earthing,safety precaution and maintenance

8/2/2019 Earthing,Safety Precaution and Maintenance

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Earthing,

Safety Precautions

and

Maintenance of Electric Machine

Thermax GraduateProgramme


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How Electricity Works

Operating an electric switch is like turning on a water faucet.

Behind the faucet or switch there must be a source of water or

electricity with something to transport it, and with a force to

make it flow.

In the case of water the source is a pump, and the force to

make it flow through the pipes is provided by the pump.

For electricity, the source is the power generator. Current

travels through electrical conductors (wires) and the force to

make it flow, measured in volts, is provided by a generator.


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Voltage, Current andResistance

Voltage increases Current increases

Resistance decreases Current increasesVoltage = Current /Resistance - Ohms Law


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The complete circuit

A complete Circuit or loop is

necessary for current to flow


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Electrical Hazards

Basically, electrical hazards can be

categorized into three types.

The first and most commonly recognizedhazard is electrical shock.

The second type of hazard is electrical burns

and

The third is the effects of blasts which include

pressure impact, flying particles from

vaporized conductors.


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A complete circuit

Complete Circuit or loop is necessary for current to flow

Current takes the path of least resistance


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Electrical Shock

Received when currentpasses through the body You become part of the

circuit

Severity of a shockdepends on: Path of current through the

body

Amount of current flowingthrough the body

Length of time the body is inthe circuit


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Step Potential


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Step potential


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Touch Potential


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Effect of electricity on human body

Burns

Surface

Deep tissue

Electric Shock

Muscular Contraction

Asphyxia

Respiratory Arrest

Ventricular Fibrillation


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Electrical Burns

Most common shock-related, nonfatal

injury.

Occurs when you touch electrical wiring

or equipment that is improperly used or

maintained.

Typically occurs on the hands

Very serious injury that needs

immediate attention


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Ventricular Fibrillation

Factors are Current / Time & Physiological Structure of body

Can occur at 30mA (0.03 A)

Causes heart to flutter(abnormally rapid heartbeat)

Muscle cannot open / close properly

Does not pump

Lack of oxygen to brain - DEATH


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Summary of Effect of Electric Current

Current Reaction1 Milliampere Perception level5 Milliamperes Slight shock felt; not painful but

disturbing6-30 Milliamperes Painful shock50-150 Milliamperes Extreme pain, respiratory arrest,

severe muscular contraction1000-4,300 Milliamperes Ventricular fibrillation10,000+ Milliamperes Cardiac arrest, severe burns and

probable death


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Clues that Electrical Hazards Exist

Tripped circuit breakers or blown fuses

Warm tools, wires, cords, connections, or junction boxes

GFCI that shuts off a circuit

Worn or frayed insulation around wire or connection


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Causes of Electrical Hazards

Most electrical mishaps

are caused by a

combination of three

factors:

Unsafe equipment and/or

installation,

Workplaces made unsafe by

the environment

Unsafe work practices


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Overload Hazards

If too many devices are plugged into a

circuit, the current will heat the wires

to a very high temperature, which may

cause a fire

If the wire insulation melts, arcing may

occur and cause a fire in the area

where the overload exists, even inside

a wall


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Preventing Electrical Hazards

Ways of protecting workers and preventing electrical

hazards are:

Safe work practices

Insulation

Grounding

Electrical protective devices (GFCI)


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Moisture provides a conductive paththat could result in death.

Never work with wet tools or clothing.

Remove Your JewelryWatch bands, bracelets.

Safe Work Practices


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Insulation

Check insulation prior to

using cables, tools, or

equipment

Remove from service any

tools or equipment with

damaged insulation


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Cabinets, Boxes, and Fittings

Junction boxes and fittings

must have approved covers

Unused openings in cabinets,

boxes and fittings must be

closed (no missing knockouts)


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Hand-Held Electric Tools

Hand-held electric tools pose a potential

danger because they make continuous contact

with the hand

To protect you from shock, burns, and

electrocution, tools must:

Have a three-wire cord with ground and be

plugged into a grounded receptacle, or

Be double insulated

29 CFR 1910.304(f)(5)(v)(C)(3)


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Panel Boxes

Panel boxes are used to house circuit breakers that

block or isolate energy

Ensure panel boxes remain clear

Label all circuits for what they control

Label panel boxes for what they control


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Improper Grounding

Tools plugged into improperly

grounded circuits may become

energized

Broken wire or plug on extension

cord

Some of the most frequently violated

OSHA standards


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Grounding and Earthing

Grounding creates a low-

resistance path from a tool to the

earth to disperse unwanted current

When a short or lightning occurs,

energy flows to the ground,

protecting you from electrical

shock, injury and death


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Electrical circuits - Earthing

Very important for safety !

Prevents conducting parts of equipment (ie. metal frames or lids),

which do not normally conduct electricity from becoming live during

faults.


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Earthing


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Types of Earthing

Plate Earthing

Pipe Earthing


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Fuses, RCDs &GCFI

Fuses essential for safety, will cut off supply at a certain current

level i.e. 16A, 10A, 5A mains supply fuse

Fuse has a fuseable wire element which heats up when

current flows Excessive current = excessive heat & wire meltspreventing current flow

RCDs Residual current device

Compares current in Live & Neutral if different and abovea certain value supply switched off


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Ground fault circuit interrupters(GFCIs)

A GFCI is not an overcurrent device like a fuse or

circuit breaker

GFCIs are designed to sense an imbalance in current

flow over the normal path


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Ground fault circuit interrupters(GFCIs)

GFCI contains a special sensor that

monitors the strength of the

magnetic field around each wire in

the circuit when current is flowing

The field is proportional to the

amount of current flow

T

G d f l i i


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If the current flowing in the black (ungrounded) wire is within

5 milliampers of the current flowing in the white (grounded) all

the current will flow in the normal path

If the current flow differs by more than 5mA +/- 1mA, the

GFCI will quickly open the circuit

Ground fault circuitinterrupters (GFCIs)


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Work on Electrical Equipment

Always ISOLATE supply

Switch off using a device that will create an air gap that

should not fail

Lock off where possible to prevent inadvertent switching ON

whilst work taking place

Always test conductors in a reliable way before touching (ie.

test the test device before and after use!)

Permit to work may be required for higher

voltage or complex installations


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Electric Shock - Treatment

Isolate supply immediately Call the doctor

If you cannot isolate DO NOT attempt to touch casualty

Physically remove victim using non-conducting implements

Check for pulse / breathing, give artificial respiration if necessary


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Electrical Rescue Techniques

Approaching the accident:

Never rush into an accident situation.

Call the doctor as soon as possible.

Get the aid of trained electrical personnel if possible.

Approach the accident scene cautiously.


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Examining the scene:

Visually examine victims to determine if they are in contact with

energized conductors.

Metal surfaces, objects near the victim or the earth itself may be

energized.

You may become a victim if you touch an energized victim or

conductive surface.

Do not touch the victim or conductive surfaces while they are

energized.

De-energize electrical circuits if at all possible.


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Methods to de-energize:

An extension or power cord probably powers portable electrical

equipment.

Unplug portable electrical equipment to remove power.

Open a disconnecting device or circuit breaker to de-energize

fixed electrical equipment.


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Hazards and solutions:

Be alert for hazards such as stored energy, heated surfaces and fire.

If you cant de-energize the power source use extreme care:

Ensure that your hands and feet are dry.

Wear protective equipment such as low voltage gloves and

overshoes if available.

Stand on a clean dry surface.

Use nonconductive material to remove a victim from the conductor.


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High voltage rescue:

Special training is required for rescues if high voltage is present.

Protective equipment such as high voltage gloves and

overshoes must be worn.

Special insulated tools should be used


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Insulated tools:

Insulated tools, with high voltage ratings, are a lifesaver!

Use devices such as hot sticks(insulated pole of fibreglass) or

(high quality fibreglass) shotgun sticks to remove a victim from

energized conductors.

In some cases, nonconductive rope or cord may be used to

remove a victim from a conductor.


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Rescuing the victim:

Stand on a dry rubber blanket or other insulating material if

possible.

Do not touch the victim or conductive material near the victim

until the power is off.

Once power is off, examine the victim to determine if they should

be moved.

Give First Aid.


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First Aid:

A victim may require Cardio-Pulmonary Resuscitation (CPR).

If the victim is breathing and has a heartbeat, give first aid for injuries

and treat for shock.

Ensure the victim gets medical care as soon as possible.

Provide medical personnel with information on voltage level, shock

duration & entry/exit points.

The treating/attending physician must have detailed specific information

to properly diagnose and care for the victim.

The physician must determine whether the victim should be sent to a

Trauma or Burn Center.


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Stay with the victim until helparrives.


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REMEMBER!

The results of a mistake with electrical energy occur at

the speed of light. There is not time to react after the

error is made. You must think ahead.

Pre-job briefs, planned work instructions, and facility

requirements are not optional. Pay attention and obey

all the rules, not just the ones that are convenient. They

provide the edge you need to be safe with electrical

energy.

Post-job reviews help keep us from repeating errors

participate in them.

You are responsible for your safety.


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Dirt and Corrosion

Lubrication

Heat, Noise and Vibration

Windings and Winding Insulation

Brushes and commutators

Maintenance of Machine


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Wipe brush, vacuum or blow accumulated dirt from the frame and air passages of

the motor. Dirty motors run hot when thick dirt insulates the frame and clogged

passages reduce cooling air flow. Heat reduces insulation life and eventually

causes motor failure.

Feel for air being discharged from the cooling air ports. If the flow is weak or

unsteady, internal air passages are probably clogged. Remove the motor from

service and clean.

Check for signs of corrosion. Serious corrosion may indicate internal deterioration

and/or a need for external repainting. Schedule the removal of the motor from

service for complete inspection and possible rebuilding.

In wet or corrosive environments, open the conduit box and check for deteriorating

insulation or corroded terminals. Repair as needed.

Dirt and Corrosion


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LubricationLubricate the bearings only when scheduled or if they are noisy or running hot. DoNOT over-lubricate. Excessive grease and oil creates dirt and can damagebearings.

Heat, Noise and VibrationFeel the motor frame and bearings for excessive heat or vibration. Listen forabnormal noise. All indicate a possible system failure. Promptly identify andeliminate the source of the heat, noise or vibration

Winding InsulationWhen records indicate a tendency toward periodic winding failures in theapplication, check the condition of the insulation with an insulation resistance test.Such testing is especially important for motors operated in wet or corrosiveatmospheres or in high ambient temperatures.


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Brushes and Commutators (DC Motors)

1. Observe the brushes while the motor is running.The brushes must ride on the commutator

smoothly with little or no sparking and no brush

noise (chatter).

2. Stop the motor. Be certain that:a. The brushes move freely in the holder and the spring tension on each brush is about

equal. Every brush has a polished surface over the entire working face indicating good

seating.

b. The commutator is clean, smooth and has a polished brown surface where the brushes

ride.

c. There is no grooving of the commutator (small grooves around the circumference of the

commutator). If there is grooving, remove the motor from service immediately as this is a

symptomatic indication of a very serious problem.


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3. Replace the brushes if there is any chance they will not last until the next

inspection date.

4. If accumulating, clean foreign material from the grooves between the

commutator bars and from the brush holders and posts.

5. Brush sparking, chatter, excessive wear or chipping, and a dirty or rough

commutator indicate motor problems requiring prompt service.


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Brushes and Collector Rings(Synchronous Motors)

Black spots on the collector rings must be removed by rubbing lightly with fine

sandpaper. If not removed, these spots cause pitting that requires regrinding the

rings.

An imprint of the brush, signs of arcing or uneven wear indicate the need to

remove the motor from service and repair or replace the rings.


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Modern motor designs usually provide a generous supply of lubricant in tight

bearing housings. Lubrication on a scheduled basis, in conformance with the

manufacturer's recommendations, provides optimum bearing life.

Bearing Lubrication


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HEAT

Excessive heat is both a cause of motor failure and a sign of other motor

problems.

The primary damage caused by excess heat is to increase the aging rate of

the insulation. Heat beyond the insulation's rating shortens winding life. After

overheating, a motor may run satisfactorily but its useful life will be shorter.

For maximum motor life, the cause of overheating should be identified and

eliminated.


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NOISE AND VIBRATION

Noise indicates motor problems but ordinarily does not cause damage. Noise,

however, is usually accompanied by vibration.

Vibration can cause damage in several ways. It tends to shake windings loose

and mechanically damages insulation by cracking, flaking or abrading the

material.


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Except for expensive, high horsepower motors, routine inspections generally do

not involve opening the motor to inspect the windings. Therefore, long motor life

requires selection of the proper enclosure to protect the windings from

excessive dirt, abrasives, moisture, oil and chemicals.

When the need is indicated by severe operating conditions or a history of

winding failures, routine testing can identify deteriorating insulation

WINDlNGSCare of Windings and Insulation


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Maintenance

Checklist to maintain motor efficiency

Inspect motors regularly for wear, dirt/dust

Checking motor loads for over/under loading

Lubricate appropriately

Check alignment of motor and equipment

Ensure supply wiring and terminal box and properly

sized and installed

earthing,safety precaution and maintenance

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