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Single and Three-phase distribution

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					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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