area of study 2: electricity part 4: home distribution

Area of Study 2: Area of Study 2: ElectricityElectricity

Part 4: Home DistributionPart 4: Home Distribution

Distribution in the HomeDistribution in the Home Households are connected to the Households are connected to the

main electrical grid and are main electrical grid and are supplied with an AC voltage of 240V supplied with an AC voltage of 240V (RMS) at a frequency of 50Hz.(RMS) at a frequency of 50Hz.

Root Mean Square (RMS) refers to a Root Mean Square (RMS) refers to a mathematical process by which a mathematical process by which a DC equivalent voltage is calculated.DC equivalent voltage is calculated.

Actual values of voltages ranges Actual values of voltages ranges between +340V and -340V between +340V and -340V

Distribution in the HomeDistribution in the Home If frequency (f) is 50Hz, current changes If frequency (f) is 50Hz, current changes

direction every 1/50direction every 1/50thth of a second. of a second.

Electricity is fed into the home from Electricity is fed into the home from overhead cables (suburbs) or overhead cables (suburbs) or underground cables (city).underground cables (city).

Electricity to the HomeElectricity to the Home

Metal pipe or earth stake

Switch-

board

Junction box

Switchboard: Contains meters, fuses/circuit breakers and neutral bar

Two wires bundled together : active (240V) and neutral (0V)

Fused mains connection box (50A)

Earth wire (defined as 0V): provides a lower-resistance conducting path during faults

SwitchboardSwitchboard

Powerpoints and lights connected in parallel

Fuses / Circuit Breakers normally set to trip at 8A, 16A or 32A depending on application.

Electrical Wiring: Colour Electrical Wiring: Colour CodingCoding

WireWire Old Old SystemSystem

New New SystemSystem

ActiveActive

NeutralNeutral

EarthEarth




ActiveActive RedRed

NeutralNeutral BlackBlack

EarthEarth GreenGreen




ActiveActive RedRed BrownBrown

NeutralNeutral BlackBlack BlueBlue

EarthEarth GreenGreen Green-Green-YellowYellow

Double-Insulated Double-Insulated Appliance:Appliance:

(2-pin plug)(2-pin plug)

Two independent Two independent layers of layers of insulation. insulation.

Eg. Hand-held Eg. Hand-held electrical tools electrical tools such as drills or such as drills or hairdryers.hairdryers.

Appliance WiringAppliance Wiring

ACTIVE

NEUTRAL

LOAD

Symbol

Normal ApplianceNormal Appliance

(3-pin plug)(3-pin plug)

Appliance casing Appliance casing connected to the connected to the earth wire.earth wire.

(Usually has a (Usually has a metal casing.)metal casing.)

Appliance WiringAppliance Wiring

ACTIVE

NEUTRAL

LOAD

EARTH

EarthingEarthing The earth wire is a safety measure for The earth wire is a safety measure for

power circuits. It connects the metal power circuits. It connects the metal chassis of an appliance to the earth chassis of an appliance to the earth (0V).(0V).

If a fault occurs where the active wire If a fault occurs where the active wire comes into contact with the metal casing, comes into contact with the metal casing, then the appliance will carry an AC then the appliance will carry an AC voltage. (Very bad!!!)voltage. (Very bad!!!)

Earth wire provides a lower-resistance Earth wire provides a lower-resistance conducting path to the earth, rather than conducting path to the earth, rather than through a person.through a person. Low resistance will produce a large current in circuit.Low resistance will produce a large current in circuit. Fuse will “blow” or circuit breaker will “trip”.Fuse will “blow” or circuit breaker will “trip”.

Symbol

Electric ShockElectric Shock An electric shock is a violent An electric shock is a violent

disturbance of the nervous system disturbance of the nervous system caused by an electrical discharge or caused by an electrical discharge or current through the body.current through the body.

Contributing factors:Contributing factors: Resistance of the human body (depends Resistance of the human body (depends

on moisture)on moisture) Amount of currentAmount of current Current pathCurrent path Duration of exposureDuration of exposure Voltage (small effect)Voltage (small effect)

Electric ShockElectric Shock The greater and longer a current flows The greater and longer a current flows

through a body, the more damage is through a body, the more damage is done to tissues within the body.done to tissues within the body.

Current Current (mA)(mA)

Time Time (ms)(ms)

EffectEffect

50mA50mA 10-200ms10-200ms Usually no dangerous Usually no dangerous effectseffects

50mA50mA >4000ms>4000ms Fibrillation possibleFibrillation possible




Time Time (ms)(ms)

EffectEffect





Electric ShockElectric Shock Fibrillation is the disorganised, rapid Fibrillation is the disorganised, rapid

contraction of separate parts of the contraction of separate parts of the heart so that it pumps no blood. heart so that it pumps no blood. death may follow!death may follow!

Defibrillation: Medical interventionDefibrillation: Medical intervention 20mA at 3000V is passed through 20mA at 3000V is passed through

the heart for 5ms.the heart for 5ms. This “jump starts” the heart.This “jump starts” the heart. Conducting gel used to improve the Conducting gel used to improve the

contact with the body.contact with the body.

Electric ShockElectric Shock Electrocution is death brought about Electrocution is death brought about

by an electric shock.by an electric shock. If the event that you witness an If the event that you witness an

electric shock, ensure that power has electric shock, ensure that power has been turned off / disconnected / been turned off / disconnected / isolated before helping the victim.isolated before helping the victim.CALL 000 straight away!CALL 000 straight away!

Safety in Household Safety in Household CircuitsCircuits POSSIBLE FAULTS:POSSIBLE FAULTS:

Overloading a circuit:Overloading a circuit: In parallel circuits, the total current In parallel circuits, the total current

flowing in the circuit is the sum of the flowing in the circuit is the sum of the individual currents through the devices.individual currents through the devices.

Too many appliance operating on a Too many appliance operating on a single power circuit will produce a large single power circuit will produce a large current.current.

The conducting wires get hot and melt The conducting wires get hot and melt their insulation, potentially causing a their insulation, potentially causing a fire!fire!

Safety in Household Safety in Household CircuitsCircuits POSSIBLE FAULTS:POSSIBLE FAULTS:

Cheap extension cords:Cheap extension cords: These are usually not designed to carry These are usually not designed to carry

more than 7.0A safely, and exceeding this more than 7.0A safely, and exceeding this may result in overheating. may result in overheating. Melting Melting insulation insulation Fire! Fire!

Short circuit:Short circuit: This can occur when frayed electrical cords This can occur when frayed electrical cords

or faulty appliances allow the current to or faulty appliances allow the current to flow from one conductor to another with flow from one conductor to another with little or no resistance. little or no resistance. Current Current increases rapidly increases rapidly Overload Overload Fire! Fire!

Safety in Household Safety in Household CircuitsCircuits PROTECTION MEASURES:PROTECTION MEASURES:

Fuse:Fuse: A short length of conducting wire or A short length of conducting wire or

strip of metal that melts when the strip of metal that melts when the current through it reaches a certain current through it reaches a certain value.value.

Circuit Breaker:Circuit Breaker: Serves same function as a fuse, but can Serves same function as a fuse, but can

be reset (after fault is rectified)be reset (after fault is rectified) NOTE: Fuses and circuit breakers are NOTE: Fuses and circuit breakers are

designed to prevent fires in buildings, designed to prevent fires in buildings, not to protect against electric shock!not to protect against electric shock!

Safety in Household Safety in Household CircuitsCircuits Circuit Breaker Types:Circuit Breaker Types:

Thermal Type:Thermal Type: Uses a bimetallic strip wrapped in a Uses a bimetallic strip wrapped in a

heating coil. heating coil. As current increases, the heating coil As current increases, the heating coil

causes bimetallic strip to bend, tripping a causes bimetallic strip to bend, tripping a switch. (slow acting)switch. (slow acting)

Electromagnetic Type:Electromagnetic Type: Uses magnetic effects of electric currents Uses magnetic effects of electric currents

in an electromagnet to lift a catch at a in an electromagnet to lift a catch at a predetermined value. (fast acting)predetermined value. (fast acting)

Circuit Breaker – ThermalCircuit Breaker – Thermal

Circuit Breaker – Circuit Breaker – ElectromagneticElectromagnetic

Safety in Household Safety in Household CircuitsCircuitsProblem:Problem: A kitchen power circuit has the A kitchen power circuit has the

following appliances operating at the same following appliances operating at the same time:time:

1000W toaster1000W toaster 312W refrigerator312W refrigerator 60W 60W juicerjuicer

1400W kettle1400W kettle 600W microwave oven600W microwave oven

The circuit is protected by a 15A fuse, and is The circuit is protected by a 15A fuse, and is connected to a 240V supply.connected to a 240V supply.

a) What is the total current in the circuit?a) What is the total current in the circuit?

b) Will the fuse “blow” if a 2400W heater is b) Will the fuse “blow” if a 2400W heater is used in the same power circuit at the same used in the same power circuit at the same time?time?

Safety in Household Safety in Household CircuitsCircuits

Earth Leakage Device (ELD):Earth Leakage Device (ELD):(a.k.a. Residual Current Device (RCD) or safety switch)(a.k.a. Residual Current Device (RCD) or safety switch)

An ELD operates by making use of the An ELD operates by making use of the magnetic effects of a current.magnetic effects of a current.

The current in the active wire flows in the The current in the active wire flows in the opposite direction to the current in the opposite direction to the current in the neutral wire.neutral wire.

Both currents pass through the iron loop.Both currents pass through the iron loop. Each wire produces a magnetic field that Each wire produces a magnetic field that

is equal in size but opposite in direction. is equal in size but opposite in direction. (balanced magnetic fields)(balanced magnetic fields)

Earth Leakage DeviceEarth Leakage Device

If IIf IAA = I = INN Nothing happens (fields are balanced) Nothing happens (fields are balanced)

If IIf IAA > I > INN Magnetic field in coil produces a current Magnetic field in coil produces a current and relay opens the switches to break circuit.and relay opens the switches to break circuit.

Safety in Household Safety in Household CircuitsCircuitsEarth Leakage Device (ELD):Earth Leakage Device (ELD):

If there is an electrical fault and a residual If there is an electrical fault and a residual current flows to the earth via earth wire or current flows to the earth via earth wire or person (rather than through neutral wire), person (rather than through neutral wire), then Ithen IAA>I>INN..

Magnetic fields are not balanced.Magnetic fields are not balanced. Magnetic field produces a current in relay circuit, Magnetic field produces a current in relay circuit,

breaking the active and neutral switches.breaking the active and neutral switches. An ELD operates in approx 30-40ms, limiting An ELD operates in approx 30-40ms, limiting

the current to about 30mA the current to about 30mA minor electric minor electric shock.shock.

Safety switches save lives!!!Safety switches save lives!!!

Safety in Household Safety in Household CircuitsCircuits

Earth Leakage Device (ELD):Earth Leakage Device (ELD): Note: An ELD is only useful when the Note: An ELD is only useful when the

current flows to earth, but not when current flows to earth, but not when the current flows through a person the current flows through a person between the active and neutral wires.between the active and neutral wires.

WHY?WHY?

area of study 2: electricity part 4: home distribution

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