HSEV 1005 Unit 2f - Electrical Hazards PDF

Unit 2f: Electrical Hazards ______________________________________________ Lecture Objectives i. Identify the hazards, and describe control measures for the use of electricity in the workplace ii. List different forms of hazardous energy and describe the procedure for LOTO, Lock T...

Unit 2f: Electrical Hazards ______________________________________________ Lecture Objectives i. Identify the hazards, and describe control measures for the use of electricity in the workplace ii. List different forms of hazardous energy and describe the procedure for LOTO, Lock Tag Try. ___________________________________________________________________ Introduction Electricity is a widely used, efficient and convenient, but potentially hazardous method of transmitting and using energy. It is in use in every factory, workshop, laboratory and office in the country. Any use of electricity has the potential to be very hazardous with possible fatal results. Electrical work from the largest to the smallest installation must be carried out by people known to be competent to undertake such work. New installations always require expert advice at all appropriate levels to cover both design aspects of the system and its associated equipment. Figure 1: Beware of electricity - typical sign Principles of electricity and some definitions. In simple terms, electricity is the flow or movement of electrons through a substance which allows the transfer of electrical energy for one position to another. The substance through which the electricity flows is called a conductor. This flow or movement of electrons is known as the electric current. There are two forms of electric current – direct or alternating. Direct current (dc) involves the flow of electrons along a conductor from one end to the other. Alternating current (ac) is produced by a rotating alternator and causes an oscillation of the electrons rather than a flow of electrons so that energy is passed from one electron to the adjacent one and so on through the length of the conductor. It is sometimes easier to understand the principles of electricity by comparing its movement with that of water in a pipe flowing downhill. The flow rate of water through the pipe is similar to _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 1  current flowing through the conductor which is measured in amperes, normally abbreviated to amps. Sometimes very small currents are used and these are measured in milliamps (mA). The higher the pressure drop is along the pipeline, the greater will be the flow rate of water and, in similar way, the higher the electrical “pressure difference” along the conductor, the higher the current will be. This electrical “pressure difference” or potential difference is measured in volts. The flow rate through the pipe will also vary for a fixed pressure drop as the roughness on the inside surface of the pipe varies – the rougher the surface, the slower the flow and the higher the resistance to flow becomes. Similarly, for electricity, the poorer the conductor, the higher the resistance is to electrical current and the lower the current becomes. Electrical resistance is measured in ohms. The voltage (V), the current (I) and the resistance (R) are related by the following formula, known as Ohms law: V = I × R (Volts) And, electrical power (P) is given by: P = V × I (Watts) These basic formulae enable simple calculations to be made so that, for example, the correct size of fuse may be ascertained for a particular piece of electrical equipment. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 2  Conductors are nearly always metals, copper being a particularly good conductor, and are usually in wire form but they can be gases or liquids, water being a particularly good conductor of electricity. Superconductors is a term given to certain metals which have a low resistance to electricity at low temperatures. Very poor conductors are known as insulators and include materials such as rubber, timber and plastics. Insulating material is used to protect people from some of the hazards associated with electricity. Electrical equipment components and an electrical power supply are joined together by a conductor to forma circuit. If the circuit is broken in some way so that the current flows directly to earth rather than to a piece of equipment, a short circuit is made. Since the resistance is greatly reduced but the voltage remains the same, a rapid increase in current occurs which could cause significant problems if suitable protection were not available. Earthing The electricity supply company has one of its conductors solidly connected to the earth and every circuit supplied by the company must have one of its conductors connected to earth. This means that if there is a fault, such as a break in the circuit, the current, known as the earth fault current will return directly to earth, which forms the circuit of least resistance, thus maintaining the supply circuit. This process is known as earthing. Electrical hazards and injuries Electricity is a safe, clean and quiet method of transmitting energy. However, this apparently benign source of energy when accidentally brought into contact with conducting material, such as people, animals or metals, permits releases of energy which may result in serious damage or loss of life. Constant awareness is necessary to avoid and prevent danger from accidental releases of electrical energy. The principal hazards associated with electricity are: Electric shock Electric burns Electrical fires and explosions Arcing Portable electrical equipment Secondary hazards Falls from heights _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 3 Electric shock and burns Current flow is a factor that causes injury in electrical shock. The severity of the electrical shock is determined by the amount of current flow through the victim. Duration of the flow and frequency also determine the extent of the injury. Human skin if wet is much less resistant to electrical current flow. Figure 2: Effects of electrical current in the human body. Electric shock is the convulsive reaction by the human body to the flow of electrical current through it. This sense of shock is accompanied by pain and, in more severe cases, by burning. This shock can be produced by low voltages, high voltages or lightning. Most incidents of electric shock occur when the person becomes the route to earth for a live conductor. The effect of electrical shock and the resultant severity of injury depend upon the size of the electrical current passing through the body which in turn depends on the voltage and the electrical resistance of the skin. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 4  Burns of the skin occur at the point of electrical contact due to high resistance of skin. These burns may be deep, slow to heal and often leave permanent scars. Burns may also occur inside the body along the path of the electric current causing damage to muscle tissue and blood cells. Electric arcing A person who is standing on earth too close to a high voltage conductor may suffer flash burns as a result of arc formation. Such burns may be extensive and lower the resistance of the skin so that electric shock may add to the ill effects. Electric arc faults can cause temporary blindness by burning the retina of the eye and this may lead to additional secondary hazards. The quantity of electrical energy is as important as the size of the voltage since the voltage will determine the distance over which the arc will travel. The risk of arcing can be reduced by the insulation of live conductors. Electrical fires and explosions Over 25 % of all fires have a cause linked to a malfunction of either a piece of electrical equipment or wiring or both. Electrical fires are often caused by a lack of reasonable care in the maintenance and use of electrical installations and equipment. The electricity that provides heat and light and drives electric motors is capable of igniting insulating or other combustible material if the equipment is misused, is not adequate to carry the electrical load, or is not properly installed and maintained. The most common causes of fire in electrical installations are short circuits, overheating of cables and equipment, this ignition of flammable gases and vapours, and the ignition of combustible substances by electrical discharges. Short circuits happen, if insulation becomes faulty, and an unintended flow of current between two conductors or between one conductor and earth occurs. The amount of current depends, among other things, upon the voltage, the condition of the insulating material and the distance between the conductors. At first the current flow will be low but as the fault develops the current will increase and the area surrounding the fault will heat up. In time, if the fault persists, a total breakdown of insulation will result and excessive current will flow through the fault. If the fuse fails to operate or is in excess of the recommended fuse rating, overheating will occur and a fire will result. The term overcurrent is also specifically used for circumstances where there is an electrical fault. Secondary hazards Secondary hazards are those additional hazards which present themselves as a result of an electrical hazard. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 5 Portable electrical equipment Portable electric tools are a very convenient aid in many occupational activities. Inevitably in their use, they receive rougher treatment than fixed appliances and are therefore more likely to develop faults. The main problem is the necessity to use flexible cables to supply electricity to power the tool. These cables are often misused and abused, resulting in damaged insulation and broken and exposed conductors. Also the tool itself could also become unsafe, if its metalwork became charged with electricity because of a fault. List of possible faults:  Damage to flexible cables  Loose cable connections in appliance or plug  Badly wired connections in appliance or plug  Damaged plugs  Insulation failure and exposed cables  Damaged, defective and misused appliances  Repairs by unqualified personnel. Repairs and adjustment without disconnection from electricity supply  Using apparatus unsuitable for the duty or conditions.  Inadequate maintenance To avoid the risks from the use of such tools one solution is substitution. This can be achieved by replacing electrically powered tools with pneumatic tools. Another approach is to use tools designed to run on a relatively low voltage. Portable tools should also be double insulated. This means that all metal work which a person may handle is, where practicable, separated from the conductors by double insulation so as to prevent such metalwork from becoming electrically dangerous. Preventative Measures In the previous section we have seen the potential for electricity to cause harm, by electric shock, burns, fire and explosion. It is clear that if these potential hazards are not controlled there is considerable risk as both the probability and consequences of injury are high. This section gives abroad outline of what can and must be done to ensure that we are not exposed to these risks. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 6 Safety by design Electrical installation and apparatus must be designed to one major parameter i.e. rating. This basically requires all electrical installations to be designed to carry out the work required of them in order to avoid overcurrent. Earthing Regardless of the quality of the insulation in electrical equipment there always exists the possibility that this protection will break down and ‘short circuit’. Ordinarily fuses and circuit breakers maybe counted upon to automatically disconnect the circuit if a short occurs, but this action cannot be guaranteed. In a single phase AC system the current flows between the ‘live wire’ and neutral through the device that is being powered. If the insulation around the ‘live’ wire becomes damaged enabling it to come into contact with a piece of metal that is itself insulated from its surroundings, then no fuse will blow and that piece of metal will become ‘live’ producing considerable hazard to anyone touching it. The system of earthing removes the risk. All exposed metal parts are connected to the local ground, via the earth lead, and if any comes into contact with the live wire a considerable current will flow, immediately operating any protective devices and making the metal surfaces safe. Fuses The fuse is the most common form of protection against overcurrent. A fuse consists of a suitable mounted conductor, usually in the form of a wire, chosen so that it will melt if the current rises above a specified value. Such a fuse, placed in the live lead of a domestic circuit, will disconnect the device being powered from the electrical supply, if the current becomes excessive.. A fuse should never be replaced until the cause of its melting can be ascertained and corrected. Such fuses will allow currents to flow, above their rated fusing value, for a short time. This is to allow for current surges often experienced when a circuit is first turned on. Plugs containing fuses are the best way to ensure that the degree of protection is suitable for all pieces of equipment. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 7 Overfusing This is the name given to the bad practice of using a fuse of a higher rating that the circuit it is supposed to protect. This is dangerous because in the event of a fault the current may flow to earth without blowing the fuse. Thus endangering the workers, the circuit and the apparatus involved. Circuit breaker A circuit breaker is a mechanical switch, or relay that is able to isolate the circuit it is protecting when the conditions become abnormal. It is used on systems with high voltages and large current carrying capacities. There are usually two types of circuit breakers: 1. Thermal – operate on basis of temperature rise. 2. Magnetic – operate on basis of the amount of current through a wire. Usually better than thermal ones because it can handle wider fluctuations in ambient temperatures. A ground fault circuit interrupter (GFCI) This is another type of protective device. This is a fast acting electronic interrupting device that is sensitive to current flow to ground. The unit operates only on line to ground current, such as insulation leakage current or current likely to flow during accidental contact with a hot wire or a 120 volt circuit and ground. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 8 Lockout Procedures Lockout of equipment is done whenever equipment needs to be inspected or for maintenance: Electrical equipment needs to be de-energized. Live circuits and equipment left in the operable mode are always a hazard. It should always be assumed that a circuit is alive unless it is proven dead. To make certain, switches and circuit breakers carrying current to or from switchboards, buses, controls, starting equipment should be checked to see that they are open. As an additional safeguard, the inspector (usually an electrician) should have the breakers and the switches locked open, grounded and tagged so they cannot be energized. This is considered for rotating and intermittent starting equipment. When do we do a lockout procedure? When doing inspection or repairs on equipment such as motors, blowers, compressors – any part of which may be remotely controlled or which will automatically start, circuit breakers or switches should be locked out, and the fuses pulled. Machinery connected to blowers or pumps without check valves, may start to rotate with a supply current. For this reason the rotor or armature should be blocked before an inspection is made. Lockout procedures should be followed on generators driven by prime movers. The throttle, starting valve or other means of controlling the energy of the driving part of the unit should be locked and tagged. THE LOCKOUT PROCEDURE 1. Alert the operator and other users of the system to be shut off. 2. Plan the shutdown to ensure that the system will be shut off. 3. Place your own padlock on the control switch, lever or valve even though someone has locked out the control before you. YOU WILL NOT BE PROTECTED UNLESS YOU USE YOUR OWN LOCK. Sign and blocks are no substitutes for locking out the source off electrical power. 4. Test the lockout to be sure the system is really off. 5. Do work with peace of mind. 6. When through working, remove your own padlock. Never permit anyone to remove your lock, and be sure you are not exposing another person to danger. Verify the equipment is clear and post a watch if necessary. 7. Re-energize the system. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 9 Static Electricity Static electricity is produced by the build-up of electrons on weak electrical conductors or insulating materials. These materials may be gaseous, liquid or solid and may include flammable liquids, powders, plastic films and granules. The generation of static may be caused by rapid separation of highly insulated materials by friction or by transfer from one highly charged material to another in an electric field by induction. Discharges of static electricity may be sufficient to cause serious electric shock and are always a potential source of ignition when flammable liquid, dusts or powders are present. Flour dust in a mill, for example, has been ignited by static electricity. Static electricity may build up on both materials and people. When a charged person approaches flammable gases or vapours and a spark ignites the substance, the resulting explosion or fire often causes serious injury. The following points illustrate some of the common hazards due to static electricity: 1. Static electricity has been the cause of many disastrous fires and explosions because it provides an ignition source for fuel-air mixtures. 2. A sudden reflex movement by a worker who has been startled by a static shock could lead to severe injuries if he is working around rotating equipment. Similarly, a worker could fall from a ladder or scaffold if he were startled by an unexpected static shock. 3. Static can build up in control instrumentation to a point where the equipment cannot function correctly. This can lead to process upsets, production losses or even damage to equipment. 4. Computers and other sophisticated electronic equipment can be badly damaged, or even destroyed by static electricity. 5. A static charge can cause paper and other non-conductive materials to stick together, or stay apart, creating production problems and losses. How can static electricity be controlled when flammable liquids are transported? 1. Many of the problems associated with static electricity can be removed, or at least reduced, by simply “draining off” the static charge as fast as it is produced. Bonding and grounding are two methods used. Bonding eliminates a potential difference between two different objects. It is accomplished by connecting two conductive objects by means of a metal conductor usually copper wire. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 10 Grounding eliminates a potential difference between an object and the earth. Grounding is accomplished by connecting a conductive object to the earth with a metal conductor, usually a copper wire attached to a ground rod. 2. Increasing the humidity – when the humidity is high, the nuisance from static electricity is reduced because the moist air provides a conducting path for electricity, and can prevent the charge from building up to a potential where a spark is produced.  Common Controls  Bonding, grounding  Reduce pouring, splashing  Use special conductive containers for storing flammables  Increasing humidity  Positive pressure inside containers to prevent items from falling in _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 11  Unit 2f – Electrical Hazards 1) Provide a brief explanation of the following terms: Voltage, Insulators, and Grounding. 2) There are four main types of electrical injuries; electrocution, electrical shock, burns and falls, briefly describe. 3) After the passing of Hurricane Marco, you have been assigned to work on checking the transformers on Lady Chancellor Hill. This requires working from a height. What are some of the control measures you will use for this job? 4) What is Electrical current measured in? 5) Who is the person responsible for placing and removing a lock and tag? 6) What are the hazards of static electricity? 7) Investigators of the cause of the fire that burnt down the recently renovated electronics appliance department store of Courts Intl Ltd. narrowed it down to electrical. What ‘electrical fault’ considerations must they now assess? 8) What makes electricity capable of causing fires? 9) Discuss, on discovering a person who has been electrocuted by high voltage electricity, the police and T&TEC should be informed. 10) Discuss, secondary hazards associated with portable electrical appliances include fire and static electricity. _______________________________________________________________________ HSEV 1005 –Intro to HSE Unit 2f –Electrical Hazards Amna S Imamshah 12