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Three phase metering and neutrals

tattyinengland
I have been asked by the owner of a saw mill: Does a three phase meter (Old rotating wheel type) need to have a neutral connected to it in order to work correctly?


This is a property that only uses three phase motors for a saw mill and some lights (That I assume are single phase but quite how that works I'm not sure..........)


This is in a far flung country and is probably a TT system with no earths at all and certainly no RCDs. 


AJ - I managed to delete my last post when I was trying to attach a picture; I know you replied to me previous post, thank you very much, and I'm sorry to have deleted your reply to me. 


It seems that there are no neutrals what so ever into this saw mill......It looks to me like the terminals for the neutral are present but no neutral is connected. 


It may be that the lights are connected to earth as I know that there has been a lot of equi-potential type bonding going on - on this property between machines, structural steelwork, in the ground, all sorts. 


I cant get out there to see whats going on or advise at all because I cant see what effect my advise would have. Certainly if there is single phase lighting going on the neutral is essential. 


The metering I could possibly, with your help advise on. I'd love to post a picture but I'm not sure how. I'll copy this message now and try again........Dammed if I know how to attach a photo........
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    I think this is what AJ was trying to tell me before I mistakenly deleted the post and his reply to me.............



    No worries!


    My thought was that the three voltage coils would usually be connected L-N, but with N unconnected the meter's N terminal would form a kind of artificial neutral point - held about mid-way between the three line voltages. So if all three voltage coils had the same impedance and all three phases had the same line voltage then it should hover at about 0V and the meter would work normally. If one of the lines had a lower voltage though (perhaps due to a larger load on that phase) then the artificial N of the star point of the voltage coils might drift away towards the other lines, making the voltage for the heavily loaded phase look a bit larger than it really was, so the meter might over-estimate the consumption a little.


      - Andy.
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  • 0

    I think this is what AJ was trying to tell me before I mistakenly deleted the post and his reply to me.............



    No worries!


    My thought was that the three voltage coils would usually be connected L-N, but with N unconnected the meter's N terminal would form a kind of artificial neutral point - held about mid-way between the three line voltages. So if all three voltage coils had the same impedance and all three phases had the same line voltage then it should hover at about 0V and the meter would work normally. If one of the lines had a lower voltage though (perhaps due to a larger load on that phase) then the artificial N of the star point of the voltage coils might drift away towards the other lines, making the voltage for the heavily loaded phase look a bit larger than it really was, so the meter might over-estimate the consumption a little.


      - Andy.
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