Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 14 replies
  • Subscribers 32 subscribers
  • Views 6647 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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
    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.
  • 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
    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
    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 without any comment as to its safe use.
  • 0
    Yes, that's why I like controls to switch lines rather than Neutral or mid-point (earthed DC current carrying conductor) - and (save very exceptional circumstances) this is required by BS EN 60204-1, which of course includes requirements for controls for machinery.
  • 0
    Thanks Graham.

    I think it is also worth bearing in mind that one of the fatalities during the construction of the Channel Tunnel was due to a control cable being crushed and shorting to earth. Because it was part of an arrangement like this, the solenoid, operating a hydraulic ram closed crushing the worker and even the emergency stop button wouldn't stop it.

    The equipment was not allowed to be restarted until the entire panel (not made in the UK) had been rewired to put all the switching on the live side of relays and solenoids.

    Now I accept that a lighting circuit switching on in the event of an earth fault is not likely to be life threatening but I really don't think that control by switching the neutral should be encouraged.

  • 0
    I'll come clean that I was playing devil's advocate to try and tease out whether there is really a safety issue with the arrangement shown.


    I don't necessarily disagree with you per-say, that it's an odd arrangement, but in my mind isolation (via the light switch) is not a real issue - switching the neutral and not line is though.
  • 0
    I was not suggesting a separate supply. All I was saying is that there are instances when a device might be reasonably expected to be safe because it is switched off but is not safe.

    One of these is where there are two supplies, and a warning notice is normal, another is where the switch is on the neutral and it does not appear that a warning notice is recommended or required.

  • 0
    Why? Surely it's safer only to have a single source of supply, than provide a separately switched line, given your argument?


    Especially true in TN systems where isolation of the Neutral is not necessary. Also, better to ensure the circuit can be isolated from a single point of isolation, because there's nothing in BS 7671 to mandate that the coil is supplied (through the light switch) from the same points of isolation as the switched line through the contactor ... which is why we end up with fan isolators with the extra pole for switched line.

Community Rules & Guidelines