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why we generally use 3.5 core cable or in other word 1/2
core for neutral and also confirm me that in balance
conditiob(3 phase)current in neutral will zero
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Answer Posted / john hancock
The choice of whether to use a neutral of equal size to the
phase conductors depends on your design criteria and load
equipment. The difference of the load being delta, floating
star or star with a neutral point
A 3 phase circuit in balance will produce little or no
neutral current contribution.
However a 3 phase circuit imbalance will produce neutral
currents as per the following formula
A^2 + B^2 + C^2 - AB - AC - BC).
A three phase circuit wired in star with the neutral at the
star point will operate with null balance if the loads are
equal, it is possible (and I have seen this) for 2 phases
to fail and leave one phase connected to the load and
neutral. The neutral current becomes equal to the phase
current, the designer of the circuit failed to account for
this, the undersized 1/2 neutral overheated and caught fire
destroying the switchgear and damaging the furnace.
I would urge anybody to carfully consider all possible
outcomes when choosing neutral cable sizes as flames coming
out of a switchboard often alarms the unwary.
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