safety in electricity

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28/01/2011

HSE COURSES

Safety in Electricity




As a source of power, electricity is

used in all aspects of life. In the

home and at work, electricity provides

the POWER for everything from

domestic appliances to industrial

machines, from office equipment to

electric tools.

When compared to the potential

dangers of machinery and chemical

substances, electrical hazards can be

easily overlooked.

Unfortunately, this neglect often causesserious accidents and loss of property.

Many of these accidents can be prevented

as they are mainly caused by electricity

users who lack basic safety awareness.




Ohm's LawA complete electrical circuit is composed of 3 elements:

Current, Voltage and Resistance.




The Ohm formula applies to the human body

during an electric shock, provided that the

voltage remains constant and there is

Resistance (R) in the human body.

A reduction in resistance in the human body

will lead to higher electrical currents passing

through it, and any injury will be more

serious.

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Hazards associated with electricity

1. Electric shock

2. Electric fire & explosion

3. Electric flash & burns

-Joules burns or electric mark

-Flash burns or arc burns




How shock occurs

By simultaneously touching the phase and neutral conductor

thus completing the circuit through his body.

By touching the phase conductor and standing on the ground

By touching the metallic part that has become live by itself

being in contact with energized wire and standing on the

ground




Electric shock refers to the

electricity passing through the humanbody, affecting the normal function of

the heart, lungs and nervous

system.

Ventricular fibrillationcaused by electricity is the main

reason for death from electric shocks.

Electric shocks may indirectly lead to

accidents, e.g. falling from heights

and bruising due to body trauma etc.




The perception threshold of electric current in humans is about 0.5 mA AC tofrequency 50 ÷ 60 Hz and 2 mA DC, it must also take into account the effectof a particular electricity varies, not only for the intensity, but also for theduration of current flow.

Note that the voltage is not relevant in the effects on human,but we need a minimun voltage to be crossed by the current,

then below about 50 VAC or 120 VDC is not at risk, but theabove does not affect the tension , the effects depend solelyon ampere intensity

With voltage greater than

those specified in the humanbody produces the following

effects:




Amperelevel

Definitions Effects

1-3 mA Perceptionthreshold There are no risks or dangers to health.

3-10 mA electrificationIt produces a tingling sensation more or less strong and cancause reflex movement

10 mAtetany of the

muscles

You have muscles contractions. If the party in power was grabbedwith the hand you can have paralysis of the muscles, making itdifficult for the posting

25 mAdifficulty

breathing

due to the contraction of muscles involved in breathing and theflow of current to the nerve centers that oversee the respiratoryfunction.

25-30 mA asphyxiatetany of the muscles of respiration may be sufficient to causedeath by asphyxiation.

60-75 mA fibrillation

If the current flows through the heart can affect the smooth

operation, causing an irregular and disordered contraction of thecardiac fibers that can lead to death

75 mA- 4 Aventricularfibrillation

usually fatal

Over 4 Asevere burns

heart paralysisDeath




Shock Protection Method

A person can come into contact with live parts, and then suffer the effects ofcurrent passing through direct contact or indirect contact.

So to avoid this must implement countermeasures imposed by the existing rule

The protection against direct contacts are realized by preventing accidental contactwith live parts:

insulation of live parts with non-removable insulating material,

covers or barriers that prevent contact with live parts,

obstacles or spacers,

High sensitivity circuit breakers with residual current threshold is ≤ 30 mA




Shock Protection Method

Protection against indirect contact is made in the following way:Neutral Earthing (System Grounding):

Neutral point is earthed – connected to ground

If neutral point is earthed ,the phase to ground voltages under earth fault

conditions do no arise to high value.

Equipment Earthing:

Connecting to earth the non-current carrying metal parts of equipment is

equipment earthing. The potential of earthed body does not reach to dangerously

high value since it is connected to earth.

The equipment Earthing is the essential safety measure




Earthing bring electricity tothe ground




Remember the word POWER in the first slide?In very simple terms we can define Power as “the measure of the

force that we can apply when we do a job”, trough this formula:

Power (Watt) = V (Voltage) x I (Ampere)x cos f(where cos f value is 0.9 ) than, we can also say that

I (Ampere) = (Power(Watt) / V (Voltage))xcos f




A non professional power tools like drill can have

1200W of power, then with the voltage to 220 V

the value of current consumption is

(1200Watt / 220Volt)x cos f = 4,9 A

What does it means ?




Means that in a simple drill we have a current

level that exceed 80 times the threshold of

fibrillation !!!

Amperelevel

Definitions Effects

60-75 mA fibrillationIf the current flows through the heart can affect the smoothoperation, causing an irregular and disordered contraction ofthe cardiac fibers that can lead to death

75 mA- 4 Aventricularfibrillation

usually fatal

Over 4 A

severeburnsheart

paralysis

Death




With the same formula I = (P/V) x0,9 applied to a standard lamp of 100 W we get

that : I = 0.45 A.Is a low value but still good enough to kill a man




Use ladder made of wood ornon-conductive material

Electrical-based work must not be

carried out near flammable substanceespecially electric arc welding




To avoid tripping cables should not

be on the ground

Inspect electrical tools before use.

Stop using unsafe tools

S f i El i i




S f t i El t i it




Before carrying out highrisk or large-scalemachinery maintenance,

the main switch should be

turned off. The powersource must be locked and

labeled with warning sign,so as to avoid any

confusion whenreconnecting the power



Report all accidents, incidents and

near miss to your Supervisor evenif no injury was caused.

They will make inquiries and takeaction to prevent it happening to

someone else.

Incident ,accident and near miss

reporting


safety in electricity

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