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Should I Switch Neutral Through a Contactor?

Good afternoon folks. I'm in the process of setting up a nano-brewery. The supply is three phase L1 L2 L3 N. I wish to control the temperature on two of the immersion elements which are 9kW and 5kW. I have been advise by the element manufacturers that they should be wired in Star. My question is, do I need to, or should I switch neutral through a contactor pole? Or should I connect neutral externally? I have a retired electrician helping me out with the important stuff but he's a little rusty! Any advice would be most appreciated. Cheers, Dan.
  • It is common to use a 3 pole contactor to switch L1 L2 L3, and let N go round the outside non-switched, that is  when it is needed at all. If all 3 elements were identical, there wont be any neutral current to switch, and the neutral only carries the imbalance.

    What is not advised, especially on 3 phase, is switching neutral and not the associated live - as a system that is still live but  not neutral, will appear to be off, but is potentially dangerous.

     In a 3 phase system, a neutral fault could leave an element with the best part of 400V across it, if highly unbalanced loads are in effect hanging between 2 phases. In such cases as solidly bolted neutral is preffered.


    If you do want to switch neutral using the contactors  for any  reason, it is best to treat the heaters as single phase loads with double pole switching for each discrete element.

  • Former Community Member
    0 Former Community Member
    If you are going to wire the heaters in star, you don't need a Neutral and one won't exist there.

    The heaters will simply be fed with 3ph and are a balanced "load" not a supply to have a neutral generated.

    That is if all is designed correctly and the heaters are three the same.

    Also, remember that BS 7671 is almost certainly irrelevant to the controls and supplies on the brewery system.