Work Shop Ppt Shock And Earthing

ELECTRIC SHOCK AND EARTHING Prepared by Shameer N & Sunder

ELECTRIC SHOCK

Electric Shock 

An electric shock can occur upon contact of a human body with any source of voltage high enough to cause sufficient current through the muscles or hair.

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The minimum current a human can feel is thought to be about 1 milliampere (mA).

Effect of Electric shock 

Effect of shock depends on 

Magnitude of the current Voltage  Body Resistance 

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Current path through the body to the earth Duration of contact

Cont.. Current Value

Effects

1mA

No Perception

1 – 5 mA

Sensation of shock, Not painful, individual can still let go

5 – 15mA

Painful shock, Reflex action, individual can still let go

15 – 20mA

Painful shock, Muscular control is lost in the current path, cannot let go

20 – 50mA

Painful muscular contraction leading to light /severe cramp, breathing difficult

50 – 100mA

Paralysis/suffocation

100 – 200mA

Can bear for a very short time. Ventricular fibrillation possible

200 – 500mA

Cardiac arrest possible. Even deaths are encountered

>500mA

Fatal by longer contact Death with internal burns.

Preventing Electrical Shock 

Electrical shock appear to be caused by a combination of three possible factors:   

unsafe equipment and/or installation, workplaces made unsafe by the environment, unsafe work practices

Precautions to be taken against Electric shock 

Avoid working on live electrical parts

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Wear proper insulating boots and gloves as necessary

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Follow Lock-out tag-out procedures as required

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When working on high voltage, make sure that the floor is not made of any conducting material

Cont.. 

Make sure that no part of your body comes in contact with earth, metallic casing, metal plates or cross arms when working on high voltage

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Avoid working at places where your head is liable to touch the live parts or mains

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Never touch an energized person with your bare hands but rather use a wooden stick or other non-conductor to push them away from the source of current

Rules for minimizing injury from Electrical shock  

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Cut the power supply If cutting the electricity is not possible, try to free the victim of electrical shock from contact with the live at once, using a dry stick, dry rope, dry clothing or nonconductor. Do not touch him with your bare hands. If victim having little or no breathing, provide rescue breathing. Never give an unconscious victim any liquids. Efforts to aid a victim should be continued until he revives or is pronounced dead by a physician. Also remember that even if electrical burns look minor at the entry and exit points, there might be invisible internal damage. Consult a doctor immediately after an electric shock.

Rescue Breathing 

Tilt the head back with victim on his back, neck fully extended.

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Elevate victim’s jaw into jutting – out position by inserting thumb between teeth, grasping lower jaw and lifting it forcefully upward.

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If air passage is not yet cleared, clear at once with several sharp blows between shoulder blades.

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Open your mouth wide and cover victims mouth completely by placing your mouth over his with airtight contact, also closing victim’s nose by pinching it between thumb and finger.

Cont.. 

Blow air into victim’s lungs until you see the chest rise (less forcefully for children); remove your mouth and let him exhale. If chest does not rise, check steps above.

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Repeat above step approximately 12 times a minute until victim revives (20 times a minute for children).

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In the case of infants, rescue breathing should be done through both the nose and mouth.

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If the victim's mouth can't opened, rescue breathing should be applied mouth – to – noise ,rather than mouth – to – mouth.

General Safety Instructions 

Only authorized, competent, and qualified persons are allowed to work on or around electrical equipment and/ or wiring.

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The person in-charge shall explain the precautions to be taken by the workman before issuing the permit.

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Purchase up-to-standard electrical equipment equipped with appropriate protective devices

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Proper maintenance of system and equipment

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Exercise care: Place yourself in a safe and secure position to avoid slipping or stumbling. Make a habit of being cautious.

Cont.. 

Think carefully before you act. Make sure you are right.

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Treat everything as live unless it is positively known to be dead and properly earthed. All voltages are dangerous. Even low voltage shocks may be fatal.

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Extra precautions should be taken when working in abnormally damp areas.

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Follow the DO’s and DON’Ts instructions. These should be displayed in all establishments at places of convenience and personnel trained repeatedly.

EARTHING

Earthing 

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Connecting an equipment to the earth so as to ensure at all times an immediate & safe discharge of electric energy due to leakage, faults etc Earth potential is taken as zero for all practical purposes Necessity of Earthing  To provide greater safety to personnel & equipment from the danger of current leakage  To maintain a constant voltage in healthy phases in the event of a fault in one phase  To serve as a return conductor in communication and traction works

Methods of Earthing 1.Plate Earthing 

Size of the plate – 60cm x 60cm & 120cm x 120cm

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Electrode is buried at a depth of at least 1.5m

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Plate material Vs minimum thickness Plate Material

Minimum thickness

Copper

3.15mm

Galvanized iron

6.3mm

Cast iron

12mm

Plate Earthing

2.Pipe or Rod Earthing 

Used in congested areas

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The normal size of the G.I. Pipe used is 2 to 2.75meter long, 38mm diameter. Where rod is used, the length of the rod is between 1.22 to 2.4 m. And the diameter varies between 13 to 19 mm.

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The pipe /rod is driven to a depth of approximately 3m in the ground.

Pipe or Rod Earthing

3.Strip or Conductor Earthing 

Used in places where the topsoil has got lower resistivity but underneath it has very high resistivity like a rock etc.

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An approximate length of the stripe is buried in the top soil at a depth of not less than 0.5m to give reasonably low earth resistance.

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Copper strip if used shall not be less than 25mmx1.6mm in cross section.

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Copper conductor shall not be less than

3mm 2

Strip or Conductor Earthing

Double Earthing  

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If one earth fails ,then second earth will do the purpose. It offer minimum resistance (as in parallel) to the flow of leakage current. The distance between the two electrode should be 5m approximately . If the electrode placed near the wall of the building ,minimum distance from the wall should be 1.5m.

Rules for Earthing 

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In the case of house wiring ,the earthing must be continuous by a single solid conductor of 14 SWG . All the metallic covering of the main switch,DB,brackets etc along with the earth point of the wall socket should be earthed. The metallic cover of all medium voltage machines should be earthed by double earthing. In any case the resistance of the earth continuity conductor should not be greater than 1 Ω throughout the system. In case of ordinary soil the earth resistance of the electrode should not be more than 3Ω and 8 Ω in rocky soil.

References 

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Electrical Estimation and Costing by N Alagappan & S Ekambaram,T T T I Madras. Hazards of Electricity and Static Electricity published by Institution of Chemical Engineers,2005.