Switching the neutral

[Labarum]

Any electricians here?

A domestic bathroom with a time-switched fan has an extra switch in circuit. A three pole isolator it fitted near the ceiling outside the bathroom.
It switches out the switched-live, the always-on live and the neutral.

To turn off both live wires is understandable, but what is the rationale for isolating the neutral?

 

[NickK]

It will be mains AC - positive and negative are 180 degree out of phase. Both are 'live'. An isolation switch will do that - it's role is to isolate rather than simply switch.

Not an electrician it may be regulatory standards for 'wet' areas such as the bathroom. However it's also a good idea due to the above.

 

[Labarum]

Thanks for your quick response NickK, but your first sentence contains errors. Neutral is at earth potential, so why switch it?

 

[PlacidCasual]

Centre tapped neutral for the 3 phase circuit does not guarantee no voltage. The phases are averaged out broadly but from second to second you could get a voltage at the neutral thanks to unequal distribution of loads.

 

[NickK]

Thanks for your quick response NickK, but your first sentence contains errors. Neutral is at earth potential, so why switch it?

The 'earth' potential for neutral is remote - it's typically the sub station or the power generation plant. A ground is local. You also get potential over the distance. It's the reason that you have to be careful of cable runs between buildings and the reason why audio equipment can have issues. It's also not wise to simply consider it non-live. It can have current flowing (if you connect neutral to earth for example) and in the cause of failure conditions elsewhere in the electrical circuit it can be 240V.

 

[lewism]

There's nothing in the regulations stating that the neutral needs to be broken. It's more a best practice and usually what building control specify. However, if the supply to the fan is taken from a lighting circuit (Usually the bathroom, upstairs or hall) and you only isolate the permanent and switched conductors, then there is the possibility of a back-feed on the neutral. Hence the triple pole being the best option.

Quick edit. You are also not using it as a switch. It is not used as functional switching (Such as a light switch) it is isolation.

 

[pp111]

Actually I don't think it does say in the regs that it needs to be switched but it is widely considered safer because of course neutral is a live conductor so potentially dangerous.