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Cost of Electricity
The electricity used in homes is measured in the unit kilo-watt-hour (kWh). One kWh is the electrical energy used by a 1kW appliance in 1 hr.
It is calculated by the product of the energy used (in kWh) and the cost per unit.
The energy used can be calculated using W = Pt ( where P is in kilo watts and t is in hours)
Eg. What is the cost of using 4, 100W lamps 5 hours a day for a 30 day month along with a 3kW cooker 1 hour a day for the same period. The cost of the electricity is $0.50 per unit.
Fuses and Circuit Breakers
Fuses and circuit breakers are connected to circuits to prevent large currents from flowing that can damage appliances and cause fires. A fuses uses a low melting point wire that melts and causes a break in the circuit when current surges beyond a particular value.
Choosing an appropriate fuse or circuit breaker for a circuit is easy. Simply determine the maximum value of safe current that can flow around the circuit and use a fuse or circuit breaker of equal value. Example if the maximum safe current for a circuit is 30A, then a 30A fuse or circuit breaker would provide the appropriate protection against burn outs.
A fuses differs from a circuit breakers in that the circuit breaker can be reset fairly quickly without having to replace the melted wire. The circuit breaker can break a circuit in less than 0.01s, a time far less than it would take for the wire in a fuse to melt.
The fuse must be fitted to the live wire and not the neutral.
The fuse is correctly placed on the live (hot) side of this circuit. Notice that when the fuse is blown no current flows to the human nor the load.
The fuse is incorrectly placed on the neutral side of the circuit. Notice that even though the fuse is blown, current can still flow through the load as well as the human.
NOTE: Earth serves as an extremely low potential, so current flows from the high potential (which is the battery) through the human to earth when the human makes contact with the circuit in the way he does.
Circuit Diagram to connect a fuse
Wiring a plug
The plug carries at least two, but most time three wires.
Live and neutral wires carry the current to and from the appliance. Switches and circuit breakers are placed on the live wire to effectively stop the flow of current.
The earth wire is connected to the earth and provided an alternate path from current to travel in case of a malfunctioned appliance.
Colour codes for wires
image: http://www.frankswebspace.org.uk/ScienceAndMaths/physics/physicsGCSE/wiringPlug.htm
Parallel connections
Appliances in house hold circuits are connected in parallel with the mains supply to provide the full potential difference required.
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