Switched Neutral: When to and when not to.

[Jocktrician]

Guys, it's a question that raises its head every now and then and the web references only ever seem to direct towards much plagiarised versions of the same paper that explains a little (but nothing) about relying upon the earth connection on the other side of the switching device. Can anyone succinctly advise me when the neutral MUST be switched, or when the neutral MUST NOT be switched?

 

[Richard Burns]

The neutral must not be switched if the line is not being switched as well.
Neutral switching (i.e. double pole switching) is useful to disconnect all live conductors and prevent transmitting faults upstream.
Nothing to do with the earth, which will not be switched.

 

[7029 dave]

One BIG ditto Richard.

 

[ChrisElectrical88]

Might as well shut this one down.

 

[telectrix]

but it is , of course , perfectly acceptable to switch the neutral when plumbers are replacing fittings.

 

[Darkwood]

As you have posted in the Commercial and industrial section their are some additional hazards regards 3ph and N switching - If you decide to have a switched N you need to get a 4pole isolation device so all Phases are isolated too as well as Neutral but the switching device should have an early make / late break Neutral pole otherwise an opening of N a fraction before Phases or energising of phases fractionally before N makes could create a floating star point and send up to 400v across 1ph loads with expensive results.

 

[Darkwood]

If the main switch is under the control of an untrained person I.E. Joe Public; then the main isolation switch for the premises must disconnect all live conductors (by definition this includes the Neutral). Hence domestic fuse boards always isolate Neutral as well as phase.

 

[Jocktrician]

Guys, thanks for the responses so far. My question is not for the thought of switching the neutral as opposed to the phase conductor, but in addition to. The matter was raised where generator changeover panels were discussed. I recall many years ago having something about rotating machines must not have their neutral switched. I can see the point too about the phase poles being broken marginally before the neutral, but where multiple pole devices are installed they don't always prescribe which pole is phase and which is neutral. My question is more fundamental. When we are using the three phase switching devices, when should we use four pole as opposed to three poles variants and are there any instances where the neutral should never be switched. Again, in the case of a generator and the earthed star point is downstream of the changeover, I can see the floating neutral causing issues.? ?