How does one get Electric Shock sensation?

Most people have experienced electric shock some time, and are aware of shock hazards. It is seen that a person senses shock differently at different times or different conditions. Electrical installations and equipment are provided with earth connection (earthing) or extra heavy insulation to protect user from coming in contact with live components, or to see that one does not get shock even if insulation of equipment fails and leakage makes its body dangerous to touch.

A shock sensation is felt whenever a current passes through some part of our body. Our body acts as a resistance whenever a voltage appears across two points on body, thereby allowing current to pass through. This current interferes with our body nervous system, and a shock sensation is felt. The interference is often enough to affect the signal system between different parts of our body, and feeling could be a slight tingling sensation to severe shock, depending on the current through the body and the parts carrying the current.

Electrical Resistance of Body

Electrical resistance of body depends on points of measurement on body, and it defers significantly with person. Measurements could be between arms, feet, hand to foot etc. Resistance is not a fixed entity, and varies with time of day, wetness, condition of body as also area of contact. Sweat is rich in salts, and causes drop in resistance. An open wound also has similar effect.

A dry skin resistance of body may be as high as 100,000 Ω, while in wet condition after bath it may drop to as low as 1000 Ω, as per National Institute for Occupational Safety and Health (NIOSH), U.S.A. It is also known that response to a current passing through body drops this resistance further, as low as 500 Ω.

Body resistance between two reference points on body varies with applied voltage. This happens because of interference of body natural reaction to these voltages. International Electrotechnical Commission (IEC) carried out a survey of population and has given average values of resistance for 95% of population at different voltages. It mentions value of resistance between extremes of hands in dry body condition at 25 V as 6100 Ω, which goes down successively to 3200 Ω at 100 V, 2125 Ω at 220 V, and 1500 Ω at 1000 Ω. These resistances will drop further greatly under wet conditions, and also will vary with time.

Electric shock

Electric shock can be seen as millions of electrons passing through our body when in contact with a voltage source. An electric shock is a painful and often dangerous physiological effect caused by passing of electric current through the body of human or animal. The shock could be mildly unpleasant phenomena, or highly dangerous life-threatening emergency. Shock is an experience of electric current passing through the body. It happens because the current interferes electric signals between different parts of body for its normal functioning. This interference differs with magnitude of current, as also parts of body it passes through.

A current up to 200 μA will not be sensed by human (this is threshold of zero sensation), beyond which a person starts sensing it. A voltage of about 30 V may be considered a threshold voltage for sensation of shock. Touching 12 V battery terminals will not make any sensation, while a 48 V battery terminals may give us a mild sensation. As current in body increases, one is able to hold on to the live wire till a certain level. At about 1 mA, one is not able to hold on, and body reflexes will let go the live wire voluntarily.  Maximum current a man may be able to tolerate for a short time may be a maximum of 10 mA for any person. One may hold on to high voltage terminals of 3 KV or even more safely if the current in body is limited far below 1 mA. Many H.V. testers are designed with this safety.

Current becomes more dangerous as it increases, and at some level, it can be fatal. One-tenth of an ampere (100 mA) passing through body for one second could be fatal. Currents above 10 mA can ‘freeze’ or paralyze muscles. The tighter the live part is held, more is the shock severity. This is why hand tools that give a shock are more dangerous. If one cannot let go of the live part, this can lead to injury, and cause breathing to stop (respiratory paralysis) at about 30 mA. Current greater than 75 mA can cause ventricular fibrillation (very rapid ineffective heartbeats), leading to death. Heart paralysis occurs at 4 Amp., meaning it stops pumping. Tissues may burn at a level of 4-5 A. A current applied for 100 mA for 3 seconds is as dangerous as 900 mA for 0.03 seconds. Even lower voltages could be dangerous if applied for long periods of time.

Protection from shock

Most devices and instruments are provided with an earth terminal, connected to the outer metal surface, and wall sockets are provided with corresponding thick earth receptors connected to supply or local earth. It is mandatory all over the world to have such protection for safety. In some European countries, home sockets are nor provided with earth terminal, but the equipment and devices are required to have double insulation of high level to avoid any possibility of leakage of current, or accidental contact with any live part. Most wall sockets nowadays have recessed contacts, with provision to access only while insertion of plug-pins of devices.

All wiring, industrial or domestic, are by law provided earth protection to guard against accidental shock, and guidelines are issued while handling equipment or installation to avoid any untoward shock hazard.  All equipment, as also devices like soldering irons etc., are always provided with dearth terminal. It is for the user to ensure that these safety measures are properly in action and working.

Applications making use of body resistance

Common line tester used by electricians to check for live wire contains a neon lamp in series with a resistor, connected between its metal clip and screwdriver head. When the screwdriver tip is connected to live part, and a finger is placed on clip, if current passes through your body, the neon lights up. The current through human body is mostly 200 micro-amps.

Many high voltage testers are designed to indicate failure when leakage current reaches 1 mA, and these can be handles safely by unskilled personnel. Most instruments are tested for leakage current to ground at 2000 V or higher, and are accepted when the leakage is below 5 mA.

Sensors used to call lift to a floor are often resistance or capacitor type, where a miniscule current passing through our body actuates the sensor and signal goes to lift. A defibrillator helps a heart recover by providing a shock to activate it or ‘reset’ into action.