32 Chapter 2 • Cable joints and switchgear
Small consumers can then receive a standard 220-volt single-phase supply. Large consumers can receive a three-phase, 4-wire supply (L1, L2, L3 and neutral) at both 380 volts between lines (phases) and 220 volts between a line (phase) and neutral.
2.5.3 Advantages of a three-phase supply Te advantages of a three-phase, 4-wire system are: • Two voltages are available i.e. a line voltage, VL
= 380V (VL = 3 Vph) and a phase voltage, Vph (Vph = VL
__ ) 3
In the case of supplies to small factories, shops, flats and residential areas the line voltage is, VL
= 380 V and the phase voltage, Vph
= 220 V (i.e. Vph = VL
__ = 380 = 220 V) 3
___ 3
• The neutral (or star point) of the transformer is earthed and earth leakage protection can therefore be obtained easily.
A three-phase, three-wire system has the advantage of requiring only three wires as opposed to the four wires of the three-phase, four-wire system but it has the disadvantage of having only one voltage and no star point, making earth leakage protection more difficult to obtain.
Tree-phase, three-wire systems are used to supply loads such as three-phase motors and transformers. It is also a common system used for transmission lines.
2.5.4 Disadvantages of a single-phase supply A single-phase distribution system has the following disadvantages:
• It has only one voltage. • The amount of power delivered to the consumer is limited. • Single-phase motors are more expensive, larger and less efficient.
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