SET Chapter 1.pdf

Learning Outcomes - Explain the two types of electric shock: o Direct contact o Indirect contact Explain the potential dangers in electrical work Understand the danger of hazardous work practices Explain the prec...

Learning Outcomes - Explain the two types of electric shock: o Direct contact o Indirect contact Explain the potential dangers in electrical work Understand the danger of hazardous work practices Explain the precautions and procedures for safe electrical work Explain the benefits of good housekeeping in electrical work Recommend measures to protect against electrical hazards 1.1.1 How an Electric Shock Happens An electric shock occurs when an electric current flows through the human body (Figs. 1.1-1 and 1.1-2). P Phase or Litive :. Electric current N : Neutral P or L N E Earth : E / # Symbol for Earth. Water Fig. 1.1-1: A person who has Fig. 1.1-2: An electric shock occurs when an received an electric shock electric current flows through a person Unit 1.1 | Electrical Safety 1  1.1.2 Types of Electric Shock There are two types of electric shock: - (a) Direct contact (b) Indirect contact (a) Direct Contact Direct contact occurs when a person comes into contact with a conductor (i.e., heating element of appliance in this example) that is live under normal conditions (Fig. 1.1-3). Supply Transformer Electric Current Heating element of P or L heater which is live Metal casing of appliance which is earthed N E Protective conductor against electric shock in the form of an earth wire Fig. 1.1-3: Electric shock due to direct contact Unit 1.1 | Electrical Safety 2  (b) Indirect Contact Indirect contact occurs when a person comes into contact with exposed conductive parts (i.e., metal casing of appliance in this example) that have become live under fault conditions (i.e., parts not normally live but have become live due to faults such as insulation failure). In Fig. 1.1-4, the metal casing of the appliance is not connected to a protective conductor (i.e., earth wire) which means it is not earthed. The person gets an electric shock when the live metal casing of the appliance is being touched. Supply Transformer fault due to insulation failure P or L Electric Current due to a stray live wire touching the metal casing Metal casing of appliance N E Fig. 1.1-4: Electric shock due to indirect contact The severity of injury from an electric shock depends on: the amount of current flowing through the body; and the length of time the current flows through the body. The severity of injury increases with a larger current and/or a longer duration of current through the body. A shock current of 50 mA can be fatal. Fig. 1.1-5 shows a hand burned by electricity. MA : milliampere Milli : x 10-3 GOnA = 50x10-s =H Fig. 1.1-5: Electrical burn Unit 1.1 | Electrical Safety 3  1.1.3 What to Do When Someone is Shocked or Burned by Electricity Switch off the electrical supply if the person is still in contact with the live circuit. At the same time, get someone else to call for help. If you cannot find the electrical supply source, do not take hold of the victim, as the current may pass through you too. Either use a dry towel or scarf to free the victim, or use a piece of wood to knock the victim’s hand free of the electrical equipment. As a last resort, take hold of the victim’s clothing – without touching the body – to pull the victim free. Do not try to move a victim who has fallen due to electric shock, except to shift the body into the recovery position, as the victim may have sustained other injuries. 1.1.4 How to Work Safely In addition to maintaining a safe work environment, we must also work safely. Safe work practices help reduce the risk of injury or death from workplace hazards. Here are some examples of safe work practices: Use and maintain tools properly. Inspect tools before using them. Switch off the power before working on a circuit. 1.1.5 Good Housekeeping In order to work safely, we should keep our workspaces tidy and well-arranged. Good housekeeping: lowers the risk of accidents and fire; improves productivity; makes better use of space; and reflects a well-managed operation. Unit 1.1 | Electrical Safety 4  1.1.6 General Safety Rules in the Electrical Laboratory Dos Use only insulated tools. All electrical work must be completed by a suitably qualified person. Always switch off the power and remove the plug before any electrical work. Ensure that your electrical equipment is properly inspected and maintained. Disconnect broken appliances from the power supply and replace frayed cords or broken power points. Use test equipment and tools correctly. Read the instruction booklet (if available) and understand the instructions before following them. Keep electrical cords off the floor to prevent them from being damaged from dragging or contact with sharp objects. A damaged electrical cord can cause a fatal electric shock. Know the location of the mains supply. Keep electrical equipment away from water and wet areas to lower the risk of electric shock. Don’ts Never take risks. Do not re-close a tripped circuit breaker or replace a blown fuse until the cause has been found and rectified. Do not misuse electrical equipment and appliances. Keep them dry. Do not use flammable solvents near an electrical apparatus unless the apparatus is labelled “flameproof”. Do not use a fire extinguisher on electrical fires unless it is an appropriate type, such as a carbon-dioxide or dry-powder extinguisher. Switch off the power as soon as possible. Do not overload circuits and fuses by plugging in too many appliances into one power point. Use a power board with individual switches instead of double adapters. Unit 1.1 | Electrical Safety 5