Electrical Safety

Domestic ‘mains’ systems in the UK use AC at 50 Hz. In the USA the systems use 60 Hz. These frequencies are used as they are efficient frequencies for transmission from power generation to the users and minimise the effect of leakage currents due to capacitance, which is discussed later in this chapter. The mains is initially generated in a power station and the power generated there (volts × amps) is enough to supply a number of hospitals and consumers. The voltage and the potential to do work must be transmitted to the user and this is usually achieved with overhead pylons, or sometimes by underground cables. Both types of cable will be designed to carry current, but the higher the current, the greater power lost to heat (I 2R). It is desirable to keep the current as low as possible to reduce this transmission heat loss, and this can be done by making the voltage as high as possible. For a given power, (V × I) there can be a high V and low I or vice versa. The transmission voltage is normally greater than 11 kV. It arrives at a domestic substation and is transformed down to (in the UK) 230 V RMS by a transformer. At the substation, as illustrated in Figure 6.1, one connection of the transformer is firmly bound to earth at what is called the star point, and this forms the start of what is called the neutral lead. The earth connection forms a vital part of electrical safety. The connection on the other side of the transformer is called the live lead and this is at 230 V RMS. These two leads are taken to the individual outlets or mains sockets, the live carrying the voltage to the load, and the neutral lead carrying the return current back to the source of the supply. The earth connection of the mains socket is connected back to the star point separately, although sometimes in older installations this can be earthed locally. In this way only one of the socket points is live, and the other is at near zero potential.