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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
    So the only the contactor and the lamp need a live supply. It is also how a lot of E7 meter switching is organised.

    And we do it for electronic switches so that the driver electronics can be at or within  half a dozen volts of neutral potential, rather than always live. Better for things like photocells outdoors.
  • 0
    This makes sense except that the electronics still sees the 230V when the device is switched off.

    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. I would have thought that the regs should mandate a warning notice whenever this is the design.
  • 0
    Harry Macdonald:

    This makes sense except that the electronics still sees the 230V when the device is switched off.

    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. I would have thought that the regs should mandate a warning notice whenever this is the design.


    If you are using a contactor, rather than a light switch, because either inrush currents or load currents are too high for the switch to handle, or you have alternative means of control such as SMART devices, then using the switch as a means of isolation for working on the contactor is at best a folly ... there will still be a live line conductor at the contactor ! I'd want to use an alternative point of isolation, such as the circuit protective device.


  • 0
    I agree absolutely, but for those who think that the switch is always on the live side of any device, a warning could be a life saver.

    Devices with two sources of supply are usually identified as such and this seems to be very similar.

  • 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.
  • 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
    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
    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
    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
    Which is why I was surprised to see in in BS7671 AMD 2 without any comment as to its safe use.

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