Environmental Engineering Reference
x /3 kVA
Phase A of a three-phase supply
3( x /3 kVA)
neutral (return) current. Because the supply voltages are balanced, the phase
currents will be equal in magnitude but displaced by one third of a cycle from
each other. The corresponding phasors will mimic the voltage phasors shown in
Figure 2.13. Hence the total return current will be zero. We may take advantage of
this by connecting the three phases with a common return, as shown in Figure 2.16.
The three neutrals have been merged. Because the resulting current is zero for
a balanced system, the neutral connection is not required. Hence, conductor
material is halved. In practice, a neutral conductor is provided, with 50-100 per cent
of the rating of a line conductor. This carries current imbalance and higher
2.4.3 Three-phase supply
A three-phase supply will always have three phase conductors, A, B and C, as
shown in Figure 2.17. There will normally also be a neutral conductor N.
A balanced three-phase supply may be defined in terms of (a) line-to-line or
line voltage V L and (b) line-to-neutral or phase voltage V PH .
It is important to establish the relationship between line and phase values of
voltage and current for common three-phase connections.
It may be seen from Figure 2.17 that the A to B line voltage is
V AB ¼ V AN V BN