Earthing,Safety Precaution and Maintenance
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Transcript of 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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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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Electrical Rescue Techniques
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