Single and Three-phase distribution The distribution of electricity to consumers locally – domestic, industrial, commercial, agricultural – is the responsibility of area electricity companies who purchase electricity from the national generating companies who control the national grid system. The national grid transmits electrical power from the generating stations to distribution centres (substations) which are designed to control and distribute electricity locally at voltages that are efficient and safe. The standard distribution voltages are: 11,000 V (11 kV) – Heavy Industry 400 V, Three-phase (3/4 wire systems) – light industrial and commercial premises 230 V, Single-phase – domestic and small commercial premises All the above are at 50 Hz (one cycle per second) Single-phase Supplies and Systems A 230 volt single-phase supply is used for domestic and small commercial premises as it is considered to be a safe working voltage which can provide electrical power for lights, heaters, cookers, portable appliances and data- processing equipment, economically and efficiently with minimum risk to the user. However, it has its limitations, examples of which are: inefficient as a power source above certain values, for example 1 kW single-phase motors of equivalent power-rating to that of a three-phase motor are physically larger conductor c.s.a. is larger compared to that of a three-phase system transmitting the equivalent electrical power Three-phase Supplies and Systems The significant advantages of three-phase supplies and systems over single phase are: generation is more efficient smaller conductor c.s.a. required for given levels of power transmission three-phase motors have better characteristics. That is to say a better speed/power output to load-current ratio, and have a greater level of reliability power losses (I2 R) are less for an equivalent circuit a three-phase, four-wire system, offers two voltages: o 230 volts – connection between one phase and neutral o 400 volts – connection between any two phases o voltage loss on long runs is less than that of an equivalent single-phase circuit supply The disadvantage of a three phase, 400 V, supply is that the voltage is too high to be considered safe for systems where the possibility of personnel having direct or indirect contact with electricity is high. For example in domestic and commercial premises where consumers regularly have physical contact with electrical appliances and equipment during their everyday activities.
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