The Hong Kong University of Science and Technology Health, Safety and Environment Office. Electrical Safety. Dr. C. M. LI Senior HSE Manager

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1 The Hong Kong University of Science and Technology Health, Safety and Environment Office Electrical Safety Dr. C. M. LI Senior HSE Manager Electrical Hazards Electrical Safety - Electrocution/ Electric Shock to personnel - Fire ignition of combustible or flammable material - Overheating & damage to equipment - Electrical explosions - Inadvertent activation of equipment which may cause subsequent injuries to personnel 1

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3 Sources of Electricity The electricity we used is mainly supplied by the local electricity companies, and is connected to our homes, working places and other areas, through distribution systems. Frequency: 50 HZ Voltage: Single-phase, 220 Volts (V) Alternate Current Three-phase, 380 Volts (V) Alternate Current The electricity uded in our home and offices is single-phase AC, and 3-phase AC normally serves the industry. Electric Shock Electric Shock refers to the electricity passing through the human body (the accidental flow of electric current through the body), affecting the normal function of the heart, lung and nervous system. Causes of Electric Shock Contact between a live conductor and earth Contact between phase and neutral conductors 3

4 The Effect of Currents Passing Through Various Parts of The Body Electricity Principles - Ohm s Law A completed electrical circuit is composed of 3 elements: Current, Voltage and Resistance 4

5 How Dangerous Are Shocks? Degree of Harmful Effect: Magnitude of current Voltage Body resistance Duration of contact The current pathway through the body Within the range of 8 to 15 ma, the victim would be able to let go and get free of the object that is causing the shock Currents exceeding 15 ma may prevent the victim from letting go because of muscular freeze Currents of 20 ma 40 ma flowing through the heart may cause arrest of breathing/respiration. Currents over 75mA are almost always fatal At levels of current flow exceeding 100 milliamps, the heart stops. This is called fibrillation. How Dangerous Are Shocks? Contact with live parts causing shock and burns (normal mains voltage 230volts AC, can kill). The normal body resistance of human being (depends on the individual) is approximately 1000 ohms. Thus, if he is in contact with a domestic electric supply (220V AC) the current flow through the body would be: 220 Volts = 220 ma (a very dangerous current) 1000 ohms 5

6 First-aid For Electric Shock Burns During an electric shock, the current passing through the body may lead to burns on the skin, muscles or internal organs. As electric shocks and burns are closely related, preventive measures should be considered together! 6

7 Fires and Explosions High temperatures caused by currents under abnormal conditions, may result in accidental fires and explosions. The common causes leading to high temperatures include: Overloading of electricity Insulator breakdowns or short circuits Improper contact of electric circuit Improper maintenance of electrical appliances or wiring Poor ventilation, etc. Fires or explosions can also occur: If flames, sparks or metallic solutions due to electric arc welding are not controlled. Where electricity could be the source of ignition in a potentially flammable or explosive atmosphere, e.g in a spray paint booth. Electrical Hazards in a Construction Site In a construction site and because of the temporary nature of the work, workers working in an unaccustomed and changing environment would be exposed to many hazards including electrical hazards. Electric fires and electric shocks given rise by improper installation such as switch boards/panels, emergency and portable electric generators and electrical wirings are common. 7

8 Electrical Hazards in a Construction Site Adequate precautionary measures should be taken to avoid short circuiting of high tension overhead power lines and underground power cables. For example: Consider the location of overhead and underground cables at the planning stage of a project. Erect clearly marked barriers to protect overhead power lines which remain live provide 6m minimum clearance or as directed by the electricity company. The precise position of buried cables must be determined from utility plans and confirmed using cable locators and hand digging. Consult the electricity company to arrange diversions, isolation of permit-to work arrangements Prevention of Direct Electrical Contact Insulation Placing out of reach Barriers/Enclosures Obstacles 8

9 Grounding for Safety Electricity always takes the easiest path to earth. A wiring installation which has been properly grounded permits excessive electrical currents (eg. lightning strikes) to travel into the earth without causing injuries to people or damage to the wiring system or property. (Grounding electrical circuits is a safety practice that enables electricity to take an alternate path back to the breaker box when an electrical device or appliance short circuits. Without a grounding path, the current would flow through someone that was holding the appliance.) Grounding for Safety In the event of a short circuit, the electricity can follow two available paths. The easy path is through the case of the drill and the grounding wire to the breaker box. The harder path is through the person using the drill. Without the grounding wire, the current would flow through the person using the drill since there is no other possible path for the current to follow. This could be catastrophic. 9

10 Safety Devices Fuses and circuit-breakers are normally employed as circuit protecting devices for ground fault, overloading or short circuit in the wiring system. Their proper functioning would automatically open or break the circuit in the event of an excessive current flow. Fuse: if the current exceeds the set fusion value, the fuse will blow and the current is cut-off, thus preventing overloading. A fuse must be installed on live wires. When replacing a new fuse, must be of the same current fusion value. Safety Devices Circuit Breakers (MCB) are based on the principle of the electromagnetic field. The current entered may enable the coils of the circuit breaker to magnetise. When the current exceeds the set value (i.e. overloading), the magnetisation intensifies, switching off the circuit breaker and disconnecting the electric source. Earth Leakage Circuit Breaker (ELCB) monitor the operation of the natural or live wires in the circuit. During an imbalance in the electrical circuit, or when not all the current flows to the electrical appliance through the live wire and returns through the neutral wire, part of the current flows away (leaks) into other sources. The earth leakage circuit breaker will immediately detect such an imbalance and cut-off the electrical source in 0.4 seconds. Rating of the tripping current shall not exceed 30mA. 10

11 Safety Devices Earth Leakage Circuit Breaker (ELCB) Safety Devices An electrical appliance with Double Insulation is protected by a supplementary insulation layer in addition to basic insulation. 11

12 Safe Use of Plugs: Safe Use of Electricity Safe Use of Electricity Safe Use of Plugs: Use correct plugs. Never insert the core of cord directly into a socket. Unplug by pulling the plug. Not the cord. Do not touch the plug with wet hands, as wet skin reduces the resistance of the body, resulting in more serious injuries. Broken plugs must be replaced immediately. Avoiding overloading! Never put too many plugs into the same socket. 12

13 Legal Requirements and Regulations The legislation involving electrical safety are: Electricity Ordinance (Chapter 406), enforced by the Electrical and Mechanical Services Department Factories and Industrial Undertakings (Electricity) Regulations (Chapter 59), enforced by the Labour Department Construction Sites (Safety) Regulations Electricity (Wiring) Regulations Electricity and electrical installations on construction sites and various workplaces must always be treated with the utmost care and be under the control and supervision of experienced competent persons. Check List for Electricity Distribution and Use On Site Comply with all statutory and electricity supply regulations for all electrical work Employees should be trained to report as soon as practicable any obvious hazard observed in connection with electrical equipment or lines. Make preliminary inspections and/or appropriate tests to determine what conditions exist before starting work on electrical equipment or lines. When electrical equipment or lines are to be serviced, maintained or adjusted, switches must be opened, locked-out and tagged whenever possible. Portable electrical tools and equipment shall be grounded or of the double insulated type 13

14 Check List for Electricity Distribution and Use On Site Extension cords being used should have a grounding wire Do not use multi-way adaptors Use the correct leads and sockets for the voltage supplied/amperage Check that fuses are of the correct rating All temporary circuits are to be protected Electrical installations in hazardous dust or vapor areas should meet the necessary safety requirements (flame proof equipment) Check List for Electricity Distribution and Use On Site Use portable electric tools powered by voltages of 110 volts is recommended Extra low voltage: means voltage between conductors or between any conductor and earth with Alternate Current (AC) not exceeding 50V or Direct Current (DC) not exceeding 120V. Low voltage: means voltage normally exceeding extra-low voltage, but between conductors with AC not exceeding 1000V or DC not exceeding 1500V, or the voltage between conductors and earth with AC not exceeding 600V or DC not exceeding 900V. High voltage: means voltage normally exceeding low voltage, but between conductors with AC exceeding 1000V or DC exceeding 1500V, or the voltage between conductors and earth with AC exceeding 600V or DC exceeding 900V. 14