SAFETY

Compiled by Er. Bibek Dahal SAFETY RULES AND REGULATIONS 1 Compiled by Er. Bibek Dahal UNIT 1: SAFE USE OF ELECTRICAL COMPONENTS SAFE USE OF ELECTRICAL TOOLS Electrical Safety refers to any safety precautions taken against electricity. Electricity is undoubtedly one of the most basic needs in modern life, but electrical hazards are very dangerous. Some of the serious injuries from include you can receive electricity include electrical shocks, electrical burns, and electrocution, which could be fatal. Electrical Safety precautions can protect against electrical shocks and burns, and authorities responsible for workplace safety ensure that workers are following the correct safety workers are following the correct safety procedures while dealing with electricity. Some Electrical devices pose a higher threat to life because of high Voltage and high electrical shocks. The most common electrocution occurs when a person comes in contact with unsafe equipment or an uninsulated wire thus becoming part of the electric circuit. Safety precautions could be different for the home and the work place, depending on the electrical devices being dealt with. To prevent any unwanted situations, the most basic electrical safety measures to be taken are: 1. Avoid touching any electrical wire without safety gloves, even if the wire is insulated and seems safe. In addition to gloves, your footwear must also be safe. It's best to always assume the electric wire is unsafe to touch. 2. If water or a chemical is spilled into equipment, shut off power of the main switch or circuit breaker and unplug the equipment. 3. Never use metallic pencils or rulers, or wear rings or metal watchbands when working with electrical equipment. 4. When it is necessary to handle equipment that is plugged in, be sure hands are dry and when possible, wear nonconductive gloves, protective clothes and shoes with insulated soles. 5. Never handle electrical equipment when hands, feet or body are wet or perspiring when standing on a wet floor. 6. Avoid touching any electrical wire without safety gloves, even if the wire is insulated and seems safe. In addition to gloves, your footwear must also be safe. It's best to always assume the electric wire is unsafe to touch. 7. Do not store highly flammable liquids near electrical equipment. 8. Never touch another person’s equipment or electrical control devices unless instructed to do so. 9. Enclose all electrical contacts and conductors so that no one can accidentally come into contact with them. 10. Minimize the use of electrical equipment in cold rooms or damp areas. If equipment must be used in such areas, mount the equipment on a wall or vertical panel. 11. Treat all electrical devices as if they are live or energized. 12. Avoid touching any electrical wire without safety gloves, even if the wire is insulated and seems safe. In addition to gloves, your footwear must also be safe. It's best to always assume the electric wire is unsafe to touch. 13. Disconnect the power sources before servicing or repairing electrical equipment. Use only tools and equipment with non-conducting handles when working on electrical devices. 14. Always refer to the safety precaution manual, and check for all the safety equipment before coming in contact with electrical wires or devices. 2 Compiled by Er. Bibek Dahal The safe use of electrical tools includes following points: A. Protect yourself from electricity a. Follow specialization procedures in wet work areas. b. Conduct inspections c. Follow proper grounding methods d. Guard energized parts B. Wet areas If you touch a live wire or other electrical components while standing on even small quantity of water, you will get shock. So to avoid getting electric shocks always use appropriate equipment and as far as possible do not stand in wet areas when operating electrical equipment. C. Inspection Visually inspect all electrical equipment before use. Remove any equipment with cracked tool casing, missing ground point etc. D. Use PPE There are many types of PPE specifically designed and approved for electrical work such as rubber, insulating gloves, blankets, covers, hard hats etc. E. Rescue If your co-worker sustains an electrical injury de-energize the circuit before getting help. Do not try to pull some one off of the electrical circuit. STATIC CHARGE IN HIGH VOLTAGE EQUIPMENT Static means not moving. Static electricity is an electrical static charge, a static charge that doesn't move. All materials are made up of atoms. An atom is the smallest particle of a material that still contains the properties of the material. Each atom consists of a positively charged nucleus around which one or more negative electrons move. In an idle state, the positive charge of the nucleus is equal to the sum of the negative charge of the electrons moving around the same nucleus. Therefore the charge is neutral. If the nucleus loses or gains electrons, an imbalance is caused and this is the answer to the question: What causes static electricity? An atom that has lost one or more electrons has a positive charge. An atom that has gained one or more electrons has a negative charge and is called an ion. There are only two types of charge: positive and negative. Atoms with the same type of charge repel one another, while those with the opposite type of charge attract one another. The difference between parts that have been charged is measured in kV (kilovolt) is called static electricity. That is also what is meant by static electricity. Static electricity is the result of an imbalance between negative and positive charges in an object. Static electricity is measured in kilovolt (kV). These electrical charges can build up on the surface of an object until they find a way to discharge the static energy. This can be done for instance with the aid of Static Eliminators. Production processes, static electricity can often be a severe process disruption, as it means that materials get stuck to machine parts or to each other. Operators do not like getting electric or static shock. The dust 3 Compiled by Er. Bibek Dahal in the surrounding area is attracted by the electric charge. In explosion hazardous zones, static charge can cause a spark, which in turn can cause a fire or even an explosion. Electrical safety tools 4 Compiled by Er. Bibek Dahal Electrical insulation techniques Assignment 5 Compiled by Er. Bibek Dahal Unit 2: Electric Shocks Electric shock introduction  An electric shock occurs when a person comes into contact with an electrical energy source.  Exposure to electrical energy may result in no injury at all or may result in devastating damage or death.  Excitation or disturbance of the function of nerves or muscles caused by the passage of significant amount of electric current through parts of body.  We’ve all felt a buzzing or tingling sensation that does not cause injury. That’s the effect of a current as low as 0.25 mili-amperes (mA) entering the body.  When a current above 10 mA travels through flexor muscles such as the ones in our forearms that close the fingers it causes a sustained contraction. The victim may be unable to let go of the source of the current, making the duration of the contact longer and increasing the severity of the shock.  If a current of 50 mA passes through the heart, it can cause cardiac arrest  When a current above 100mA passes through the body, it leaves marks at the points of contact with the skin.  Currents above 10000 mA (10A) cause serious burns that may require amputation of the affected limb. Possible Damages due to electric shock  Pain at the point of enter and exit of the current  Contraction of muscles pain and in torn muscles and ligaments due to severe contractions.  Loss of muscles control resulting in the inability to take self-protective action such as letting go the source of electricity  Contraction of the chest muscles diaphragm and throat resulting in breathing difficulties  Interference with heart action Reasons behind Electric Shocks  Faulty outlet/ switch When an outlet or switch is faulty or malfunctioning, electrical shock can ensue. Outlets and switches receive their electrical currents through a box, further connected to wiring. If any screw or wiring is loose on the box, wiring or outlets/switch, electricity becomes unstable. This can lead to electrical shock if you plug in an appliance or flip the light switch.  Outdated outlets Outdated outlets usually possess two-prong instead of the now common three. Two- prong outlets possess no ground wire. Ground wire acts as an additional safety barrier in the case of an unstable electrical current.  Faulty Appliances You may receive electrical shock by using faulty appliances when you plug faulty appliances in, the unstable electricity can ruin your appliance as well as give you an electric shock. So always check your appliances before plugging them in. 6 Compiled by Er. Bibek Dahal  Electricity touching water Electricity and water make a dangerous combination as the water’s ions are extremely conductive. This leads to electrical shock, possibly at a more severe level.  Mishandling electricity Incomplete information about the electrical equipment and no knowledge can lead to mishandling electricity which may result to shock. Bio-Physical characteristics of human body against electric shocks When an electric currents flows through any physical objects they absorbs the energy and when the amount of energy absorbed in sufficiently high damage occurs. Effect of electricity to the any living organism/ human body is governed by a combination of inter related physical and biological phenomenon. Like any other physical irritant i.e. heated body, sound, light flash, etc. the electric current produces not only a local effect by damaging the tissues but also creates instantaneous response of the organism Electric shocks effects the blood circulation. The effects of injurious factors will always be at heart activity and consequent death ensues. Typical value and characteristics of human body against Electrical current     The current drawn depends on the resistance of the body. Under dry conditions, the resistance offered by the human body may be high as 100000 ohms. Wet or broken skin may drop the body’s resistance to 1000 ohms. High voltage electrical energy quickly breaks down human skin reducing the human body’s resistance to 5000 ohms.  The resistance of human body noticeably drops with increase in voltage.  The body resistance drops with increase in voltage  The body resistance depends upon duration of impressed voltage dropping with the time. A Smurov (around 1915) and his followers did an experiment on determination of the electrical resistance of the human body. They concluded the two points.  The resistance of the human body noticeably drops with an increase in voltage.  The body resistance depends upon duration of the impressed voltage dropping with the time. Attempt no. 1 Duration of exposure First 15 minutes Applied voltage 2V Body resistance (Ohm) 16x104 2` 3 4 15- 30 minutes 30-45 minutes 45-60 minutes 6V 10V 2V 4x104 8x103 4x104 From above table, first three attempts state that body resistance depends on voltage applied and drops with the increment in the voltage. While from attempt no. 1 and 4 it is clear that the body resistance also varies with duration of voltage exposure. Hence in overall, it can be stated that the body resistance is nonlinear and varies with both voltage and time. 7 Compiled by Er. Bibek Dahal Effects of environmental factors The effect of surrounding medium on the outcome of the electrical injury has been proven both by experiments with animals and by investigation of electrical accidents. These are summarized below:  Elevated temperature is dangerous not only because a worker starts perspiration due to which electrical resistance of his body drops. Apparently any heat increases sensitivity in the electric current of the human body.  The excessive humidity produces nearly the same effect. It is true that the excessive humidity of air decreases the general resistance of the organism to the electric current.  Pressure of surrounding medium also influences the sensitivity to electric current. It has been experimentally shown that a drop in the atmospheric pressure increases the electrical hazards for the living organism.  The increased partial content of oxygen in the air reduces sensitivity of the organism to the electric currents and, on the contrary, the reduced partial content of oxygen increases the sensitivity. Effect of state of organism The effect of electric current also depends on the state of the affected organism. 1. The industrial fatigue occurring by the end of the workday dulls the alertness and not only increases the likely hood of injury but might aggravate its consequences. 2. Any illness leading to the nervous exhaustion will no doubt aggravate the injury. 3. Alcoholic intoxication also produces the same effect. It has been observed that sensitivity of the organism to current effect may be changed by application of different drugs. Safe value of electric current and voltage through human body: Dalziel and others (1972) have done extensive experimental work on animals and observed the results of many accidents with humans and have proposed that the lower limit for “safe” currents for shocks lasting from 5 to 8 milliseconds is given by: I =0.116/√T Where, T is the actual time duration of current in seconds, this is illustrated in graph below; Current is hazardous Shock is felt but harmless 8 Compiled by Er. Bibek Dahal As the resistance of the human body to power frequency currents is highly variable depending on the areas of contact, skin surface dryness and the actual voltage acting. Resistance from hand to hand or hand to foot ranges from few hundred ohms with 2000 volts applied to a few thousands of ohms with 12 volts applied. Experimental evidence indicates that the voltage required to produce various degrees of shock at 50 Hz is as shown in table below Minimum threshold of feeling 10-12V Minimum threshold of pain 15V Minimum threshold of severe pain 20V Minimum threshold of hold -on 20-50V Minimum threshold of death 40-50V Range for ventricular fibrillation 50-2000V Approximate threshold of shock voltages The physiological sensations observed by test subjects for various levels of 50 Hz currents are summarized in table below. The values are currents in mA for the particular sensation observed by 5%, 50% and 95% of subjected tested. 5% 50% Just perceptible in palms 0.7mA 1.2mA Prickle in palms 1.0mA 2.0mA Prickle perceptible in wrists 1.5mA 2.5mA Slight vibration of hands 2.0mA 3.2mA Slight spasm in upper arm 3.2mA 5.2mA Letting go possible 4.2mA 6.2mA Prickle over arm surface 4.3mA 6.6mA Physiological sensation for 50Hz currents (hand-body-hand) 95% 1.7mA 3.0mA 3.5mA 4.4mA 7.2mA 8.2mA 8.9mA First aid for electric shock The effects of electrical energy on the human body may include stoppage of the respiratory and/or cardiac functions, burns or wounds caused by direct contact of an electrical conductor with the body, injuries caused by electric arcs, and injuries incurred in falls or other reactions to electrical shocks. The possibly severe effects of electric shock on the cardiopulmonary system makes it extremely important that emergency personnel well trained in the proper procedures for performing cardiopulmonary resuscitation (CPR) respond to electrical emergencies. The following life-support procedures are recommended for the rescue and care of shock victims. 1. Remove the victim from further danger (move only if necessary) but “DO NOT ENDANGER YOURSELF”. This can be achieved by: a. De-energizing the equipment from the supply switch b. Cutting cable or wire to apparatus using a wooden handle axe. Protect your eyes against any flash. c. Use of a dry stick, dry rope, leather belt, coat, blanket, or any other electrical nonconductor to move the victim away from contact. 2. Ascertain whether the patient is breathing and whether the hearts is still beating. If indications are favorable, keep the patient in a reclining comfortable position and loosen 9 Compiled by Er. Bibek Dahal 3. 4. 5. 6. all clothing about the neck, chest and abdomen. Protect from exposure to cold, covering with blanket. Do not leave the patient unattended. If the patient’s condition persists, keep him moving about. Do not give stimulants or opium derivatives. Send for medical assistance at once. If the victim is not breathing, utilize CPR at once If the heart has ceased beating, have a qualified person administer appropriate first aid immediately. In case of serious electric shock, the time for commencing first aid is the vital importance. If treatment is commenced within the first 3 minutes after the accident, the chance of recovery of the person is enormously increased, compared to the case if first aid is delayed for 5 or more minutes. Therefore, regardless of any other consideration, remember that, when victim has received a severe electrical shock, second count. The sooner you begin first-aid measures, the better the chances of saving the victim. Remember, an electrical current can paralyze the nervous center of the brain which controls breathing. Breathing is stopped. This leads to temporary paralysis. Cardiopulmonary Resuscitation (CPR) In as much as severe electric shock may cause a stoppage of breathing or heartbeat, or a stoppage of both. Under some unforeseeable circumstances, a person’s life may be saved by the timely execution of the following. A program for applying cardiopulmonary resuscitation (CPR) involves the following three basic steps. 1. The most important factor for successful resuscitation is immediate opening of the airway. This can be accomplished easily and quickly by tilting the victim’s head backward as far as possible without harming. Sometimes this simple maneuver is all that is required for breathing to resume spontaneously. To perform the head tilt, the victim must be lying on his back. The rescuer places one hand beneath the victim’s neck and the other hand on his forehead. He then lifts the neck with one hand and tilts the head backward by pressure with his other hand on the forehead. This maneuver extends the neck and lifts the tongue away from the back of the throat. Obstruction of the airway caused by the tongue dropping against the back of the throat thereby is relieved. The head must be maintained in this position at all times as shown in figure. 10 Compiled by Er. Bibek Dahal 2. If the victim does not promptly resume adequate spontaneous breathing after the airway is opened, artificial ventilation, sometimes called rescue breathing, must be started. Mouth to mouth breathing and mouth to nose breathing are both types of artificial ventilation as in figure. Adequate ventilation is ensured on every breath by the rescuer: a) Seeing the chest rise and fall. b) Feeling in his own airway the resistance and compliance of the victim’s lungs as they expand. c) Hearing and feeling the air escape during exhalation. The initial ventilator maneuver should be four quick, full breathes without allowing time for full lung deflation between breaths as in figure. In accident cases, it is imperative that caution be used to avoid extension of the neck when there is a possibility of neck fracture. 3. Provide artificial circulation by use of external compression (heart massage). The patient always must be in the horizontal position when external cardiac compression is performed since, during cardiac arrest, there is no blood flow to the brain when the body is in the vertical position. It is imperative, therefore, to get the cardiac arrest victim into a horizontal position as quickly as possible in situation where he is vertical, such as in a dental chair, trapped in a vehicle, stricken on a telephone pole, while in a stadium seat, stricken on a telephone pole, or in any similar situation. Elevation of the lower extremities, while keeping the rest of the body horizontal, may promote venous return (rate of blood flow back to heart) and augment artificial circulation during external cardiac compression as in figure. 11 Compiled by Er. Bibek Dahal Safety precautions and regulations: Insulation is one of the oldest and best methods of protection against shock. Insulation of the conductor, insulation of the worker, or isolation of the conductor can protect worker. Any of these methods however can fail. Therefore continuous maintenance and checks become essential. The rules are different for local people and electrical employee, following are some common precautions, suggested from different standards. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) Always switch off the mains switch before replacing a blown fuse. Always use correct size of fuse, while replacing blown fuse. Always maintain earth connections in satisfactory conditions. Safety depends upon good earthing. Beware of live wires or conductors bare or insulated. Before replacing a lamp or handling a table fan be sure that the switch is in ‘OFF’ positon. Before switching on current to any portable equipment, make sure that it is properly earthed and insulation is sound. Be sure that all the connections are tight. Before working on inductive circuits or cables; discharge them (through short circuit or earth). Do not forget to put on safety belt before starting work above ground level or pole. Never disconnect a plug point by pulling the flexible wires. Never tamper unnecessarily with any electrical apparatus, unless you are authorized to handle it. Never touch an overhead line unless you are sure that it is dead and properly earthed. Never energize a line unless you are sure that all is clear and there is no one working on that. Never tamper with electrical protective or inter-locking gearing unless you are specially authorized for them, taking all precautions. While handling any portable appliances (table fan etc.) see that it is disconnected from the supply, switching ‘OFF’ may not be enough, leakage of insulation’s can give you a serious shock. While working on any Motor/ Generator (Rotating Machines) make it sure that no one can put it to ‘ON’ position without your permission. When cells are being charged in a room, always maintain good ventilation. Never bring a naked light near a battery. Smoking is also prohibited in the battery room. Rubber mats are placed in front of electrical panels and switch boards. 12 Compiled by Er. Bibek Dahal Unit: 3 Equipment Earthing Definition: Earthing is the first step towards electrical safety. The body of electrical equipment, appliances etc. is to be connected to earth for safe discharge of electric current to the earth, in the event of any leakage, fault etc. The earth is considered to be zero potential. Earthing is done to provide safety to user from electric shock. The other name for Electrical Earthing is Grounding. Electrical earthing is done to protect ourselves from shock during fault conditions. The common electrode used for Earthing is copper, which is used for grounding. The basic requirement of every household or industry is to maintain proper Earthing.      It is a set of conductors connected in series or in parallel in order to dissipate the potential difference immediately into the ground. The wire connected from equipment to earth called earthing wire. The resistance of earthing wire is low value to provide least resistance path for leakage or fault current. The area of cross section of conductor is more as resistance is inversely proportional to area of cross section. That is why size of earthing wire is always more. All metallic part of wiring system must be earthed using one continuous earthing wire in the form of bare copper wire/strip throughout the wiring circuit. Electrical System with Earthing 13 Compiled by Er. Bibek Dahal Necessity of earthing: 1. To save human life from danger of electrical shock or death by blowing a fuse i.e. to provide an alternative path for the fault current to flow so that it will not endanger the user. 2. To protect buildings, machinery & appliances under fault conditions i.e. to ensure that all exposed conductive parts do not reach a dangerous potential. 3. To provide safe path to dissipate lightning and short circuit currents. 4. To provide stable platform for operation of sensitive electronic equipment i.e. to maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment . 5. To provide protection against static electricity from friction 6. It prevents the risk of fire that could otherwise be caused by current leakage through unwanted path As per IS standards the max permissible values of earth resistances for various places are,     Large power station- 0.5 Ω Major power station-1.0 Ω Small substation- 2.0 Ω In all other cases- 5.0 Ω Qualities of Good Earthing: 1. Must be of low electrical resistance 2. Must be of good corrosion resistance 3. Must be able to dissipate high fault current repeatedly. Factors influencing earth resistance:        Condition of soil Temperature of soil Moisture content of soil Size and spacing of earth electrodes Depth at which the electrode is embedded Material of conductor Quality of coal, dust and charcoal in the earth electrode pit. 14 Compiled by Er. Bibek Dahal Types of Earthing: 1. Pipe earthing 15 Compiled by Er. Bibek Dahal         Pipe earthing is best form of earthing. It is cheap in system of earthing a GI pipe of 38mm diameter and 2 meters length is embedded vertically in ground to work as earth electrode, but depth depends on soil conditions. Wire is embedded up to the wet soil. The earth wire is fastened to the top section of the pipe with nut and bolts. The pit area around the pipe is filled with salt and coal mixture for improving soil condition and efficiency of earthing systems. It can take heavy leakage current for same electrode size in comparison to plate earthing. The earth wire connection with GI pipes being above the ground level can be checked for carrying out continuity test as and when desired where it is difficult for plate earthing. In summer season to have effective earthing 3 or 4 buckets of water is put through the funnel for better continuity of earthing. Galvanized iron pipe of 38mm diameter, 2m length placed to depth of 4.75mtrs. The depth depends on conditions of moisture. The size of pipe depends on the current to be carried and type of soil. The powdered charcoal salt is put 15cm around the pipe. A funnel is connected at the top of pipe and water is poured at regular period of for maintaining resistance as low as possible. 2. Rod earthing This type of earthing is similar to pipe earthing, but a rod made of galvanized steel is used in this case. The rod used for this purpose is buried inside the earth at a certain depth. As it is of low resistive material, the short circuit current will be diverted to the ground safely. 3. Plate earthing:     In this type of earthing plate either copper or G.I is buried into the ground at depth of not less than 3 meter from the ground level. The earth plate is embedded in alternative layer of Coke and salts for minimum thickness of about 15cm. The earth wire (Copper wire for copper plate earthing and G.I. wire for G.I. plate earthing) is securely bolted to an earth plate with the help of bolt nut and washer made of copper, in case of copper plate earthing and of G.I. pins in case of G.I. plate earthing. In this type of earthing the pipe earthing along with additional earth plate is provided at the bottom of the earth rod. If earth plate is made up of iron, size of earth plate is 60cm X60cm X6.3mm. If it is made of copper, size of earth plate is 60cm X60cm X3.15mm. The plate is laced at the depth of 3meter and then alternative layers of charcoal and salt is put below and top of earth plate. 16 Compiled by Er. Bibek Dahal 17 Compiled by Er. Bibek Dahal Extra: 4. Strip earthing: In these types of earthing, strip electrodes of cross-section not less than 25 mm x 1.6 mm is buried in horizontal trenches of a minimum depth of 0.5 m and if it is a galvanized iron or steel then their cross-section area should not be too small as less than 6.0 mm2. The length of the conductor should not be less than 15 m and this length buried in the ground would give a sufficient earth resistance. 5. Earthing Through The Water main: Before connecting an earthing system to a water main, it must be ascertained that all throughout galvanized iron pipes have been used- If any portion of the water main is made of cement concrete, earthing resistance will be highland the earthing arrangement will not be effective. While connecting earth wire with the water main, care must be taken to see that the contact resistance is minimum. For this purpose specially made earthing clamps are used. If a stranded conductor is used as earth wire, all the strands should be made straight and placed parallel and soldered together to make it solid at that end which is to be connected with the water main. A lead strip is bent round the pipe so that the earth wire can be properly placed over it and the contact between the wire and the pipe is perfect. Before placing the lead strip the surface of the pipe should be thoroughly cleaned, and all traces of dirt and grease are removed. Afterwards some more lead strip packings are placed in between earthing clamp and the wire, and the screws are tightened to complete the earth connection as shown in fig. 18 Compiled by Er. Bibek Dahal Concept of 3-pin plug for high rating equipment The LIVE wire is represented as BROWN or RED This is connected to a fuse on the live pin, the electric current routes through the live wire. The NEUTRAL wire is represented as BLUE or BLACK The electric current routes out through the Neutral wire, i.e. this is the route the electric current takes when it exits an appliance; it is for this reason the neutral wire has a voltage close to zero. The EARTH wire is represented as GREEN & YELLOW This is connected to the earth pin. This is used when the appliance has a metal casing to take any current away, in case if the live wire comes in contact with the casing. A 3-pin plug consists of three pins (hence the name). It is important to know how to wire a 3-pin plug correctly. The 3-pin plugs are designed so that electricity can be supplied to electrical appliances safely. Each wire has its own specified color, as shown above and each pin must be correctly connected to the three wires in the electrical cable. How the Earth pin is connected in the appliances: In a 3-pin plug, the wire that is connected to the ground pin of a plug is also connected to the metal body of the equipment. If any current leakage is appearing on the metal body of the equipment then, it must be discharged safely to ground. It will prevent the users from any electric shock while using the equipment. If there is any serious fault on the equipment, it also makes the fuse blow, hence protecting the user. Therefore, the system should be properly earthed or grounded to prevent electric shock. 19 Compiled by Er. Bibek Dahal The earth pin or the 3rd- pin on a plug is generally bigger and thicker than the rest pins, probably the live and neutral pins allowing the earth pin to be the first to connect and the last to disconnect. Let us also identify the reasons for making the earth or the 3rd-pin like that: The length is made larger, because this allows the Earth pin to make the contact first and later when the supply is disconnected and it breaks the contact ensuring high safety. Insertion of a longer earth pin first helps in opening the shutters, which thereafter facilitates the insertion of the other two pins. In many wall sockets, there are safety shutters on the live and neutral lines to prevent people especially the children from inserting any conducting materials which may result in an electric shock. These types of sockets are known as Earth-pin operated shutters. The earth pin is made thicker so that even by mistake, it cannot be inserted into the live or a neutral hole. The smaller pins on the plug, the live and neutral pins can go into the larger hole, but the larger pin will never go into a smaller hole and any live wire cannot be connected to the body of the equipment. Hence, the earth pin is made thicker so that there is no chance for it to be inserted into the live or neutral hole of the socket. This also prevents earth pin from establishing an electrical contact with the live terminal. Technically speaking, we can say the thickness of earth pin is more in order to decrease its resistance and more the fault current will pass through low resistance path. Let us explain this via the mathematical formula, provided below: R= (ρ * L) /A where R= resistance, ρ (also called as Row) = Resistivity of the material, L= Length of the material, A=Area of cross section and R=resistance of the material Hence resistance (R) is inversely proportional to the cross-sectional area of conductor area (A), hence the resistance of the earth pin decreases due it being thicker than the other two pins, because it will have more area in this case. So more the area, less the resistance and more the fault current will pass through low resistance path. Earth pin carries fault current, which is several times the normal current. Therefore, to ensure proper safety the current carrying capacity of an earth pin should be more than other pins. 20 Compiled by Er. Bibek Dahal Touch and Step Potential When a fault occurs, the fault current flow to the ground due to which the potential gradient near the electrical equipment developed. This potential gradient may affect a person in two ways, either by stepto-step contact or by touch contact. Step-Potential – Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 0.5 m spacing between the one steps, during the flow of fault current through the ground system. Due to this potential, the living body may get an electric shock. This is mostly occurring in Electrical switchyard. Touch-Potential – Touch potential is a potential difference between the fingers of a raising hand touching the faulted structure and the feet of the person standing on substation floor. The person should not get a shock even if the ground structure is carrying fault current, i.e.; the touch potential should be very small. When operating personnel touch electrical equipment during short circuit condition, fault current flows through the human body. To avoid this, we use stone on the high voltage switchyard and Transformer yard. The resistance of the human body is highly variable, and the current that may pass through the body depends on the resistance of the body and the position in which it touches the ground. The resistance of the body is taken as 1000 ohms. 21 Compiled by Er. Bibek Dahal Various types of electrodes used for earthing: Assignment: Earthing mat: A solid metallic plate or a system of closely spaced bare conductors that are connected to and often placed in shallow depths above a ground grid or elsewhere at the earth surface, in order to obtain an extra protective measures minimizing the danger of the exposure to high step or touch voltages in a critical operating area or places that are frequently used by people.” An Earthing Mat is the interconnection of Horizontal and Vertical electrode. The vertical Electrodes are for the dissipation of fault current into the ground while horizontal electrodes are laid for suppressing the dangerous Touch and Step voltages which are generated due to heavy fault current. After the construction of Earth Mat, Crushed Gravels (of resistivity around 5000 ohms) are laid above the whole mat area. This make high resistive path for the dangerous voltages generated (Touch, Step Voltage), which in turn prevents a person from electric shock standing above the Earth mat area 22 Compiled by Er. Bibek Dahal . Concept of instruments used for earthing resistance measurement ASSGINMENT 23 Compiled by Er. Bibek Dahal Unit 4: Fire Hazards and Firefighting Techniques in Electrical Equipment Causes of fire hazards due to electricity: 1. Faulty electrical outlets and aging appliances. This can include faults in appliance cords, receptacles and switches. If an appliance has a worn or frayed cord it can generate dangerous levels of heat, igniting surfaces like rugs and curtains, starting a fire. 2. Using ungrounded plugs. Appliances have the third prong so they can be only used in outlets designed to handle the demands of higher wattage appliances. Never circumvent the ground on an appliance or power cord. 3. Overloading light fixtures. Installing a high wattage bulb into a lamps or light fixtures that it is not designed for is a leading cause of electrical fires. Always install a bulb that is within the recommended wattage. 4. Placing flammable material near light fixtures. Placing cloth or paper over a lampshade can cause the material heat up and ignite, causing a fire. 5. Extension cord misuse. Large appliances should not be plugged into an extension cord. If you do not have a nearby outlet for your appliances, you should have one installed. 6. Space heaters. Placing electric heater too close to combustible surfaces such as curtains, bedding and furniture is a leading cause of house fires. Coil space heaters should be avoided if possible. Liquid filled “soft heat” electric heaters are safest. 7. Old wiring. If your home is over twenty years old, it may not have the capacity to handle today’s energy intensive homes. In addition, outdated breaker boxes often have worn connectors that do not work, causing the system to overload. Fire classification: Class A: These types of fire extinguishers are used in fires which are a result of the burning of wood, glass fiber, upholstery, and furnishing. Usually, Water, DCP (Dry Chemical Powder) and Foam fire extinguishers smother the Class A fire by removing the heating factor of the fire triangle. Foam agents also help in separating the oxygen part from the other aspects. 24 Compiled by Er. Bibek Dahal Class B: These fire extinguishers are used for fires which occur from fluids such as lubricating oils, fuels, paints, cooking oil etc. A portable co2 fire extinguisher or a portable DCP (Dry chemical Powder) extinguisher can be used in this class. Class C: Fires resulting from involvement of energized electrical equipment such as motors, switches, wiring etc. are extinguished by Class C type of fire extinguishers. Usually, CO2 or DCP portable fire extinguisher is used in such fires. Class D: Fires occurring as a result of combustible materials such as magnesium and aluminum are extinguished by this type of fire extinguishers. These elements burn at high temperatures and will react vigorously when coming in contact with water, air, carbon dioxide and/or other chemicals. For extinguishing this class of fire, Dry Powder extinguishers are used which is similar to dry chemical besides they extinguish the fire by isolating the oxygen from the fuel or by eliminating the heat factor of the fire triangle. The dry powder extinguishers are only used for class D fires and they cannot be used for other classification of fire on board ship. Class E: This type of fire extinguisher on a ship is used for subsiding fire resulting from any of the abovementioned materials along with high voltage electricity. Hence, if a portable extinguisher with a conductive agent is used for fighting the class E fire, it may lead to a risk of shock to the operator. CO2 or DCP portable fire extinguisher is used in such fires. 25 Compiled by Er. Bibek Dahal Firefighting techniques IF AN ELECTRICAL FIRE STARTS 1. Cut off the electricity. If the device that is causing the electrical fire is found, and you can reach the cord and outlet safely, unplug it. 2. Add sodium bicarbonate. If the fire is small, you may put it out by smothering it with baking soda. 3. Remove the oxygen source. It may also be put out by removing the oxygen source with clothing or a heavy blanket if the fire is small and it is safe to do so. 4. Don’t use water to put it out. Water is a natural conductor of electricity and if you throw water on an electrical fire, you can get shocked or electrocuted. Also, water may enable to fire to spread by conducting electricity throughout the room and potentially igniting flammable materials. 5. Check your fire extinguisher. Electrical fires are a Class C fire, which means that you will need an extinguisher that is appropriate for this type of fire. Most residential fire extinguishers are multi-purpose and labeled ABC, but it is crucial to verify this before using it on an electrical fire. END OF UNIT 4 26