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What unit of power is normally used for domestic electricity?
The Kilowatt (kW).
What is the unit of energy associated with the kW?
The Kilowatt Hour (kWh).
State the equations used to calculate the cost of electricity. Give appropriate units.
Energy (kWh) = Power (kW) × Time (h) Cost = Units used × Cost per unit energy (cost per kWh)
Is mains electricity an a.c supply or a d.c supply? What do each of these stand for?
It is an a.c supply. a.c: Alternating Current d.c: Direct Current
Define direct current.
Direct current: One-directional current flow.
Define alternating current.
Alternating current: Current that continuously changes direction at a specific frequency.
What is the frequency and voltage of the UK mains electricity supply?
Frequency: 50 Hz Voltage: 230V
How many wires are usually in the cables connecting electrical appliances to the mains? Name these wires.
Live wire Neutral wire Earth wire
State the insulation colour used on the Earth wire.
Green and Yellow Stripes.
State the insulation colour used on the live wire.
Brown.
State the insulation colour used on the neutral wire.
Blue.
Explain when the Earth wire does and doesn’t carry a current.
Under normal circumstances, no current flows through the Earth wire. If a fault occurs in the appliance (such as a surge or the casing becoming live), current will flow.
What potential is the neutral wire at?
0 Volts.
State the potential difference between the live and earth wires.
230 Volts.
What is the purpose of the neutral wire?
To complete the circuit by connecting the appliance back to the mains supply.
What does MCB stand for?
Miniature Circuit Breaker.
What does RCCB stand for?
Residual Current Circuit Breaker.
For metal appliances, where is the Earth wire connected to? Why?
The Earth wire is connected to the metal casing of the appliance. If the live wire becomes loose and touches the casing, the casing will become live. The current will flow through the Earth wire, preventing electrocution./
