Wow....I think I start to see the light...

So when the regs say U0=230V this also covers the 3 phase circuits that have 400V between lines.

So U0 is the voltage between line and neutral and not generally the voltage at the terminals of a load

So when the regs say U0=230V this also covers the 3 phase circuits that have 400V between lines.

Correct.

So U0 is the voltage between line and neutral and not generally the voltage at the terminals of a load

Yup, well strictly speaking Uo is line to Earth rather than N - but usually it's much the same (sometimes there isn't a N as such). Have a look at the "Symbols used in BS 7671" in part 2 of the regs - there are at least 12 variations of "U" for voltage (and 30-odd versions of I for current) - the regs will make a lot more sense if you know which particular one they're talking about.

   - Andy.

Thank you so much.

This question really REALLY puzzled me for some good time

Cheers