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Two Phase CU

richard64
Good morning,


Came across a strange one doing an EICR yesterday.

The fuse board is fed with two phases and one neutral...phases L2 and L3 are linked.

I'm looking for a coding or deviation.

As I see it, best way is it's unbalanced loadings with pressure on the neutral.

Worst way is two supply circuits with a shared neutral.

The spare phase is being used to supply a flat in the building.

Loading wise, it would be possible to link out all phases in the main CU, making it a single phase board...but that would only push the problem back to the intake.

Any suggestions please.
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    It is not clear to me if you mean the incoming supply is 3 phase, but only 2 of the phases are used, or really it is a single phase supply, fed as if it was 3 phase  that has twice as much cable cross-section in the live path as the neutral.

    Does the neutral show signs of distress (like over heating) ? Unless it does it is probably a non-problem.


    Edit, if loads are mostly resistive, then when only 2 phases are loaded up but broadly balanced, then completing the triangle shows that the current in neutral is equal to that whcih would have flowed in the missing phase (as if the 3rd phase was present and loads more or less balanced, the neutral current would be near zero). If there is a lot of waveform distortion to the current from electronic loads, that is not the case, but that is not lilkley to be a large part of a residential load anyway.
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  • 0
    It is not clear to me if you mean the incoming supply is 3 phase, but only 2 of the phases are used, or really it is a single phase supply, fed as if it was 3 phase  that has twice as much cable cross-section in the live path as the neutral.

    Does the neutral show signs of distress (like over heating) ? Unless it does it is probably a non-problem.


    Edit, if loads are mostly resistive, then when only 2 phases are loaded up but broadly balanced, then completing the triangle shows that the current in neutral is equal to that whcih would have flowed in the missing phase (as if the 3rd phase was present and loads more or less balanced, the neutral current would be near zero). If there is a lot of waveform distortion to the current from electronic loads, that is not the case, but that is not lilkley to be a large part of a residential load anyway.
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