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Switched neutral

Harry Macdonald
Looking at the BS7671 AMD2 out for consultation, I noticed clause 463.1.2. This shows an arrangement whereby the control device for a lighting circuit is switched by a switch on the neutral side of the device.

Clearly this works but why would anyone want to do this?
  • 0
    Also anyone who uses the light switch as a means of isolation before working on the contactor will find out the hard way why it is a bad idea.


    BS 7671 is probably even more out of step with common sense that you'd imagine - table 537.4 decrees that an ordinary lightswitch (probably to BS EN 60669-1) isn't suitable for isolation, no matter which conductor it's in. Since recent changes to 537.3.2 it's not even suitable for switching off for mechanical maintenance (as the requirement is now that it must provide full isolation for that function) - so you can't even use the light switch to switch off to replace or even clean the lamp!


       - Andy.


  • 0
    Which is why I was surprised to see in in BS7671 AMD 2

    It's not new to AMD 2 - it's been there since the start of the 18th. As others have said it saves an extra wire out to the rooms - and I suspect brings us in line with continental practice (where it's usual to use retractive switches and a step relay in "CU" for multi-switch arrangements rather than 2-way/intermediate switches and loads of extra wires).

       - Andy.
  • 0
    Thanks AJJewsbury. I can see that the step relay saves a wire (but adds a relay) but I am not seeing how switching the neutral "saves an extra wire out to the rooms".

    Give that the whole purpose of BS6761 is safety, I would have thought that any practice that reduces safety should be actively discouraged if not forbidden.

  • 0
    but I am not seeing how switching the neutral "saves an extra wire out to the rooms".

    If switching in L for a relay you'd need L to the switches and SW back to the relay coil, then SL and N to the lights - so 4 conductors in all (plus any c.p.c.s). By allowing N switching you only need N & SW to the switches then SL to the lights - no 4th wire as you already have N handy - you save on the perm L.


       - Andy.

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