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18 Cards in this Set
- Front
- Back
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What is a fuse? |
Fuses protect electrical circuits and appliances. The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. |
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How does a fuse work? |
If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit. Fuses in plugs are made in standard ratings. The most common are 3 A, 5 A and 13 A. The fuse should be rated at a slightly higher current than the device needs: |
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What is a circuit breaker? |
Circuit breakers are similar to fuses, but they are more sensitive, operate much faster and are resettable. |
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What is a residual current circuit breakers, RCCB? |
Residual current circuit breakers, RCCBs, protect some circuits. They detect a difference in the current between the live and neutral wires. In a normal circuit the current i both wires is the same. RCCBs work much faster than fuses do. |
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What does an earth wire do? |
Many electrical appliances - including cookers, washing machines and refrigerators - have metal cases. The earth wire creates a safe route for the current to flow through if the live wire touches the casing. |
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What will happen if the live wire comes into contact with the metal casing of an appliance? |
The earth terminal 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 (or trips the circuit breaker) and disconnects the appliance. |
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What is double insulation? |
Some appliances - such as vacuum cleaners and electric drills - do not have an earth wire. This is because they have plastic casings, or they have been designed so that the live wire cannot touch the casing. As a result, the casing cannot give an electric shock, even if the wires inside become loose. |
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What is the symbol for double insulation? |
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Explain why it is more important to fit an RCCB to a hedge trimmer than to a lamp used in a bedroom. |
A hedge trimmer is much more likely to cut through its cable than a bedside lamp. |
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Why is it that extension cables have the current ratings that they are designed for clearly marked on their packaging? |
If too high a current flows through them then the cable may overheat and melt/catch fire. |
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Even double-insulated appliances have a fuse fitted. How can a fuse work without an earth wire? |
Even a double-insulated appliance can have a fault. There is no risk of getting a shock, but a large current could flow causing a fire. The fuse would blow and prevent the fire. |
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Cables for cookers are more expensive than those used for a kettle. Explain why this is. |
Cookers take more current than kettles, so thicker copper is needed. This makes the cable more expensive. |
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When you wire a plug, it is important to use the correct fuse. Why do you think you should use a 3 amp fuse and not a 13 amp fuse for an appliance that usually takes only 0.5 amps? |
The fuse should blow if an unusually large current flows. If the normal current is 0.5 A and a current of 10 A flows, there must be something wrong, but a 13 A fuse would not blow. |
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An RCCB operates much more quickly than a normal fuse. Why might this be important in the case of an accident with a lawnmower? |
If the lawnmower cuts through the electric cable, the handles and body of the lawn mower could be live for a short while before the fuse blows. An RCCB would cut the power off more quickly making the situation safe more quickly. |
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Old houses used to have fuse boxes containing fuse carriers with fuse wires in them. If a fuse blew, the fuse wire had to be replaced. Most modern houses have circuit breakers that can be reset by flipping a switch. Why do you think circuit breakers have been introduced? |
It is safer for non-technical people to flip a switch rather than replace a fuse wire, and it is much quicker. |
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Evaluate the use of circuit breakers compared with fuses in domestic power supplies. |
Circuit breakers respond to electrical faults more quickly and if they are tripped can be reset far more easily than a fuse being replaced. |
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Replacement fuses can be bought in standard current ratings. The most common ones are 3 A and 13 A. Why do you think that there is not a 5.4 A fuse on sale for an appliance that uses 5.3 A of current? |
It is much cheaper to manufacture components such as fuses in large quantities. If there are only a few fuse rating options then large numbers of each rating can be made, which keeps costs down. It is not necessary to have fuses of different ratings for appliances of slightly different powers as the safety benefit is very small. It is good enough to have just higher power fuses and lower power ones. |
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What are the key points to remember? |
The earth wire and fuse in a three-pin plug together help make appliances safe. Residual current circuit breakers can protect people from electrocution. Both circuit-breakers and fuses act by breaking the circuit. Circuit-breakers are more sensitive than fuses and they act more quickly. |
