4 Electricity - Home Distribution

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Transcript 4 Electricity - Home Distribution

Area of Study 2:
Electricity
Part 4: Home Distribution
Distribution in the Home
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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
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If frequency (f) is 50Hz, current changes
direction every 1/50th of a second.
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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)
SwitchJunction box
board
Metal pipe or
earth stake
Two wires bundled together :
active (240V) and neutral (0V)
Earth wire (defined as 0V):
provides a lower-resistance
conducting path during faults
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
Active
Neutral
Earth
Old System
New System
Electrical Wiring: Colour Coding
Wire
Old System
Active
Red
Neutral
Black
Earth
Green
New System
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)
Symbol
ACTIVE
Two independent layers
of insulation.
Eg. Hand-held electrical
tools such as drills or
hairdryers.
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
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The earth wire is a safety measure for
Symbol
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.
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Low resistance will produce a large current in circuit.
Fuse will “blow” or circuit breaker will “trip”.
Electric Shock
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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)
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Electric Shock
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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
50mA
>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
50mA
>4000ms
Fibrillation possible
100mA
10-100ms
Usually no dangerous effects
100mA
>600ms
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
50mA
>4000ms
Fibrillation possible
100mA
10-100ms
Usually no dangerous effects
100mA
>600ms
Fibrillation possible
500mA
>40ms
Fibrillation possible
Electric Shock
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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
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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
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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!
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Safety in Household Circuits
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POSSIBLE FAULTS:
Cheap extension cords:
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These are usually not designed to carry more than
7.0A safely, and exceeding this may result in
overheating.  Melting insulation  Fire!
Short circuit:
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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
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PROTECTION MEASURES:
Fuse:
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Circuit Breaker:
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A short length of conducting wire or strip of metal
that melts when the current through it reaches a
certain value.
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
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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)
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Electromagnetic Type:
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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.
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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.
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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.
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WHY?