Electricity: Mains Electricity Flashcards
What is the mains voltage supply in the U.K.?
230V a.c. at a frequency of 50 Hz.
What are the three wires in a cable and what are their colours?
Live (brown), neutral (blue) and earth (green and yellow)
What is the purpose of the live wire?
The live wire is connected to the National Grid. Its potential varies from about +340V to -340V.
What is the purpose of the neutral wire?
It is simply a return wire. It has potential 0.
What is the purpose of the earth wire?
The earth wire is a low resistance connection straight to the earth. Because the live wire and neutral wires should form a complete circuit no current should normal flow In the earth wire.
Why must the live wire be insulated from the other two wires in electric plugs?
If the live wire comes Into contact with the neutral or earth wires a short circuit will result that will cause a large current to flow for a short time before safety devices are activated.
Name 5 safety safety hazards
Frayed cables Too many cables in one socket Water around sockets Long cables Pushing metal objects into sockets
What is the use of fuses and circuit breakers?
Fuses and circuit beakers are designed to cut the circuit when the current exceeds a certain value.
What do fuses consist of?
Traditional fuses consist of a thin wire contained in a glass or ceramic tube: when the current gets too large the wire melts or breaks. This type of fuse can be replaced when it ‘blows’.
What are the most common fuses used?
3A for lighting and 13A for everything else.
How do circuit breakers work?
Circuit breakers get too hot when the current exceeds a certain value. When this happens a switch that controls the circuit will switch. These have to advantage of being re-settable. You don’t need to replace anything.
Why do fuses and circuit breakers have to be connected in the live side of circuits?
The circuit will be disconnected and no current will flow if they are not.
What are the dangers of an exposed metal part of an electrical appliance?
A person touching it would recurve an electric shock as a current flowed through them to earth - if no safety measures were in place.
Why are appliances earthed?
If a fault develops a large current will then flow to earth and that will blow a fuse or activate a circuit breaker, thus disconnecting the circuit. However the earth wire is no use if it’s not connected to a fuse, the person will still receive a shock if it’s not.
What is double insulation?
As well as the wiring inside the appliance wing insulated the appliance itself will have a casting made out of an insulating material, usually plastic.
Do you need double insulation and an earth wire?
No, if there’s nothing metal on the exterior or the appliance that can be pushed there is no need for earth connection.
What are some of the main uses of electricity in homes?
Heating and cooking (Lighting)
What is the equation for power?
Power=current times voltage P=I times V
What is power measured in?
Watts (W)
What is current measured in?
Amps
What is voltage measured in?
Volts
What is the equation for energy transferred?
Energy transferred = current times voltage times time E = I times V times t
What is energy measured In?
Joules (J)
How do you work out how much energy has been converted in kW hours?
Multiply the power of the appliance in kW by the time it was operating in hours.
How much do electric companies charge per kW hour?
12p
What is direct current?
Current that’s flow is always in the same direction, though it does not have to have the same value all the time. This is supplied in batteries.
What is alternating current?
The current flows in one direction and then another. This is what the mains supply is, at 50 Hz.
Why are frayed wires dangerous?
You could touch the live wire and become electrocuted as it is no longer insulated.
Why is water near electrical components dangerous?
Water is a good conductor of electricity so you could easily get electrocuted.
