What is mains supply??

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

Rotating magnet along the wire Rotating magnet along the wire

flow of electrons:

Steady magnetism 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

Mains supply

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.

Distribution and grid system

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.

Wiring of the houses

Distribution in a house

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

Light & power circuits

• 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

Distribution in a house – Ring main

• 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

Fuses

• 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

Fuses cont.

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

Fuses cont.

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

Power Plugs

• 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

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

Switches

• 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

Dangers of electric current

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

Severity of shocks

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 painful but disturbing Slight shock felt; not painful but disturbing Extreme pain, respiratory arrest, severe muscular respiratory arrest, contraction Ventricular fibrillation Cardiac arrest, severe burns and probable death

Effects of electric shock

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

Treatment of electric shock

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?

Causes of earth shock

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

Precaution for earth shock

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 on off switch. This type can be used to switch the power supply to a circuit. W hen used with mains electricity this type of switch 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. 3. Make sure the soles are oil resistant if the shoes are subject to oils and grease requires lamp in one position and a green lamp in the other position. A SPDT toggle connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled.

ON-OFF-ON SPDT Centre Off

A special version of the standard SPDT Rubber boots for damp locations off. Momentary (ON)-OFF-(ON) versions are also available where the released. Photographs © Rapid Electronics SPDT toggle switch SPDT slide switch (PCB mounting) A hat or cap. Wear an approved safety helmet (hard hat) if the job

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