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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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    if the photo is on your machine locally, compose a reply to your post, and click on the picture icon (looks like a patients temperature chart with a dot above it. Maybe a mountain by moonlight, god knows, to the right of the ohms sign.)  and browse to your file and 'upload' It is for some reason critical to put some text (any text 'aaa' will do) in all 3 description fields, or the upload will fail. Then 'post reply'.



    Mean while 3 phase 3 wire meters do exist that expect no neutral, but in the UK 3 phase 4 wires is the standard.

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    a111a160e16c5aa2b0ec2bcf413dbb99-huge-dads-metering.png


    It does say three phase 4 wire..........
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    That meter is 3 phase 4 wire - it says so on the meter. Where in the world is it ? See image from



      here 

    fcf4552b5f3fddfcd686d563f267e9b3-huge-3-phase-meter.png


    Clearly with no neutral it would work OK and self  centre the 3 phase voltages.

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    Its in Zimbabwe - in the capital city - which is having 19 odd hours per day of load shedding at the moment....if you're lucky........so the immediate electrical dangers are limited to only a few hours per day, apparently between 1 and 6 in the morning. 


    Load shedding = no power supply areas of the capital city at all - let alone the rest of the country..........Frowning2. I assume they shift the limited power available around so that each area gets a short period of early morning power supply.
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    Hmm. Probably not really a problem then. Just poor workmanship. Single phase loads can be wired via 400-230 transformer, but I can confirm that load matched pairs of of light bulbs between phases will work and last quite a while longer on ~ 200V each  (amazing what you see when you travel.)

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    Hi Mike - so you recon the meter would work OK? I think this is what AJ was trying to tell me before I mistakenly deleted the post and his reply to me.............


    What do you mean by, ".................and self  centre the 3 phase voltages."


    It would get its voltage reference from between any two phases or would it only log 400V as its voltage reference? Then work out power used accordingly......
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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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    Thanks guys......
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    A three phase meter of the type illustrated is intended to have a neutral connected at the meter even if the load requires no neutral.

    Without any neutral connection, the meter will still work for the reasons already given, in that the three equal voltage coils, one for each phase will in effect create an artificial neutral. I suspect that the accuracy would be reduced.


    What is the supply voltage ? if it is 220/380 as marked on the meter, then no standard or common type of light fitting can be used between phases. Options include series pairs of filament lamps each of 220 volt to 240 volt rating, or if the nominal 380 volts between phases is on the low side, then series  triplets of 115 volt or 120 volt lamps can work well. Some switch start fluorescent lamp circuits can be worked two in series on 380 volt circuits. Or imported multi voltage ballasts intended for use on supplies of up to 480 volts.


    Or off course the actual supply might be 3 phase, 3 wire with 220/240 volts between phases, in which case standard 220/240 volt lights may be used between any two phases.
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    In my post above is a simplified sketch of the meter internals - 3 current sensing coils, and 3 voltage sensing coils one from each phase to neutral  If there is no neutral the terminal will drift to a mid point set by the three voltage sensing coils, and that is at or near the correct neutral / ground potential, as the 3 coils are equal, and the 3 voltages are supposed to be too.

    The meter reading is the sum of the 3 lots of current each multiplied by it's respective voltage.



    edit, beaten to it as I had a visitor and did not press send until later - apologies for duplication.

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