In your house, the more things that are turned on, the more current is requested at 240V. Thus the meter in the meter box ticks over quicker and it costs your parents more money.
In the above, not very good, outline of a house, two mains leads come into the meter box through a main fuse or circuit breaker. In the meter box, the incoming lead called ACTIVE, a brown (or red in old houses) wire, is split into parallel systems each passing through a meter, a fuse, then out into the house.
In the house, the active wire is split further into parallel circuits, passed through a switch and then into the hotplate, light or whatever. When it emerges, the wires recombine and are called the NEUTRAL or RETURN line which is coded blue (or black in old houses). These are taken back out of the house to the HEC poles.
The reason for the unusual names, active and neutral, is the energy level of the lines. Compared to the surrounding earth and things in contact with earth, eg you, the active line is at high energy, 240V, while the neutral line should be at 0V. The active line current is potentially lethal from your point of view but, after passing through the resistance, the current is at zero energy with respect to earth, it has lost its energy in the resistance.
Switches are always placed in the active line even though they would work in the neutral line. Why?
The Earth line
An additional line is used in houses, the earth line, a green coated wire (with a yellow stripe), which is supposed to be a safety wire. This wire runs to a copper rod deeply inserted into the soil outside your house. It connects to the frame of your appliance through the plugs and sockets, and exists if the active wire comes loose and touches the metal frame. The idea is that then the current will go directly into the earth instead of through you to earth.
active - brown
neutral - blue
earth - green / yellow
Many plastic electrical goods only have the active and neutral wires because the plastic is an excellent insulator protecting the user from faults with the active wire.
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