Noadswood Science, 2012 Electrical Safety

Electricity Safety To be able identify safety features with electrical components Tuesday, August 11, 2015

Mains Electricity There are various electrical hazards within the home – most are common sense, and can be eliminated easily, with a basic list compromising of: - Long or frayed cables Cables in contact with something hot or wet Children and pets (hamsters, rabbits etc…) Water near sockets Shoving things into sockets Damaged plugs / too many plugs within a socket Lighting sockets without bulbs in Appliances without covers

3 Pin Plug – the cable A mains electricity cable contains two or three inner wires – each has a core of copper, because copper is a good conductor of electricity The outer layers are flexible plastic, because plastic is a good electrical insulator The inner wires are colour coded: - Blue – neutral Brown – live Green / yellow stripes – earth

Plug The features of a plug are: - The case is made from tough plastic or rubber, because these materials are good electrical insulators The three pins are made from brass, which is a good conductor of electricity There is a fuse between the live terminal and the live pin The fuse breaks the circuit if too much current flows The cable is secured in the plug by a cable grip – this should grip the cable itself, and not the individual wires inside it

Wire Placement There is an easy way to remember where to connect each wire Take the second letters of the words blue, brown and striped – this reminds you that when you look into a plug from above: - Blue goes left Brown goes right Striped (green / yellow) goes to the top

Wiring Practice wiring up the 3-pin plugs… Can you identify the deliberately damaged plugs and explain how they are damaged / how this could be rectified? Blue goes left Brown goes right Striped (green / yellow) goes to the top

Earthing Many electrical appliances have metal cases, including cookers, washing machines and refrigerators – the earth wire creates a safe route for the current to flow through if the live wire touches the casing You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing 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 and disconnects the appliance

Fuses & Circuit Breakers Fuses and circuit breakers protect electrical circuits and appliances

Fuses The fuse breaks the circuit if a fault in an appliance causes too much current flow, protecting the wiring and the appliance 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 Fuses in plugs are made in standard ratings (3A, 5A, 13A etc…) The fuse should be rated at a slightly higher current than the device needs: - If the device works at 3A, use a 5A fuse If the device works at 10A, use a 13A fuse etc…

Circuit Breakers The circuit breaker does the same job as the fuse, but works slightly differently – a spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt An electromagnet is arranged so that it can pull the bolt away from the switch If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position Circuit workingCircuit broken

RCCBs Residual current circuit breakers (RCCBs) are a type of circuit breaker used instead of a fuse or Earth wire Normally exactly the same current flows through the live and neutral wires – if the live is touched a current flows through them to the Earth (which is extremely dangerous) This also causes the neutral to carry less current than the live which is detected by the RCCB and it cuts the power by opening a switch RCCBs operate a lot faster than fuses making them safer, and they also work for a small change in current (unlike a fuse which needs to melt so requires a large current change). RCCBs can also be flicked back on!

Shock Prevention If a fault develops in which the live touches the metal case, then because the case is earthed a big current flows through the live, through the case, and out down the earth wire This surge in current also blows the fuse (or trips the circuit breaker) which cuts off the live supply This two systems isolate the appliance, making it impossible to get an electric shock from the case (as well as preventing fire due to the heating effects of large currents) *If an appliance has a plastic casing with no metal parts showing it is double insulated (and it doesn’t need an earth wire)

Plug Safety Features Complete the plug safety features worksheet Key words: - Large Harm Safety Melt Live Circuit Fuse