Starter Review your filament lamp experiment and make corrections (in green pen) Review your test and complete the front cover sheet, correct any mistakes as we go over. Put your test mark into the tracking sheet (you may have to make a row).

Lesson Goals Describe and explain how a fuse works as safety device. Describe and explain how a circuit breaker works as a safety device. Explain why electrical items need to be earthed.

Fuses Using the circuit symbol for a fuse, guess what the internal structure of a fuse is? Fuses contain a thin metal wire How is this a safety feature? What would happen is a large current was to pass through the fuse? How is this a safety feature? A current that is too large for the fuse would cause the wire I the fuse to melt and break, thereby breaking the circuit. Current would no longer be able to flow through the appliance

Fuses Fuses indicated that something is wrong and prevent the appliance from being used unsafely. They have to be replaced and the cause of the larger current identified

Circuit Breakers The electrical wiring in a building must be protected from being overloaded so that it does not overheat. This is the job of a ‘fuse box’, which used to contain fuse wire but now contains circuit breakers to protect the wiring. A house has several circuits and each one is protected by a separate circuit breaker.

Circuit Breakers Circuit breakers do the same job as fuses but they are electromagnetic switches which are easy to reset. Circuit breakers have different ratings as shown by the coloured dots.

Circuit Breakers Each circuit breaker is an electromagnetic switch which is designed to break the circuit when the current gets too high. The current creates an electromagnet when coiled around a core of soft iron. As the current increases the magnet becomes stronger… Until it is strong enough to open the switch (circuit breaker) This breaks the circuit and stops a dangerous current from flowing It can be easily reset.

RCCD Residual Current Circuit Device (Sometimes just referred to as RCD) These are the devices on the plug sockets in the physics lab. You might also find them in plug sockets in garages or out-buildings. RCCD’s detect if there is a difference between the current flowing in the live and neutral wire. If there is, then there is a fault and the RCCD will “trip” and break the circuit.

Earthing Earthing is a process used to increase the safety of electrical appliances. Cables contain an earth wire to provide an alternative path for current if appliances develop faults. An earth wire is essential for an appliance with a metal case.

Earthing When an appliance is working correctly, no electricity flows through the earth wire. If the live wire becomes loose and touches the metal case, a very large current flows to earth and blows the fuse, breaking the circuit.

Electrical Safety Another way of improving the safety of electrical appliances is to make them double insulated. Double insulated appliances have plastic cases, without any wires connected to the case. This means that the case cannot become live, because plastic does not conduct electricity. So, if one layer of insulation is damaged the appliance is still safe for the user.

Now it’s your turn… Complete the electrical safety sheet.