Area of Study 2: Electricity

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Area of Study 2: Electricity
Part 4: Home Distribution

Distribution in the Home
Households are connected to the main electrical grid and are supplied with an AC voltage of 240V (RMS) at a frequency of 50Hz. Root Mean Square (RMS) refers to a mathematical process by which a DC equivalent voltage is calculated. Actual values of voltages ranges between +340V and -340V

Distribution in the Home
If frequency (f) is 50Hz, current changes direction every 1/50th of a second. Electricity is fed into the home from overhead cables (suburbs) or underground cables (city).

Electricity to the Home
Switchboard: Contains meters, fuses/circuit breakers and neutral bar Fused mains connection box (50A) Switch- board Junction box Metal pipe or earth stake Earth wire (defined as 0V): provides a lower-resistance conducting path during faults Two wires bundled together : active (240V) and neutral (0V)

Switchboard Fuses / Circuit Breakers normally set to trip at 8A, 16A or 32A depending on application. Powerpoints and lights connected in parallel

Electrical Wiring: Colour Coding
Wire Old System New System Active Neutral Earth

Electrical Wiring: Colour Coding
Wire Old System New System Active Red Neutral Black Earth Green

Electrical Wiring: Colour Coding
Wire Old System New System Active Red Brown Neutral Black Blue Earth Green Green-Yellow

Appliance Wiring Double-Insulated Appliance: (2-pin plug)
Two independent layers of insulation. Eg. Hand-held electrical tools such as drills or hairdryers. ACTIVE LOAD NEUTRAL

Appliance Wiring Normal Appliance (3-pin plug)
Appliance casing connected to the earth wire. (Usually has a metal casing.) ACTIVE LOAD NEUTRAL EARTH

Earthing The earth wire is a safety measure for power circuits. It connects the metal chassis of an appliance to the earth (0V). If a fault occurs where the active wire comes into contact with the metal casing, then the appliance will carry an AC voltage. (Very bad!!!) Earth wire provides a lower-resistance conducting path to the earth, rather than through a person. Low resistance will produce a large current in circuit. Fuse will “blow” or circuit breaker will “trip”.

Electric Shock An electric shock is a violent disturbance of the nervous system caused by an electrical discharge or current through the body. Contributing factors: Resistance of the human body (depends on moisture) Amount of current Current path Duration of exposure Voltage (small effect)

Electric Shock The greater and longer a current flows through a body, the more damage is done to tissues within the body. Current (mA) Time (ms) Effect 50mA 10-200ms Usually no dangerous effects >4000ms Fibrillation possible

Electric Shock The greater and longer a current flows through a body, the more damage is done to tissues within the body. Current (mA) Time (ms) Effect 50mA 10-200ms Usually no dangerous effects >4000ms Fibrillation possible 100mA 10-100ms >600ms

Electric Shock The greater and longer a current flows through a body, the more damage is done to tissues within the body. Current (mA) Time (ms) Effect 50mA 10-200ms Usually no dangerous effects >4000ms Fibrillation possible 100mA 10-100ms >600ms 500mA >40ms

Electric Shock Fibrillation is the disorganised, rapid contraction of separate parts of the heart so that it pumps no blood.  death may follow! Defibrillation: Medical intervention 20mA at 3000V is passed through the heart for 5ms. This “jump starts” the heart. Conducting gel used to improve the contact with the body.

Electric Shock CALL 000 straight away!
Electrocution is death brought about by an electric shock. If the event that you witness an electric shock, ensure that power has been turned off / disconnected / isolated before helping the victim. CALL 000 straight away!

Safety in Household Circuits
POSSIBLE FAULTS: Overloading a circuit: In parallel circuits, the total current flowing in the circuit is the sum of the individual currents through the devices. Too many appliance operating on a single power circuit will produce a large current. The conducting wires get hot and melt their insulation, potentially causing a fire!

Safety in Household Circuits
POSSIBLE FAULTS: Cheap extension cords: These are usually not designed to carry more than 7.0A safely, and exceeding this may result in overheating.  Melting insulation  Fire! Short circuit: This can occur when frayed electrical cords or faulty appliances allow the current to flow from one conductor to another with little or no resistance  Current increases rapidly  Overload  Fire!

Safety in Household Circuits
PROTECTION MEASURES: Fuse: A short length of conducting wire or strip of metal that melts when the current through it reaches a certain value. Circuit Breaker: Serves same function as a fuse, but can be reset (after fault is rectified) NOTE: Fuses and circuit breakers are designed to prevent fires in buildings, not to protect against electric shock!

Safety in Household Circuits
Circuit Breaker Types: Thermal Type: Uses a bimetallic strip wrapped in a heating coil. As current increases, the heating coil causes bimetallic strip to bend, tripping a switch. (slow acting) Electromagnetic Type: Uses magnetic effects of electric currents in an electromagnet to lift a catch at a predetermined value. (fast acting)

Circuit Breaker – Thermal

Circuit Breaker – Electromagnetic

Safety in Household Circuits
Problem: A kitchen power circuit has the following appliances operating at the same time: 1000W toaster 312W refrigerator 60W juicer 1400W kettle 600W microwave oven The circuit is protected by a 15A fuse, and is connected to a 240V supply. a) What is the total current in the circuit? b) Will the fuse “blow” if a 2400W heater is used in the same power circuit at the same time?

Safety in Household Circuits
Earth Leakage Device (ELD): (a.k.a. Residual Current Device (RCD) or safety switch) An ELD operates by making use of the magnetic effects of a current. The current in the active wire flows in the opposite direction to the current in the neutral wire. Both currents pass through the iron loop. Each wire produces a magnetic field that is equal in size but opposite in direction. (balanced magnetic fields)

Earth Leakage Device If IA = IN  Nothing happens (fields are balanced) If IA > IN  Magnetic field in coil produces a current and relay opens the switches to break circuit.

Safety in Household Circuits
Earth Leakage Device (ELD): If there is an electrical fault and a residual current flows to the earth via earth wire or person (rather than through neutral wire), then IA>IN. Magnetic fields are not balanced. Magnetic field produces a current in relay circuit, breaking the active and neutral switches. An ELD operates in approx 30-40ms, limiting the current to about 30mA  minor electric shock. Safety switches save lives!!!

Safety in Household Circuits
Earth Leakage Device (ELD): Note: An ELD is only useful when the current flows to earth, but not when the current flows through a person between the active and neutral wires. WHY?

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