PHYSICS – Dangers of electricity

LEARNING OBJECTIVES Core State the hazards of: – damaged insulation – overheating of cables – damp conditions State that a fuse protects a circuit Explain the use of fuses and circuit breakers and choose appropriate fuse ratings and circuit-breaker settings Explain the benefits of earthing metal cases Supplement

In the UK, domestic electricity is supplied at a voltage of 230V (+/- 6%) Mains electricity can be dangerous – possible hazards?

In the UK, domestic electricity is supplied at a voltage of 230V (+/- 6%) Mains electricity can be dangerous – possible hazards? Old, frayed wiring might mean strands of wire with a higher resistance at one point. A heating effect may be enough to melt the insulation and cause a fire.

In the UK, domestic electricity is supplied at a voltage of 230V (+/- 6%) Mains electricity can be dangerous – possible hazards? Old, frayed wiring might mean strands of wire with a higher resistance at one point. A heating effect may be enough to melt the insulation and cause a fire. Extension leads may overheat if used when coiled up. The current flowing will warm the wire, but the tight bundle means that the heat has less room to escape.

In the UK, domestic electricity is supplied at a voltage of 230V (+/- 6%) Mains electricity can be dangerous – possible hazards? Old, frayed wiring might mean strands of wire with a higher resistance at one point. A heating effect may be enough to melt the insulation and cause a fire. Extension leads may overheat if used when coiled up. The current flowing will warm the wire, but the tight bundle means that the heat has less room to escape. If water leaks into a plug or socket there is a risk of someone getting electrocuted as water will conduct the current.

In the UK, domestic electricity is supplied at a voltage of 230V (+/- 6%) Mains electricity can be dangerous – possible hazards? Old, frayed wiring might mean strands of wire with a higher resistance at one point. A heating effect may be enough to melt the insulation and cause a fire. Extension leads may overheat if used when coiled up. The current flowing will warm the wire, but the tight bundle means that the heat has less room to escape. If water leaks into a plug or socket there is a risk of someone getting electrocuted as water will conduct the current. When using a lawnmower of electric hedgetrimmer there is always the danger of cutting through the cable. A plug-in RCD should always be used to avoid electrocution.

Mains electricity

Plastic insulated casing.

Mains electricity Plastic insulated casing. Neutral wire (blue)

Mains electricity Plastic insulated casing. Neutral wire (blue) Earth wire (yellow and green). Stops metal appliances becoming live if there is a fault.

Mains electricity Plastic insulated casing. Neutral wire (blue) Earth wire (yellow and green). Stops metal appliances becoming live if there is a fault. Live wire (brown) – goes alternately negative and positive, making current flow backwards and forwards through the circuit.

Mains electricity Plastic insulated casing. Neutral wire (blue) Earth wire (yellow and green). Stops metal appliances becoming live if there is a fault. Live wire (brown) – goes alternately negative and positive, making current flow backwards and forwards through the circuit. Fuse – always connected to the live wire. If the current gets too high the fuse will ‘blow’ and break the circuit.

Mains electricity Plastic insulated casing. Neutral wire (blue) Earth wire (yellow and green). Stops metal appliances becoming live if there is a fault. Live wire (brown) – goes alternately negative and positive, making current flow backwards and forwards through the circuit. Fuse – always connected to the live wire. If the current gets too high the fuse will ‘blow’ and break the circuit. Cable grip – keeps the wires inside the plug securely.

Mains electricity Plastic insulated casing. Neutral wire (blue) Earth wire (yellow and green). Stops metal appliances becoming live if there is a fault. Live wire (brown) – goes alternately negative and positive, making current flow backwards and forwards through the circuit. Fuse – always connected to the live wire. If the current gets too high the fuse will ‘blow’ and break the circuit. Cable grip – keeps the wires inside the plug securely. Plastic wire outer covering

Mains electricity In some countries the plug may have only two pins (live and neutral). The earth connection is made by two metal contacts at the edge

Mains electricity In some countries the plug may have only two pins (live and neutral). The earth connection is made by two metal contacts at the edge Some appliances, for example radios, do not have an earth wire. This is because their outer case is made of plastic rather than metal. The plastic acts as an extra layer of insulation around the wires.

Mains electricity Mains current is 230V AC (in the UK). AC means alternating current. The current flows backwards and forwards 50 times per second. We say that it has a frequency of 50 hertz (50Hz). Some appliances, for example radios, do not have an earth wire. This is because their outer case is made of plastic rather than metal. The plastic acts as an extra layer of insulation around the wires.

Mains electricity Fuses

Mains electricity Fuses How does the fuse and earth wire working together prevent shocks?

Mains electricity Fuses 1. If a fault develops (eg. Live touches the earthed metal case) then a big current flows in through the live, through the case and out down the earth wire

Mains electricity Fuses 1. If a fault develops (eg. Live touches the earthed metal case) then a big current flows in through the live, through the case and out down the earth wire 2. This surge in current blows the fuse (or trips the circuit breaker), which cuts off the live supply

Mains electricity Fuses 1. If a fault develops (eg. Live touches the earthed metal case) then a big current flows in through the live, through the case and out down the earth wire 2. This surge in current blows the fuse (or trips the circuit breaker), which cuts off the live supply 3. This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents fire risk from the heating effect of a large current.

Mains electricity Fuses 1. If a fault develops (eg. Live touches the earthed metal case) then a big current flows in through the live, through the case and out down the earth wire 2. This surge in current blows the fuse (or trips the circuit breaker), which cuts off the live supply 3. This isolates the whole appliance, making it impossible to get an electric shock from the case. It also prevents fire risk from the heating effect of a large current. 4. Fuses should be rated as near as possible but just higher than the normal operating current.

Mains electricity Fuses What fuse to use?

Mains electricity Fuses What fuse to use? 3A, 5A, 13A

Mains electricity Fuses What fuse to use? 3A, 5A, 13A WARNING: if the wrong fuse is used then if a fault develops, the circuit might overheat and catch fire without the fuse blowing.

Mains electricity Fuses What fuse to use? 3A, 5A, 13A WARNING: if the wrong fuse is used then if a fault develops, the circuit might overheat and catch fire without the fuse blowing. To work out the fuse required we need to know the power rating of an appliance and the voltage rating. We can then work out the current used by the appliance using this equation: I = P/V

Mains electricity Fuses What fuse to use? 3A, 5A, 13A Eg. Hair dryer. Power rating = 1.1kW Voltage rating = 230V

Mains electricity Fuses What fuse to use? 3A, 5A, 13A Eg. Hair dryer. Power rating = 1.1kW Voltage rating = 230V I = P/V I = 1100 / 230 I = 4.8A Use the fuse rated a little higher than the normal current. In this case, use a 5A fuse.

Mains electricity Fuses What fuse to use? 3A, 5A, 13A Eg. Television. Power rating = 150W Voltage rating = 230V

Mains electricity Fuses What fuse to use? 3A, 5A, 13A Eg. Television. Power rating = 150W Voltage rating = 230V I = P/V I = 150 / 230 I = 0.7A Use the fuse rated a little higher than the normal current. In this case, use a 3A fuse.

Mains electricity Fuses The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit. The fuse then has to be replaced.

Mains electricity Fuses The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit. The fuse then has to be replaced. A circuit breaker does the same job as a fuse, but it works in a different way. Circuit breakers are automatic switches that ‘trip’ (turn off) when the current rises above a specific value. The circuit breaker can be reset by pressing a button (no need to replace).

LEARNING OBJECTIVES Core State the hazards of: – damaged insulation – overheating of cables – damp conditions State that a fuse protects a circuit Explain the use of fuses and circuit breakers and choose appropriate fuse ratings and circuit-breaker settings Explain the benefits of earthing metal cases Supplement

PHYSICS – Dangers of electricity