Electrical safety

Dangers of electricity

1
 Hazards associated with using mains
electricity supply
1- damaged insulation
2- overheat cables
3- Damp conditions
4- excess current from overloaded plugs
5- extension leads
6- single and multiple sockets

2
 Electric shock
Damaged insulation or faulty wiring may cause electric shock
The current flows from electric circuit through a person’s body to earth

Resistance of dry skin = 10000 Ω Resistance of wet skin = 1000 Ω

p.d. at home = 240 V p.d. at home = 240 V
I = 240 V/ 10000 Ω = 0.024 A I = 240 V/ 1000 Ω = 0.24 A
24 mA 240 mA (lethal current)

Strength of electric shock depends on 100 mA through

1- size of current not the voltage the heart is likely
2- length of time for which it acts to be fatal
3- the path the current takes through the body
4- damp conditions ( water decreases the
resistance of the body).
5- wearing shoes with insulating rubber

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 Electric shock
To avoid the risk of getting an electric shock

4
 Electric shock
In case of electric shock, take the following action

5
 Fire risk
A fire may start
1- if flammable material is too close to a hot appliance
2- if the electrical wiring in the walls of the house becomes overheated by carrying a
large current
3- damaged insulation or faulty wiring leads to a large current flowing to earth
through flammable material
To reduce the risk of fire through overheated cables, the maximum current should be
limited by the following:

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Fire and electric shock

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 Electric lighting
LED lights

LEDs
- are semiconductor devices
- 40-50% efficient in transferring electrical
energy to light. As a comparison the efficiency of
the filament lamps only about 10%

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 Electric lighting
Fluorescent strip lamps
Fluorescent strip lamps are
1- long lasting
2- efficient

The glass tube of

Fluorescent lamps contains
1-two electrodes
2- mercury vapour
3- fluorescent powder

How does it work

1- When it switches on the mercury emits invisible ultraviolet radiation
2- this will make the powder fluoresce (glow). Visible light is emitted.
3- type of powder determines the color of the light

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 Electric lighting
Fluorescent strip lamps

Compact energy-saving fluorescent lamps are modified to fit

straight into normal light sockets

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 House circuits

What does
earthed
Electricity comes to our homes by a cable containing two wires mean??
1- Live (L): contains the power. For a.c. it is alternately positive and
negative.
2- Neutral (N): it is earthed at the local sub-station so no p.d. between
it and the earth
3- A third wire (E) is the earth wire connects the top socket on the
power points in the home to earth.

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House circuits

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 1-Circuits in parallel
Every circuit is connected in parallel with supply, (Across the live and neutral) in order
to receive the full mains p.d..

The advantages of having appliances connected in parallel, rather than in

series

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 2-Switches

-Switches are always connected in the live wire.

-If they were in the N wire, light switches and power

sockets would be Live when switches are off.

-This will cause a fatal shock

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 3-Ring main circuit
-The live wire as well as the neutral wire run in a
complete ring round the house and the power sockets
are tapped off from them.
-the current rated at 13 A
-A house may have several ring circuits to serve a
different area WHY???

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 4-Fuses
A fuse protects a circuit
- It is always placed in the live wire
- It is a short length of wire of material with a low melting temperature
- Often tinned copper
- It melts and breaks the circuit when current exceeds a certain value.

Reasons for excessive currents

1- short circuits
due to worn insulation on connecting wires
2- Overloaded circuits.

Without a fuse the wiring would

become hot and could cause a
fire.

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 Fuses
Fuse is used to ensure that the current-carrying capacity of the wiring is
not exceeded

The thicker the cable is, the more current it can carry
Each cable size has a limit.

If the live ring in the home has a 30 A fuse, then the maximum current can
be used by the ring is 30 A. otherwise the wires overheated. So, there are
several rings to serve different areas in the home.

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 Fuses
To replace a fuse
1- switch off the circuit before
replace
2-replace with one of the same
value as recommended by the
manufacturer of the device

For appliance with power up to 720 W, For appliance with power more than 720
use 3 A fuse (red) W and up to 3 kW, use 13 A fuse (brown)

How can we chose the size of

the fuse???

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 Fuses
To replace a fuse
1- switch off the circuit before
replace
2-replace with one of the same
value as recommended by the
manufacturer of the device

For appliance with power up to 720 W, For appliance with power more than 720
use 3 A fuse (red) W and up to 3 kW, use 13 A fuse (brown)

How can we chose the size of the fuse???

-Calculate the current
-Chose the size of the current a little higher
than the current of the device.
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 5-Trip switches (circuit breakers)
1-Trip switches are used instead of fuses.
2- it contains electromagnet which become
strong magnet when the current exceeds the
rated value of the circuit breaker.
3- at this moment it separates a pair of
contact and break the circuit.
4- the iron bolt is attracted far enough for a
plunger to be released, allowing the push
switch to open the circuit

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Comparison between fuse and circuit
breakers

Circuit breakers operate much faster

Circuit breaker can be reset by pressing a button

How can we chose the size of the circuit

breakers ???
-Calculate the current
-Chose the size of the current a little higher
than the current of the device.

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 The Residual Current Circuit Breaker
The Residual Current Circuit Breaker (RCCB)
Also called
The Residual Current Device (RCD)
- It is used to detect any difference between the live and neutral
wires due to an earth fault
- It works when the resistance of earth path between the
consumer and substation is not small enough for a fault-current
to blow the fuse or the circuit breaker. RCD breaks the circuit
- It has high sensitivity and quick response.
RCD should be used with any portable appliance ( lawn mower or
hedge trimmer)
The risk is greater because of a good earth connection through the
feet 22
Example

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Example

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 6-Earthing
The third wire which connects the top socket on all power points to the earth
A metal water pipe entering the house can be used

Earthing is used to prevent electric shock

In the three-pin plug, the earth pin is connected to the metal case of the appliance,
which is joined to earth by a path of almost zero resistance.

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 7-Double insulation

The appliance are usually double insulated cable with

no earth wire.
1-Two-core insulated cable is used.
2- The appliance is enclosed in a non-conducting
plastic case.
There is no risk of a shock should a fault develop.

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