Flashcards in Fuses and Earthing Deck (22):
Earthing and fuses prevent [ ]
Step 1: If a fault develops in which the live wire touches the meta case, then because the case is [ ], too great a [ ] flows through the live wire, through the case and out down the [ ]
Step 2: This surge in current [ ] when the amount of current is [ ] than the fuse rating. This cuts off the live supply and [ ]
melts the fuse
breaks the circuit.
Step 3: This [ ] the whole appliance, making it impossible to get an [ ]. It also prevents risk of fire caused by the heating effect of a large [ ]
Step 4: As well as people, fuses and earthing are there to protect the [ ] in your appliance from getting fried if there's a [ ]
circuits and wiring
Describe the rating of fuses
Fuses should be rated as near as possible but just higher than thee normal operating current
Describe the relationship between cable thickness, current and fuse rating
The larger the current, the thicker the cable you need to carry it therefore the fuse rating need for cables usually increase with cable thickness
Give a brief description of what happens when a fuse/earth wire is used
1) Fault, allows live to touch metal case
2) Big current now flows through out earth
3) Big current surges to earth
1) Fault then big surge in current blows fuse
2) Which isolates the appliance fro the live
All appliances with metal cases are usually "earthed" to ...
reduce the danger of electric shock
What does "earthing" mean?
Means the case must be attached to an earth wire
An earthed conductor can never...
What does double insulated mean?
If the appliance has a plastic casing and no metal parts showing then it's said to be double insulated
If an appliance is double insulated it doesn't need an [ ] wire.
What are two-core cables?
Cable that carry only a live and neutral wire
What are circuit breakers?
An electrical saftey device used in some circuits. Like fuses, they protect the circuit from damage if too much current flows
When circuit breakers detect a surge in current in a circuit, they...
Break the circuit by opening a switch
What's an advantage of using a circuit breaker instead of a fuse?
A circuit breaker can easily be reset by flicking a switch on the device. More convenient than fuses - which have to be replaced once they've melted
What's an disadvantage of using a circuit breaker instead of a fuse?
Circuit breakers are a lot more expensive
Name a type of circuit breaker
Residual Current Circuit Breaker (RCCB)
What are the 3 ways of coping with a current surge?
3) Circuit breakers
How do RCCBs work?
Normally exactly the same amount of current flows through live and neutral wires.
If somebody touches the live wire, a small + deadly current will flow through them to the earth. This means the neutral wire carries less current than the live.
The RCCB detects the difference and quickly cuts off the power by opening a switch