Physics
Electricity and Magnetism: Electricity in the home
Edited by Miss La Tesha Howes
References: Physics for CSEC Study Guide & Collins Concise Revision Course for CSEC Physics
Advantages of parallel connection of domestic appliances
Appliances can be switched on and off without affecting each other. If, instead, they were connected in series,
switching off one would switch off all.
Appliances can be designed to operate on a single voltage. This voltage is about 115 V in Barbados and Trinidad.
If appliances were connected in series, they would have to share the 115 V and would each have a smaller pd
across their terminals.
Fuses
A fuse is a short piece of thin wire which is placed in a circuit in series with one or more devices. When too high a current
flows, it melts and breaks the circuit, preventing damage to the devices. An appropriate fuse should melt at a current
slightly greater than that which the circuit takes under normal operation. This value is known as the current rating, in A,
of the fuse. See also 'Fuses in plugs or appliances' below.
Thickness of wires
High-powered electrical appliances, such as water heaters, fridges, cookers and washing machines, should use thick wires
because they need high currents - the currents they draw are proportional to the power they consume.
Distribution box
A LIVE and a NEUTRAL wire enter the distribution box from the electrical power company, delivering
alternating current. There is a fuse or circuit breaker in each live wire. The international colour code is shown in
Figure 29.17, together with a distribution box.
If the supply voltage is 115 V, there is a pd of 115 V between each live and neutral wire leaving the box.
The rating of the fuse or breaker depends on the total possible current that should flow when all the lights or
appliances connected to it are being used. •
Since cookers and water heaters usually take large currents, they have a fuse or breaker specially assigned to them
in the distribution box.
An actual distribution box usually has many more wires exiting. A separate fuse or circuit breaker may be
connected to the supply for each room in the building.
Ring-mains circuit (socket circuit)
This is illustrated in Figure 29.18. socket wires from distribution box Figure 29.18 Ring-mains circuit with sockets
The circuit uses thick wires because they carry current to several sockets.
If the supply voltage is 115 V, each socket will provide a pd of 115 V because they are all in parallel.
If the socket is viewed with the EARTH at the top and centred, then the LIVE is on the right.
Function of the earth wire
A short circuit can occur if a piece of metal bridges the gap between a circuit and the metal case of a device. If there is no
earth wire and the case is touched, current may pass from it through the person to the ground. Since some of the resistance
is now bypassed, the current is larger than before, so the person will receive an electrical shock and the fuse will blow.
Figure 29.19 shows an electric toaster resting on insulating supports. To avoid electrical shock, an earth wire is connected
between the case of the toaster and the ground in the yard (see Figure 29.17). Under normal conditions (Figure 29.19(a»
current flows in the live and neutral wires only. If there is a short circuit inside the toaster and some of the resistance is
bypassed, a large current will flow via the live and earth wires (Figure 29.19(b», and the fuse (or breaker) will blow (or
trip).
No current flows in the earth wire unless a malfunction occurs.
When current flows in the earth wire, the neutral wire is bypassed.
The short circuit should be fixed before the fuse is replaced (or the breaker reset).
Placement of fuses, breakers and switches in the live wire
Fuses 'blow' or breakers 'trip' when there is
a) a short circuit in the appliance so that current flows in the live and earth wires, or
b) a power surge for reasons external to the appliance so that excess current flows in the live and neutral wires.
By placing fuses or breakers in the live wire, the appliance is protected for both situations.
Switches should also always be placed in the live wire. If a switch in the 'off' position is in the neutral wire
connected to a lamp, of course no current can flow. If, however, an attempt is made to change the lamp and the
live terminal is touched, the person can receive an electric shock. By placing the switch in the live wire, once it is
in the 'off' position, no current can reach any of the terminals of the appliance.
Fuses in plugs or appliances
A fuse in the plug of the appliance or in the appliance itself can offer better protection than the fuse in the distribution
box. Its rating should only be slightly higher than the current normally taken by the appliance. Say the device requires 2.5
A for normal operation, a fuse with a rating of 3 A will be more suitable than the larger fuse in the distribution box, which
will not blow unless the current rises to a much higher value.
Adverse effects of connecting electrical appliances to an incorrect or fluctuating supply
Appliances are normally designed to operate on ac frequencies of either 50 Hz or 60 Hz. If operated on an
incorrect frequency they are likely to malfunction or even be damaged.
If an appliance is supplied with a voltage higher than it is designed for, overheating due to excessive current may
result in it being destroyed.
If a motor is supplied with a voltage that is too low, the current in its coils will rise (due to reasons beyond the
scope of this syllabus) and the device can be damaged due to overheating.
Review Questions
1. State TWO reasons for using parallel connection of domestic appliances.
2. What is the purpose of a fuse or circuit breaker?
3. In which wire from an ac supply should a fuse always be placed?
4. In which wire from an ac supply should a switch always be placed?
5. Describe and explain the function of an earth wire in a domestically wired system.
6. Describe the adverse effects of connecting electrical appliances to incorrect or fluctuating voltages.
