Sunday, April 10, 2011

Electric shock

 Instrument Transformer and Power Management (P1) Course
 Chapter (2) : Safety Basic
2.5 Electric shock
       The electrical shock hazards is the other main concern while working in a substation. A person will receive a shock whenever there is a potential difference across their body.
       This potential difference; , or voltage can be developed as a result of direct contact with a live piece of equipment, getting to close to piece of live equipment, the flow of fault current, current induced a magnetic coupling or current induced by cap skydiver coupling.


2.5.1 The physiological effects of electric current :
       A human body has resistance generally in the range of 1,000-10,000 ohms, so when a voltage is impressed across the body, current will flow through it.
       Depending on the magnitude of this current there can be several effects. The following chart shows a range of current levels and the results that can typically be expected.
Effect of electrical current on the body
       From one to milliamperes, the average person usually begin to feel a slight "tingle" or 'mild sensation". Any amount of current over ten mA is capable of producing a painful to sever shock.
        As the current is increased the shock will become increasingly sever.
       When current flows through the body, the muscles in its path will contract, this contraction may be so strong  that the victim is not able to release his hold in the live circuit.
       The fact that the person can not let go is important because in a few seconds blisters will form in the skin at the contact points, reducing the skin resistance further and causing higher currents to flow through the body.
       As current increase, the victim will begin to have difficulty breathing. Current as low as 20 mA can cause difficulties with breathing, while current in the range of 75 mA  have been known to stop it altogether.
       The human body uses small electrical signals to control its muscles. The most important muscle in the human body is the heart, as it is responsible for circulating all the blood. When current s reaches about 100 m A the control signals to the heart are overridden and the heart can no longer be controlled by the body. An uncontrolled twitching or vibration of the heart occurs at the power line frequency which is too high for the heart to operate at. As a result very little blood is bumped, and death can occur quickly.
       At current, breathing generally stops completely and the body actually begins to burn. The points of contacts  will show obvious signs of burring, while less obvious will be internal damage.
       Currents always flows through the path of least resistance and your internal organs have very resistance (due to the electrolytes that are in the body )
       Victims of sever electrical shock often do not survive because of the the damage to their internal rgans.

2.5.2 Preventive process :
       In order to prevent the damage due to electrical shock in substation ;
  • We must raise the resistance of the worker to minimize the current path through him in case of receiving a shock by ;
  1. Wearing safety shoes which have high resistance, raising the total resistance of the working personal
  2. Using gravel under and around the equipment to isolate the worker from the ground, and raise the resistance of current path
  • We must make a second low resistance path for the current in parallel with personal current path. This is done by using low grounding resistance connections at each metallic structure that can carry a voltage.
2..6 Hazards due to flashover :
       Every substance is to some degree a conductor of electricity. Metals are good conductors, insulators are very poor conductors. Under the stress of an electrical potential, some poor conductors can take on changes that make them good or at least better conductors. Air is a good example of this. Under normal conditions, dry air is a fairly good insulator, but when it is sufficiently stressed by an electrical potential, it becomes "Ionized" and turns into a very good conductor.
        Electrical systems are designed so that, there are normally ample clearance between live parts and the ground with air used as an insulator.
       If a person brings the ground plane closer to the live equipment, with tools or parts of their body, the air will be placed under a greater stress, and will eventually break down. This will cause the air to ionize, providing a path for current to flow and resulting in an electrical arc, commonly called a flashover.

2.6.1 Preventive process : 
       To ensure that employees remain safe from hazards due to flash over many electrical authorities have issued what are called ' safe limits of approach ' in their safety rules.

 Typical limits are as follows :

Personal with special training
General public
Operating voltage
1M
3M
750 V     to     15 KV
1.5M
3M
15 KV     to     50 KV
2M
3M
50 KV     to     150 KV
2.5M
3M
150 KV     to     250 KV
4M
3M
250 KV      to     550 KV

       This chart does not list safe limits for voltage less than 750 V as these voltages do not generally present the same flashover. This does not mean that one should work on low voltage equipment without taking precautions, however, as their is always the chance of sever shock even at these low levels.

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