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3 Phase + Neutral 400/230V unbalance:

Nick Parker

I came across a circuit drawing for lighting distribution panel 3P+N (400/230V) .

There are 3 phase and 1 phase (Phase to Neutral) loads connected to this Distribution panel. Apparently, there would be unbalance current between the phases due to the single phase loads as it is not connected evenly across all the phases.

Do the breaker trips for the unbalance current due to the current flowing in the neutral? Do I need to increase the earth fault setting? Typically what is the earth fault setting for this configuration? Do I have to care of any other issues associated with it?

  • 0

    AFAIK, 3-phase MCBs don't include the neutral.

    3-phase RCBOs monitor all three phases and neutral. If the three phases are not balanced, it will not trip because the current in the neutral is equal and opposite to the imbalance. In other words, the 4 windings in the RCBO sum to zero.

  • 0 in reply to Chris Pearson

    But there are 4P MCCBs that do have facility for neutral overcurrent protection (often 0.5x, 1x or 2x the phase conductor setting), which may be what the OP has on the board incomer.

    Normally that only comes into play if you have a reduced neutral CSA, or increased neutral current due to load harmonics.

    As Chris says though, assuming N and E are separate, the earth fault protection is not affected by neutral current / phase imbalance since the sum of currents is still zero.

  • 0 in reply to Jam

    Thats interesting.

    A good few years ago now i remember an electrician friend of mine was a bit worried. 3 phase supply to a pub kitchen with a few guest rooms too.

    He had connector blocks from the 3 phase incommer and was worried that the N should be about three times the csa of the Ls because he had mostly single phase loads. It took some convincing that they do not add up arthithmetically but just vectorally , no harmonics to speak of in this application either.

  • 0 in reply to ebee

    Yep, the peak currents ( or for that matter the R.M.S. currents ) do not all occur at the same time, as they are 120 degrees out of phase with each other...

    VIEW ME, VIEW ME, VIEW ME............

    www.youtube.com/watch

    Z.

  • 0

    On a standard three phase four wire system as described in the O/P some unbalance is normal and almost impossible to avoid. Even if the loads are perfectly balanced, there is no way to ensure that all lights will be used at the same time.

    A three pole circuit breaker wont trip on this unbalance, neither will a three phase and neutral circuit breaker. A true 4 pole circuit breaker MIGHT trip, but only if the neutral current exceeds the rating of the device, this is most unlikely with a full sized neutral conductor and circuit breaker to suit.

    Presuming that the earth and the neutral are entirely separate, as they should be, then the correct setting of any earth fault protection is not influenced by neutral current.

  • 0

    It is usual these days to have a 'full size' neutral, i.e the same size as any one phase. As such, the largest load on any one phase is the worst that the neutral may carry, if there is no load on the other phases, but in general it is less. There are some problems with electronic loads that only draw current for part of the mains cycle - usually the crests, as then the cancellation does not occur, as the current  bursts are at different times.

    Mike.

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