Electricity has very much importance in human life. To run an electric appliance or electronic circuit we need the help of electricity either in the form of AC or DC. Even though electricity is useful to human life, careless handling can cause lethal effects. Electrical safety measures are necessary while dealing with high voltage AC available from the power grid. Ignorance can lead to a fatal shock or fire hazards. Let us see how electricity poses lethal effects on the human body and the safety measures to prevent it. Remember, Electricity travels at the speed of light. Its flow is around 299,460 kilo meters per second so you will not get time to react because you can’t move faster than electricity. So the best way is staying out of its way.
As you know, electricity is the flow of electrons through a conductor from its source to the destination.
If the conductor offers less resistance, the flow current will be fast and if the conductor has some resistance, the flow of electrons will be obstructed leading to slow movements which generate heat through friction. Human skin is a semiconductor for electricity and current passes through the body to the ground just like the flow of current through the electric wire. This flow of current causes burning and shock. If the skin resistance is low, the chance of shock hazard is very high.
Effect of current on the body
Human body passes current to the ground just like other conductors. The amount of current passing through the body is equal to the voltage applied between two points of the body divided by the resistance between the points. If the resistance is more, less electrons pass between the points. The resistance of the human body is not a fixed value and it varies from person to person and from time to time depending on the physiological states. Body offers less resistance in high emotional state and high resistance in calm state. Body resistance to electricity also depends on the fat content and the quantity of ions especially that of sodium chloride in the blood. Sweating increases the electrical conductivity since sweat contains salts. Approximately 1 Meg ohm resistance appears across the body between two hands. This may drops to few Kilo ohms in various physiological conditions.
Let us see how current becomes harmful to Human body. Individual body chemistry has significant impact on the effect of current on the body. Some people are more sensitive to current while others are less sensitive. The table below indicates the effect of AC and DC on the human body.
AC DC Effect
0.3-0.4 mA 0.6-1 mA Slight sensation
0.7-1 mA 40-60 mA Pain feeling
10-15 mA 50-70 mA Voluntary contraction. Cannot withdraw hands
15 – 20 mA 60-100 mA Difficulty in breathing. Pain
100 mA 500 mA Heart fibrillation
It is important to note that a low AC current of 15-20 mA across the chest can induce heart fibrillation in some physiological conditions. Ohms law can be used to determine the amount of voltage required to pass 20 mA current through the body. I x R = V.
If the skin resistance of the dry body is 1 M and current is 20 mA then 20 mA x 1 Meg ohms is 20, 000 volts or 20kV.The condition worsens if the body is wet. Skin resistance drops to 20 K or less when the body wet. In this state the voltage required to cause 20 mA current to pass through the body is
20mA x 20K = 400 volts.
If the body has metal objects or in contact with a metal object, the skin resistance drops to 1 K or less. In this condition only 20 volt is sufficient to pass 20 mA current through the chest. A current around 17-20 mA is enough to cause heart fibrillation. In short, the effect of current worsens if the body is wet or in contact with a metal object.
Role of AC
Alternating Current is used in domestic supply with an alternating cycles of 50Hz or 60Hz. Low frequency AC is more dangerous than high frequency AC and the DC of same voltage and current. Low frequency AC causes muscle contraction and tetany which can cause freezing of hand on the conductor. DC on the other hand, causes a single contraction of muscle causing jerking reflex. That is why pulsating DC is used in Defibrillator to restore the heart function if fibrillation occurs.
Effects of AC on the Human body
- Effect on the Skin
Human skin act like a conductor and offers resistance to oppose the flow of electrons. This opposition effect causes dissipation of energy leading to the generation of heat. Electricity has the ability to burn skin and underlying tissues and organs.
- Effect on the Nervous system
Nervous system is composed on electrically active nerve cells which carry minute current and voltage generated from the cells itself to conduct impulses. If an electric current of sufficient magnitude passes through the nerves, it overrides the small signals of the nerves leading to the involuntary contractions of the muscle.
- Effect on the Muscle
Shock induced muscle tetany is the most important effect of current on the muscles. If a person touches the energized conductor, forearm muscles contracts forcefully and fingers bend. So the person involuntarily holds the conductor. So that the victim will be unable to take the hand from the conductor. Shock induced tetanus can be removed only if the current flowing through the victim is stopped.
- Effect on the internal organs
Internal organs such as Diaphragm, Lungs, Heart etc are also can be frozen due to the flow of current. Human heart is most susceptible to such current induced freezing. Heart has conducting fibers called SA node (Sinuatrial Node) and AV (Auriculoventricular) node for initiating heart beats by producing electric signals. These specialized tissues are called Pacemakers. When external current passes through the heart, the normal functioning of the pace makers will be affected leading to heart fibrillation and ineffective pumping of heart. External current can cause sudden contraction of lungs leading to Asphyxiation.
Effect of DC
High voltage DC is also dangerous if it exceeds 30 volts. Even the DC from the car battery can cause a mild shock if the engine is running. Telephone line uses 48 volts DC in standby which rises above 100 volts when the phone rings. Touching the wire with fingers can cause mild shocks. A Car battery can cause shock hazards or fire. Do not try to remove the battery terminals or spark plugs if the engine is on. Very high voltage generates when the engine runs. Do not keep any metal objects over the battery. Accidental shorting of terminals can cause fire.
Here are some possible sources of shock hazard and safety measures to prevent it.
Touching electrical appliances with wet hands or feet especially in sweating condition is dangerous. The body is more prone to shock when it is wet. If there is more sweat on the hands, current passes more freely since the salt in the sweat increases the conductivity of skin. Bathroom is one of the more likely places where wet people may contact with switches, water heaters etc. Switches, power receptacles, telephone sockets etc should not be installed inside the bathroom. Water heater should not be placed close to water taps and tubs. Telephone plugged into socket is also a source of shock even though it uses DC. Open circuit voltage of telephone line is 48 volts and the ringing voltage is 150 volts DC. Remember an excess of 30 volts AC or DC can cause a shock.
2. Extension cord
Most of us use extension cords to power more than one device. Since it powers many gadgets, make sure that its current handling capacity, gauge of wire, strength of contacts etc are suitable for powering many gadgets. Otherwise, due to overloading, contacts and wires heat up and insulation may break . The abberated or cracked wires pose potential dangers. If a cracked point is found, unplug the extension cord and cut the broken point, rejoin and make shock free with insulation tape.
3. Power cords
Power cords used to connect appliances like Mixer, Grinder, TV etc are another source of shock. Now most of the electric appliances use molded cords. If the quality of the insulation cover of the cord is not good, cracks may develop near the plug since the cord bends several times while plugging. Through the cracked insulation, copper wires may expose so that when we plug in the device, there is chance for touching with the cracked point. Remember, a single copper wire can discharge heavy electricity into the body. Check the point of cord joins with the plug. If there is any insulation crack, use insulation tape there or replace the power cord.
Another possible source is TV and cable. The TV power cord and the Cable inlet are placed nearby on the back side of the TV. Close observation is not possible while inserting the antenna cable into the receptacle. Always unplug the TV while inserting the balloon of antenna into the receptacle. The antenna cable also becomes a source of shock in rainy seasons. If the antenna cable is in contact with trees, electric wires, earth pipes etc during rainy season, there is chance for passing electricity through the cable. So do not try to repair the antenna cable during rainy season without adequate precautions.
Some safety measures
It is better to avoid conditions that may leads to shock hazard. Adequate safety measures should be adopted white repairing, testing or troubleshooting the electric appliances. The following tips will help you to avoid electric shock.
- Always unplug the electrical appliances before opening its cover.
- Wear rubber foot wears while doing electrical works. Rubber is safer than leather since leather holds some moisture in it.
- Check the presence of current in all bare points using a tester before unscrewing the wire contacts.
- Use a rubber sheet on the floor below the work bench so that the feet will rest on it while doing work.
- Wear rubber gloves before working with high volt AC.
- Hand-to-hand current path is more dangerous since current passes through the chest and affects heart. The best way to prevent this is to keep the left hand behind the body or insert into the pocket so as prevent accidental holding of left hand with a metal object while the right hand is in use.
- While installing television antenna or dish antenna, make sure that it is not close to any electric lines or wires.
- Close all the unused power sockets with Safety caps to prevent shock hazards. Children may insert metal objects into the sockets while playing.
- Always use switch for controlling the plug sockets. Use 3 pin plugs with correct wiring.
- Make sure that the switch is provided in the phase line. Installing switch through the Neutral line has no use. If body of the appliance makes accidental contact with the phase line, body becomes charged even if the switch in the off position.
- DO NOT use multi-way adaptors. Over loading can cause fire. One socket outlet is for one appliance only.
- Install MCB, ELCB etc. in the electrical circuits with proper earthing. Earth pipe should be piled deep into the soil to discharge electricity.
- Unplug all the appliances if they used occasionally.
- ENSURE the switch is in “OFF” position before changing bulbs.
- DO NOT drive nails carelessly on the wall. There may be concealed wiring inside.
- Avoid using handheld appliances when your hand and/or body is wet.
- Check accessories such as plugs attached to appliances for cracks and burnt marks and have them replaced. If undue overheating occurs or burnt marks appear in any electrical appliance, have it checked and serviced.
- Switch off and take the Immersible water heater before using the water.
- Take care while using Washing Machine. Switch off or unplug the machine before removing wet cloths. Use a dry rubber sheet in front of the washing machine to stand on it.
- Never use Mixer near Wash tub in the kitchen.
- Switch off or Unplug Wet Grinder before removing the contents. Do not touch the metal parts while it is operating. Before adding water, switch off the grinder.
- Inside the TV there very high voltage especially in the HT section. So do not touch any parts near the HT section while trouble shooting.
- Unplug the inverter system and switch off its output while removing or cleaning the battery terminals.
- Do not touch the electrical appliances when there is lightning. If possible, unplug all the electrical appliances when the thunder strikes start. Remember, during lightning, very high volt around 5000-50000 volts may pass through both live and neutral lines. Fuse protection is available only in the phase line so when the neutral charges, current passes into the device leading to its permanent damage. This kind of lighting breakdown is common in TV since most people switch off the TV through remote leaving the power cord charged.
- Do not park vehicles below high tension power lines such as 110KV for long time. Vehicle battery may discharge deeply since the metal body of the vehicle, may charge through induction.