Domestic uses and safety (2.3) (R) Flashcards
What type of supply is mains electricity
ac
What is the frequency of the domestic electricity supply in the UK?
50Hz
What is the voltage of the domestic electricity supply in the UK?
230V
What is the difference between direct and alternating potential difference?
Alternating p.d. constantly changes direction. Direct p.d. always goes in same direction.
What does an ac supply look like on oscilloscope/voltage-time graph?
What does an dc supply look like on oscilloscope/voltage-time graph?
What is often used to connect most electrical appliances to the mains?
three-core cable
What does a three-core cable look like? Which way does each wire point?
neutral to the left, earth down the middle, live to the right
What colour is the live wire?
live wire – brown
What colour is the neutral wire?
neutral wire – blue
What colour is the earth wire?
earth wire – green and yellow stripes.
Why is the insulation covering each wire colour coded?
for easy identification
What is the role of the live wire?
The live wire carries the alternating potential difference from the supply
What is the role of the neutral wire?
The neutral wire completes the circuit
What is the role of the earth wire?
The earth wire is a safety wire to stop the appliance becoming live
What is the potential difference between the live wire and earth (0 V)?
230V
What is the voltage of the neutral wire?
The neutral wire is at, or close to, earth potential (0 V)
What is the potential difference of the earth wire?
0V
When will the earth wire only carry a current?
if there is a fault
Is the live wire still dangerous when a switch in the mains circuit is open?
yes
still carries alternating potential difference of high voltage (230V), you may complete a circuit between the live wire and the earth (because you’ll be standing on the floor), so you get a shock.
What happens if the live wire touches metal casing?
the appliance becomes live
What is the danger of providing a connection between the live wire and earth?
This would make a complete circuit of 230V from your mains supply to the ground (earth). A shock or fire would be highly likely.
Why is ac sometimes used over dc supplies? (2 ways)
Cheaper to generate ac is more useful for transporting current over a long distance as there are less energy losses
Why do plastics not need an earth wire?
They are insulators (so the appliance will not become live)
symbol for double insulation (with insulator/plastic outer layer) shown below
How do fuses work once an appliance has become live?
The earth wire is connected to the metal casing
so that the current goes through the earth wire instead of causing an electric shock.
A strong current surges through the earth wire because it has a very low resistance.
This breaks the fuse and disconnects the appliance.
How does earthing work once an appliance has become live?
The earth wire is connected to the metal casing
so that the current goes through the earth wire instead of causing an electric shock
Carries current to ground with 0V, thus earthing it
How does a fuse work?
The fuse contains a (low-resistance) thin wire that will melt if the current gets too high.
Once the fuse has melted, the circuit is broken and no more current flows through the device
