Referred by several names household power, household electricity, power line, domestic power, wall power, line power, AC power, city power, street power, grid power etc.

• Currents for mains supply is produced by dynamos at power stations
• EMF is induced by movement of conductor & magnetic field in relation to each other
• Mechanical energy must be available
• Water power, coal, nuclear energy

Dynamos can be constructed to produce either alternating current or direct current
First electricity was produced as DC
But now it’s replaced by AC

Alternating Current Direct Current
Flow of Electrons:
Electrons keep switching directions - forward and backward
Electrons move steadily in one direction or 'forward'
Alternating Current Direct Current
Cause of the direction of flow of electrons:
Cause of the direction of flow of electrons:
Rotating magnet along the wire Steady magnetism along the wire
Amount of energy that can be carried:
Safer to transfer over longer city
Voltage of DC cannot travel very far distances and can provide more until it begins to lose energy power
Amount of energy that can be carried:
Safer to transfer over longer city distances and can provide more power
Voltage of DC cannot travel very far until it begins to lose energy

Why AC not DC?
– Greater voltage can be produced with AC than DC.
– Voltage of AC can be alter with the transformers which is more suitable for long distance transmission.
– Thin cables which cost less can be used for transmission of current because EMF is stepped up to several thousand volts and reduce the current. When necessary EMF can be stepped down.
– If higher voltage is carrying the lost in voltage can be negligible when compared to the total and original level can be restored by using step-up transformers.
– The construction of modern apparatus is such that they work only on AC supply.

1. Each dynamo has three coils of wire
2. One end of each coil is connected to live distribution line
3. Other ends are connected together and earth
4. Distribution of current is by three live cables & one neutral cable
5. These four cables are observed on the pylons
6. Each of the consumer receives one of the live wires & the neutral wire

• Advantage of the grid system
– All areas supplied receive the same voltage and type of current.
– Large demand in one area do not put an excess load on any particular power station.
– Breakdown of one power station does not cut off the supply to any area.
– It is not necessary for all generators to be in operation all time.

Current on entering the house passes through the main fuses & the meter
Next comes the main switch
& the house main fuses, then the various circuits which are parallel to each other
In this method each circuit receives the full voltage of the supply

• Circuits in the house are divided into two
1.Light circuits
2.Power circuits
Power circuits may be arranged in different ways e.g.Similar to light circuits
Ring main
Sub circuits

• Complete loop is taken from each of the two supply cables
• Supply points are wired in parallel with each other between loops
• Fused plugs are used
• No fuses are incorporated, but 30A fuse is placed on the live wire
• Each wire carries at least 15A

• Designed to a weak point in a circuit
• It blows if a current of too great intensity is passed
• It consist of a short length wire of low melting point
• If current passing through it exceeds a certain value the heat generated melts the wire preventing further current flow
• It prevents damage to another part & gives warning of the defect

There are different types of fuses
1. Cartridge fuse
Fusible element made of silver wire
Runs between metal caps through tube of glass
It has color cords
5 A – white
15 A – blue
20 A – yellow
30 A – red
45 A – green
2. Normal fuse

In many cases there are fuses on both wires of the circuit
But if only one is provided it must be on live wire
In physiotherapy departments fuses are included in circuit of each apparatus
The blowing of fuse is due to passage of too great current
It may arise from
1. Too low resistance
2. Too high voltage
3. When several parallel circuits are taken from one supply

• Apparatus working on a power circuit should be connected to the supply by three pin wall plug.
• Pins are arranged in a triangle
• Two similar pins are for connect apparatus to the circuit
• Marked “L” and “N’: live & neutral
• The wire connected to pin marked
“E” is to connect apparatus casing to earth.
Brown wire to L
Blue wire to N
Yellow wire to E

Earthing an apparatus casing is a precaution against earth shock
Sometimes insulation on live wire become worn the wire come in contact with casing
If apparatus casing is not earthed connection between casing & earth complete a circuit
If the connection is through a person he receives an electric shock
By correct earthing
When live wire gets contacted with casing current passes by the earth wire
This sis a pathway of low resistance
The current flow is great
Fuse on live wire blows
This stops current flow giving a warning of defect

• Current is turned on & off by a switch
• Switches vary type according to currents that pass through
• Commonly used in houses & physio depts. has two metal blades which fit into metal sockets
The principle is
When switch is on, the blades are gripped on the socket & circuit is completed
When circuit is broken a spring ensures the sudden separation of socket & blades

Type of Switch
ON-OFF
Single Pole, Single Throw
Circuit Symbol
(ON)-OFF
Push-to-make
ON-ON
Single Pole, Double
Throw
Dual ON-OFF
Double Pole, Single
Throw
Example

Electric shocks: painful stimulation of sensory nerves caused by a sudden flow, cessation or variation in the current passing through the body.

1. Lower the resistance of the skin greater the current passes e.g. – if circuit touched with wet hands shock is severe than when hands are dry
2. Greater the current the shock is severe
3. Path taken by the current e.g. – strong current through head, neck or heart is fatal
4. Type of current e.g. - shocks are severe with alternating current than with direct current

1 Milliampere
5 Milliamperes
Current
Current
5 Milliamperes
50-150 Milliamperes
1000-4,300 Milliamperes
1000-4,300 Milliamperes
Reaction
Perception level
Slight shock felt; not
Slight shock felt; not painful but disturbing
Extreme pain, respiratory arrest, contraction
Ventricular fibrillation
Cardiac arrest, severe burns and probable

Minor electric shock
Victim gets frightened & distressed
No loss of consciousness
Major or severe electric shock
There is fall of blood pressure
Patient may become unconsciuos
There could be cessation of respiration followed by ventricular fibrillation & cardiac arrest

1. Current should be switched off immediately
2. Victim should be disconnected from the source of supply
3. If there is no switch, the victim must be removed from contact with the conductor
4. Following minor shock patient must be reassured that every thing is alright
5. Water may be given to drink, but hot drinks avoided
6. Tight clothing loosened
7. If respiration has ceased the airway must be cleaned and artificial ventilation commenced immediately
8. CPR may also be given
9. Oxygen therapy
10.Patient shifted to a hospital after primary care

Preventing Electrical Hazards
1. Inspect wiring of equipment before each use. Replace damaged or frayed electrical cords immediately.
2. Use safe work practices every time electrical equipment is used.
3. Know the location and how to operate shut-off switches and/or circuit breaker panels. Use these devices to shut off equipment in the event of a fire or electrocution.
4. Limit the use of extension cords. Use only for temporary operations. In all other cases, request installation of a new electrical outlet.
5. Use only multi-plug adapters equipped with circuit breakers or fuses.
6. Minimize the potential for water or chemical spills on or near electrical equipment.

Earth shock when a shock is due to a connection between the live wire of the main and earth.
– How the patients and the therapists can get earth shocks?
– How can we prevent the earth shocks?

1. Connection to live wire when wire is not properly insulated live wire is touched to metal casing live wire is touched to any wet thing
2. Connection to earth if the floor is made up of stone if the conductor is touching any radiated metal casing or metal wire

1.Proper arrangement of physiotherapy dept.
2.Proper flooring done with rexin
3.Proper insulation
4.While on treatment patient should not touch any machine part
5.Metal casing should be connected to earth
6.Floor kept dry

Clothing and Personal Protective Equipment
Type of Switch
ON-OFF
Single Pole,
Single Throw =
SPST A simple onoff switch. This type can be used to switch the power supply to a circuit.
W hen used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral.
Photograph ©
Rapid Electronics
Circuit Symbol Example
SPST toggle switch
(ON)-OFF
Push-to-make =
SPST Momentary
A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around
ON. This is the standard doorbell switch.
Photograph ©
Rapid Electronics
ON-(OFF)
Push-to-break =
SPST Momentary
A push-to-break switch returns to its normally closed (on) position when you release the button.
Photograph ©
Rapid Electronics
Push-to-make switch
Push-to-break switch
ON-ON
Single Pole,
Double Throw =
SPDT This switch can be on in both
1. Thick-soled work shoes for protection against sharp objects such as nails. case. It is often called a changeover switch.
For
2. Wear work shoes with safety toes if the job requires. lamp in one position and a green lamp in the other position.
A SPDT toggle 3. Make sure the soles are oil resistant if the shoes are subject to oils and connecting to COM and one of the A or
B terminals shown
SPDT toggle switch
SPDT slide switch
(PCB mounting) grease in the diagram. A and B are interchangeable so switches are usually not labelled.
ON-OFF-ON
SPDT Centre Off switching position in the centre which is off. Momentary
(ON)-OFF-(ON) versions are also available where the A hat or cap. Wear an approved safety helmet (hard hat) if the job released.
Photographs ©
Rapid Electronics requires