Power Transmission

Domestic Wiring

Two wires come from the power pole to the house. The ACTIVE wire is 240 VRMS. The NEUTRAL wire is 0 V and is connected to the ground. The cable from the pole goes first to a mains connection which contains a fuse in the active wire. Then it is connected to the switchboard or fusebox.

A main switch is placed in the active wire so that the power can be easily cut in case of emergency. Several lighting and power circuits originate at the switchboard and each has a fuse or circuit breaker (again in the active wire) so that individual circuits can be isolated and cut. The neutral is connected to the neutral bar in the switchboard which is earthed. The neutral bar has connections to each power and lighting circuit. A third wire, an earth, also goes to each circuit.

The earth wires are connected into the ground, often by a water-pipe or metal stake. They serve as a safety feature in that the earth wire is connected to the metallic case of an appliance. If this case accidentally becomes electrified, the current flows harmlessly to earth and possibly blows a fuse in the process. The current will flow to the earth, because it will always take the path of least resistance.

Active Neutral

Earth

Switch

The fuse is designed to 'blow' (break), and break the circuit if too much current flows at any time. The earth pin on three pin plugs is always longer than the Active and Neutral, to ensure that the appliance is always earthed when plugging in.

RCD (residual current devices) can also be used on the circuit board. They do not provide protection against overload or short-circuit conditionsTo be effective, the RCD must operate very quickly at a low earth leakage current. Those designed to protect human life are engineered to trip out with an earth leakage current of 30mA within 200mS and at a higher earth current of 150mA, they will trip in less than 40mS. These limits are well inside the safety zone, within which electrocution or fire would not be expected to occur. In Australia, residual current devices have been mandatory since 1991 in new houses on all power and lighting circuits.

In order to transmit large blocks of power (for Melbourne up to 5 109W) over transmission lines, it is necessary to use a high voltage (500kV), only then can the current be kept down to a reasonable level (1 104A). If the power was transmitted at 250V it would require a current of 2 107 amps to supply the power to Melbourne.

The power is transmitted in AC current because of its guaranteed voltage level over long distances. AC is also advantageous in its ability to be transformed either up or down. In contrast DC current is unable to be transformed necessitating transmission of large amounts of power, which would increase the energy loss resulting from large voltage drops.

At various stages, power is taken from the system and rectified into 600 V (trams) or 1500 V DC (trains) for public transport. Power is also transformed into other AC voltages for specific uses in industry. Power losses in transmission lines need to be considered in some questions. The power is not actually lost it is degraded into heat, which becomes unusable. The power supplied to the consumer is equal to the power supplied at the start minus the power lost in transmission