I think that the reasoning is as follows.


Under some fault or failure conditions, the neutral may become energised at a dangerous voltage, so isolating the neutral as well as the phase eliminates this already small risk.


On a 3 phase system, isolating the neutral carries the risk that if the neutral opens before the phase conductors, then single phase circuits could be briefly subjected to 400 volts or so with the potential for serious damage.

Eliminating this risk of overvoltage by not switching the neutral is considered to be the best choice.

On single phase there is no such risk, so one might as well switch the neutral.


In my view, it would be better practice to switch all four conductors on a 3 phase, 4 wire system. Special switches are made for this purpose that open the neutral last, and reconnect it first.

I agree with the above, but would add that it's not just the design of the timing of the switch contacts that's the issue but the reliability of the switching mechanism - moving parts being inherently less reliable than static solid joint. So even a switch that's designed to open N last and close N first could allow N to be opened when L is closed should a moving contact break open (N) or weld close (L).

   - Andy.