What is mains

supply??Referred by several nameshousehold 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 currentFirst electricity was produced as DCBut 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'

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

distances and can provide more

power

Voltage of DC cannot travel very far

until it begins to lose energy

Alternating Current Direct Current

Flow of Electrons:Electrons keep

switching directions - forward and backward

Electrons move steadily in one

direction or 'forward'

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 distances and can provide more

power

Voltage of DC cannot travel very far until it begins to lose energy

Mains supplyWhy 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 wire2. One end of each coil is connected to live

distribution line3. Other ends are connected together and earth4. Distribution of current is by three live cables &

one neutral cable5. These four cables are observed on the pylons6. 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 housesDistribution in a house

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

Light & power circuits

• Circuits in the house are divided into two1.Light circuits2.Power circuitsPower circuits may be arranged in

different wayse.g.Similar to light circuits

Ring mainSub 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 fuseFusible element made of silver wireRuns between metal caps through tube of glass

It has color cords5 A –white15 A – blue20 A –

yellow30 A – red45 A – green

2. Normal fuse

Fuses cont. In many cases there are fuses on both wires of the circuitBut 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 currentIt may arise from1.Too low resistance2.Too high voltage3.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 LBlue wire to NYellow wire to E

Earthing Earthing an apparatus casing is a precaution against earth shockSometimes insulation on live wire become worn the wire come in contact with casingIf apparatus casing is not earthed connection between casing & earth complete a circuitIf 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 wireThis sis a pathway of low resistanceThe current flow is greatFuse on live wire blowsThis 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 completedWhen circuit is broken a spring ensures the sudden separation of socket & blades

Type of Switch Circuit Symbol Example

ON-OFFSingle Pole, Single Throw

(ON)-OFFPush-to-make

ON-ONSingle Pole, Double Throw

Dual ON-OFFDouble Pole, Single Throw

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 shocks1.Lower the resistance of the skin

greater the current passese.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 currente.g. – strong current through head, neck or heart is fatal

4.Type of currente.g. - shocks are severe with alternating current than with direct current

Current Reaction

1 Milliampere Perception level

5 Milliamperes Slight shock felt; not painful but disturbing

6-30 Milliamperes Painful shock; "let-go" range

50-150 Milliamperes Extreme pain, respiratory arrest, severe muscular contraction

1000-4,300 Milliamperes Ventricular fibrillation

10,000+ Milliamperes Cardiac arrest, severe burns and probable death

Current Reaction

1 Milliampere Perception level

5 Milliamperes Slight shock felt; not painful but disturbing

6-30 Milliamperes Painful shock

50-150 Milliamperes Extreme pain, respiratory arrest, severe muscular contraction

1000-4,300 Milliamperes Ventricular fibrillation

10,000+ Milliamperes Cardiac arrest, severe burns and probable death

Effects of electric shockMinor electric shockVictim gets frightened & distressedNo loss of consciousness

Major or severe electric shockThere is fall of blood pressurePatient may become unconsciuosThere could be cessation of respiration followed by ventricular fibrillation & cardiac arrest

Treatment of electric shock1. 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 loosened7. If respiration has ceased the airway must be

cleaned and artificial ventilation commenced immediately

8. CPR may also be given9. Oxygen therapy10.Patient shifted to a hospital after primary care

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.

Preventing Electrical Hazards

`Dangers of electric current

Earth shockwhen 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 shock1.Connection to live wire

when wire is not properly insulatedlive wire is touched to metal casinglive wire is touched to any wet thing

2.Connection to earth

if the floor is made up of stoneif 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 insulation4.While on treatment patient

should not touch any machine part

5.Metal casing should be connected to earth

6.Floor kept dry

Type of Switch Circuit Symbol Example

ON-OFFSingle Pole, Single Throw = SPST A simple on-off switch. This type can be used to switch the power supply to a circuit. When 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

SPST toggle switch

(ON)-OFFPush-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

Push-to-make switch

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-break switch

ON-ONSingle Pole, Double Throw = SPDT This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position. A SPDT toggle switch may be used as a simple on-off switch by 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-ONSPDT Centre OffA special version of the standard SPDT switch. It has a third switching position in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. Photographs © Rapid Electronics

SPDT toggle switch

SPDT slide switch(PCB mounting)

Clothing and Personal Protective Equipment

Clothing should fit snugly to avoid danger of becoming entangled in moving machinery or creating a tripping or stumbling hazard. See Figure 1-26.

1. Thick-soled work shoes for protection against sharp objects such as nails.

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 greaseRubber boots for damp locationsA hat or cap. Wear an approved safety helmet (hard hat) if the job requires

Thank You..