Human Safety with Earthing and shock


Human Safety with Earthing and Shock



In an electrical installation an earthing system or grounding system connects specific parts of that installation with the Earth's conductive surface for safety and functional purposes. The point of reference is the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary considerably among countries, though many follow the recommendations of the International Electrotechnical Commission. Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants.

In addition to electric power systems, other systems may require grounding for safety or function. Tall structures may have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines may use the Earth as one conductor of a circuit, saving the cost of installation of a return wire over a long circuit. Radio antennas may require particular grounding for operation, as well as to control static electricity and provide lightning protection.


Protective earthing


An earth ground connection of the exposed conductive parts of electrical equipment helps protect from electric shock by keeping the exposed conductive surface of connected devices close to earth potential, when a failure of electrical insulation occurs. When a fault occurs, current flows from the power system to earth. The current may be high enough to operate the over current protection fuse or circuit breaker, which will then interrupt the circuit. To ensure the voltage on exposed surfaces is not too high, the impedance (resistance) of the connection to earth must be kept low relative to the normal circuit impedance.

An alternative to protective earthing of exposed surfaces is a design with "double insulation" or other precautions, such that a single failure or highly probable combination of failures cannot result in contact between live circuits and the surface. For example, a hand-held power tool might have an extra system of electrical insulation between internal components and the case of the tool, so that even if the insulation for the motor or switch fails, the tool case is not energized.



Electric shock is the physiological reaction, sensation, or injury caused by electric current passing through the (human) body. It occurs upon contact of a (human) body part with any source of electricity that causes a sufficient current through the skin, muscles, or hair.

Very small currents can be imperceptible. Stronger current passing through the body may make it impossible for a shock victim to let go of an energized object. Still larger currents can cause fibrillation of the heart and damage to tissues. Death caused by an electric shock is called electrocution. Wiring or other metalwork at a hazardous voltage which can constitute a risk of electric shock is called "live," as in "live wire."


 Electric shock is often used to describe an injurious exposure to electricity. Electrical currents can travel through the nervous system and burn out tissue in patches along the way. This can leave bizarre symptoms anywhere on the body and may lead to complex regional pain syndrome.


 Shocks can be caused by direct or indirect contact. Contact with an exposed conductive part under fault conditions is called direct contact. IEC requires certain degrees of ingress protection against direct contact. Indirect contact protections can be achieved by earthed equipotential bonding and automatic disconnection of supply by using Residual Current Devices



The minimum current a human can feel depends on the current type (AC or DC) as well as frequency for AC. A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. At around 10 milliamperes, AC current passing through the arm of a 68-kilogram (150 lb) human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.


 The current may, if it is high enough and is delivered at sufficient voltage, cause tissue damage or fibrillation which can cause cardiac arrest; more than 30 mA of AC (rms, 60 Hz) or 300 – 500 mA of DC at high voltage can cause fibrillation. A sustained electric shock from AC at 120 V, 60 Hz is an especially dangerous source of ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source. However, the potential seriousness of the shock depends on paths through the body that the currents take. If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however. If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs. The protection offered by the skin is lowered by perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.


 If an electrical circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established. This is known as a microshock. Currents of only 10 ยตA can be sufficient to cause fibrillation in this case with a probability of 0.2%.



Signs and symptoms

Burns

Ventricular fibrillation

Neurological effects

Arc-flash hazards


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