Electrical Hazard4

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Electrical Hazard


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There are, however, certain hazards connected

with the installation, maintenance, and use ofelectric wirings and generating equipments.

Unless precautions are observed other hazards

may crop-up offsetting those minimized or

removed by the use of electric equipment.


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Hazards from Electricity fall into

General Categories:

1.To human by:

b)

Electric shockc) Direct burns

d) Secondary injury from non-lethal shocks

2. To property from:g) Fires

h) Explosion


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Hazards to human

The amount of tissue damage or organ malfunction causedby electricity is proportional to the current that flows

through the body measured in amperes. The oldsaying: It is the amps that kill, not the volts is still avalid and tragic truism. There are however other factorswhich can cause varying damage or injury. There are:

2. Voltage - the higher the more fatal

3. Resistance of skin or internal organs

4. Type of current - alternating or direct current

5. Path of the current through the body

6. Duration of the current flow

7. Areas of contact


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It is not possible to specify values of voltage or

current w/c may be lethal due to so manyfactors, but estimates have been made. That if a

current of about 100 amps passes through the

body for 1-2 seconds, the shock will be lethal.

For sensitive people values of about 30 mAhave been reported as being fatal. Voltages as

low as 40 V are said to be potentially

hazardous.


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Resistance to current flow is important in electrical safety.

The skin is a poor insulator but does afford some

defense against electric shocks. Laboratory test

showed that the pain of the hand can have a resistance

of about 45,000 ohms cm. This, however, can

dramatically drop to about 1000 ohms cm when the skin

is wet, w/c indicates danger of touching electricalswitches or equipments when the hand is perspiring. It

was further found that the skin gives more protection

from direct current than from alternating current, but

both can cause severe injury.


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Alternating current can cause tetany in voluntary

muscles become locked in a contracted state.This occurs between frequencies of 15 to 150

Hz. With lines operating at about 110 to 230

volts, AC, at 50 to 60 Hz, the hazards is always

present. Accidents generally occurs withalternating current, but this is because most

lines used are alternating currents.


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The current generally follows the shortest route

from the point of entry to exit. The mosthazardous route is across the chest, where

the respiratory and hearth organs are

vulnerable targets. This condition happens if

the hands is the point of entry and exit for thecurrent. If it is necessary to work in live

equipment, it is considered wise to keep one

hand in a laboratory coat pocket.


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There are 3 primary mechanism by w/c

electrocution can occur. They are:

1. Respiratory arrest

2. Ventricular fibrillation

3. Asphyxiation


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Ventricular fibrillation

Is a condition of the hearth that result in its loss

of ability to act as a pump blood. The majorityof death from electric shock occurs from this

condition. A current of about 50 mA for 3

seconds can produce ventricular fibrillation


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Asphyxiation

Its take place when the diaphragm musclesare in state of permanent contraction (tetany)by a persistency current resulting in lungfailure. This condition can be produced by acurrent of a about 20 mA from the supplyline. Moreover, if the hands touch a live wire,

contraction of the finger muscles can lock thefingers into the wires and can not let go thewire. the let go thresholds is about 6-9 mA.


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Severity of direct burns from an electric current is

dependent upon the voltage and current density.

High voltages of around 30,000 V are able toproduce spark in air across a gap of about 1 cm.

once sparking occurs, ionization of the air enables

an arc to form, which can extend to several

centimeters in length and be sustained by much

lower voltage. Temperatures of up to 4000 degree

Celsius can be produced w/c can cause the flesh

to evaporate and bone to melt.


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High temperatures can be produced when a

large current flows through a small area ofcontact. The resulting high current density

converts electrical energy to heat energy

causing burning. This type of injury can even

be obtained at low voltages.


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Static Electricity

When electrons are displaced in the surface ofa material, static electricity is produced. Italso produced due to friction when twosurfaces move over each other or whensolids or liquids flow through pipes andgasses flow through an orifice. For instance

hydrogen gas flowing through a smallopening can produce sufficient static chargeto cause self-ignition in air.


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Practical Guides

All electrical equipment and devices must be

carefully selected, installed and maintainedto minimized hazards. When this is ignored

various accidents like: Electric shock and

equipment failure are likely to occur, w/c may

lead to fire and explosion in the presence offlammable materials like gasses, and liquids.


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The rating of all equipment is in watts or Hp, w/c

indicates the quantity of energy available fromthem. For instance, a 1000 watts soldering iron

will produce more heat energy than a 700

watts. Wattage is a function of voltage, ampere

and resistance, expressed as follows:Power(watts)=I2R =V2/R =V x I =volts x amperes

Where: Volts(V) =I x R and Amperes(I)= V/R


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Cable and Plugs

The most common way of connecting electrical

devices and apparatus to the line is by theuse of plugs. As such they must be wired

properly. Plugs are rated by the amount of

current that they can safely carry. As much a

15 amperes plug must not be made to carrymore than 15 amperes.


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Guides for Connecting Plugs

1. Make the connection as follows to figure 7.1

2. Make sure all screw are tight.

3. He cord must be securely held by a grip.4. Never use a 2 pin plugs to connect a 3 wire cord.

5. Do not use the earth terminal when connecting a 2 wirecord to a 3 pin plugs.

6. When appliances has a metal case, always use a 3 wirecord and a 3 pin pugs unless appliance is doubleinsulated.

Never use silver paper as a substitutes of proper fuse. Thiswill permit more current than is necessary, causing

overheating of lines, burning of the wiring insulation andpossibly fire.


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Maintenance and Repair Work

Maintenance and repair work should be done only

by qualified engineers or technicians. When repair

work is being done on motors, or the machinerythey drive, these should be deenergized by

opening the necessary switches and locking them

in the open position. If a switch cannot be locked

open, it should be blocked and a tag attached

showing that the switch is to be closed only by the

man whose name appears on the tag. Warming

sign should be placed within the repair area.


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Wiring

The type of wiring depends upon the type of

building construction, the size and distribution of

electrical load, exposure to dampness orcorrosive vapors, location of equipment, and

other factors. In many plants, rigid metal conduit,

effectively grounded, is satisfactory. Other

wirings w/c may be used in other electricalinstallation includes armored cable, non-metallic

sheathed cable, flexible metal conduit, raceway,

open wiring on insulator, & concealed knob and

tube wirin .


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Over-Current Devices

Over-current devices like fuses, and circuits

breaker, should be installed in every circuits.protection of this kind both for personnel and

equipment is important. These devices open

the circuit automatically if excessive current

flows due to accident ground, short circuit, oroverload.


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These types of fusesLink fuses - a strip of fusible metal between the terminals of a fuse

block. it may scatter hot metal when it blows.

Expulsion fuses for use in central stations, power houses or on

overhead lines. They are so designed that when they blow thegases generated aid in quenching the arc.

Plug fuses are for circuits w/c do not exceed 30 amperes at not morethan 150 volts to ground. The type w/c cannot be bridged inside theholder is recommended.

Cartridge fuses- fusible metal strip enclosed in fiber tubes. Those w/cindicates when the fuse is blown & the refillable types in w/c fusibleelements may be replaced are available.

Circuit breaker- those for high voltage or large current capacitycircuits, & are becoming more common in many kinds of circuits.They may be instantaneous in operation, equipped with timing

devices, manual, or power operated.


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Switches

Switches comes in varied forms & design from

toggle, push-button to snap switches. Mostswitches are flush with the wall with all live parts

enclosed. When open wiring is used, surface

switches are often used. In this case switches

should preferably be mounted on porcelainbase. The wiring must be enclosed in wooden

or metal moulding where they are accessible.


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Cords and lamp sockets

All main & extension cords of electrical devices

should conform to the requirements of theNational Electrical Code. They must be properly

insulated. And when available select one with

double insulation. As a rule kinks should be

avoided to prevent breakage of the insulationand for long life. Lamp cords, should, moreover,

not be exposed to moisture or damp floors.


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Socket must be good quality for long life.

Porcelain sockets are recommendable for fixedinstallation but non-conducting plastic or rubber

covered sockets are more appropriate for

outdoors and uses at different locations. Metals

sockets when ungrounded are inadvisable touse.


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Electricians Equipment

Servicemen of electrical installation and equipment use

varied kinds of tools and devices. These devices may

be for testing, measuring, operating or repairing. Anumber of these tools are standard while others are of

special design for a specific purpose. Tools used for

energized equipment must be insulated to minimized

short circuits and the danger of shock to theservicemen. When operating or working with high

voltage lines and equipment, insulated tools may

afford adequate protection.


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Therefore , electricians and electrical servicemen

must wear personal protective devices like:linemans rubber gloves, rubber coats, rubber

line hose, rubber tools pouches, and plastic

helmets. When a linemans ladder is used to

have access to an overhead job, the ladder mustbe of wood or hard and stiff plastic.


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Electrical Equipment

Todays offices and factories are cleaner, moreorderly, and in many ways, safer because of the

almost universal use of electricity. The flexibilityof electrical power permits installation of motorson individual machines, or in strategic places fordriving groups of machines. This has made it

possible to dispense with shafting, belts andother transmission equipment, bringing greatimprovement in lighting, ventilation, andhousekeeping.


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Certain hazards are, however, connected with theinstallation, maintenance and use of electric

wiring, generating & utilization equipment. Unlesswell known precautions are observed newhazards may offset those removed by use ofelectric equipment.

Elimination of these hazards is not difficult orexpensive. Precautionary measures developedand approved will greatly control most of theelectrical hazards in industry if applied.


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Electric equipment location is important,

transformers, control boards, switches, starting

rheostats, and other apparatus should be

placed where there is the least danger of

accidental contact with energized conductors.

Where operating conditions permit, they shouldbe placed in less congested areas. All exposed

current-carrying parts should be further

protected by enclosure, railings, or special

guards.


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Enclosing Equipment

Transformers, control boards and accessories,

when possible should be placed in special

rooms to allow only authorized persons to have

access to them. If a separate room is not

feasible, enclosures should be built around

equipment having exposed conductors.Enclosures made of metals should be effectively

grounded.


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Barriers may used to prevent accidental contact withelectrical equipment. Frames may be made of woods,

rolled metals shapes, angle iron or pipe. Filler may be ofwood strips, sheet metals, perforated metals, wiremesh, or scatter-proof transparent materials.

Danger signs should be displayed near exposed currentcarrying parts, especially high-voltage installations.

Many standards machine-guarding practices apply toelectric equipment, but there are certain hazardspeculiar to electricity. Particular attention should begiven to the National Safety Code and NationalElectrical Codes.


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Equipment Grounding

Exposed non-current carrying component ofelectrical utilities must be grounded. Grounding

is likewise needed on electrical machineriesinstalled in a wet locations, and when itoperates with any terminals at more than 150volts to ground. Parts to be grounded include

motor frames, cranes, cases of transformersand oil switches, wiring conduit, and metal lampsockets. Frames of all portable w.c operates atmore than 50 volts to ground should be

grounded.


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Electric Motors

All electric motors installed should be of the type

and size required for the load and for conditions

under w/c it must operates. Overloading for long

period, use of non-approved motors in areas

containing flammable vapors of dusts, and

defective wiring should be avoided.


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Winding of motors must be protected from metal

particles, dust, dirt, lint, or others materials w/c

may damage the windings or become ignited. In

areas containing flammable materials, such as

dusts and gases, motors designed for hazardous

locations should be installed. The NationalElectrical Code should be followed.


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Starters, switches, rheostats, fuse panels and other

operating accessories should be enclosed, preferably in

grounded metal cases. Both switch and fuses may beenclosed in a cabinet so arranged that the switch can be

operated without opening the cabinet. The switch is

interlocked so that the fuses are inaccessible until the

switch is opened. Another type of enclosed switchpermits the door of the cabinet to be opened with a key,

even though the switch is closed. It is possible to padlock

the door open or closed, and the switch can be

padlocked in an open position.

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Electrical Hazard4

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