Things to talk about in this session: 1. ELECTRICAL SAFETY 2. 3. 4. 1. Why electricity is dangerous? Why electricity is dangerous? What kinds of dangers can be encountered with electricity? Under what situations would electricity be dangerous? What should/can be done to minimize the risk of the dangers? Electricity can be dangerous because: It is a form of energy, and People need to use or work with electricity all the time. The Danger is Invisible 2. What kinds of dangers (hazards) can be encountered with electricity? As a form of Energy It can: – Seriously harm people when electrical current passes through the human body » Electrocution/ Electric Shock A Nepalese worker killed by electricity when performing electric arc welding on a ship. A young electrician, killed by electricity when working in a switch room. A technician was killed by electricity when installing a cable TV decoder in a domestic dwelling. As a form of Energy Electricity can: – Cause fire » Electric faults (short circuit) » Overheating (resistance in conductor) » Sparks – Accounts for about 15% of all fires each year Harmful Effects of Electrocution Sufficient current flowing through the body will create serious harm: – Ventricular Fibrillation 心室纖維性顫動– heartbeats disrupted by electric current. The heart flutters rather than beats. The heart pumps little or no blood thru’ the circulation system. (need a defibrillator 電擊器 to resume heartbeats). – Suffocation 窒息– electric current causes the lung to contract violently, affecting respiration – Cell damage – by electrical energy – Burns – by heating effect of electric current Fire caused by electric faults in an electric fan killed a young man in a domestic dwelling. Types of burn Effects of Electrocution – Contact Burn Degree of Harmful Effect: Magnitude of current (associates with electrocution) – Voltage – Body resistance Duration of contact The current pathway through the body Effects of Current Thru Body Current Effects 1 mA Faint sensation 1 – 8 mA Clear sensation 10 mA Loss of muscle control 100 mA Paralysis/ Suffocation 100 – 200 mA Fibrillation > 200 mA Severe Burn Expected Current thru’ body Current passing through the body depends on: Voltage applied Resistance of body – moisture of skin – other factors (e.g. size, weight etc.) Resistance of other insulators Body Resistance Current through body Varies depending on many factors: (mainly on the wetness of the body) 500 kilo-ohms (dry skin) 300 ohms (wet skin) Worst Condition (wet): 220V / 300 ohms = 733 mA Best Condition (dry): 220V / 500,000 ohms = 0.44 mA Don’t take the chance! Secondary Hazards Falls – jerk reaction Arc Flash Accidental shorting of the circuit Energy released due to the development of an electric arc Current flashes through air from one conductor to another or to ground, forming an arc Temperature rises rapidly and metallic conductors melt and vaporize, forming a plasma for sustaining the arc Vaporized metals expands rapidly causing shrapnel to shoot out Arc flash accident Racking of electrical switch – breaking the contact Arc Flash – One common cause for arc flash is accidental shorting of a circuit Personal Protective Equipment for guarding against possible “Arc flash” hazards An electrician accidentally dropped a torch into a switch cubicle – shorting an electric circuit – resulting in an explosion. – Arc Flash 3. Under what situations would electricity be dangerous? Another common cause is : when metallic tools are in contact with high voltage cable underground Electricity is dangerous: When there is significant risk for people to get into contact with electric current. – Electrocution When “excessive” heat is generated by the electric current – Fire When electrical works are not carried out according to safe work procedure Electrocution Risk Electricity is safe when it is properly shielded from contact by human body, by means of: – Insulation – Enclosure Under certain (faults) conditions, people can get into contact with electric current, directly or indirectly. indirectly Direct Contacts How can people get in contact with electric current? What are those fault conditions? – People get into contact with current carrying parts (conductors), due to: » Need to open up enclosure, e.g. for repair and maintenance works Power source not properly shut down Maintenance people are more at risk Direct Contacts Examples: – Damaged casing/cable/plugs etc. where live conductors are exposed. – People get into contact with current carrying parts due to defective/damaged enclosure or insulation materials Direct Contacts Examples: – Inadequate insulation (Fray wire) Due to Unsafe design of equipment Examples: – Exposed conductors Examples: – Exposed conductors Exposed Conductors Indirect Contacts (with energized conductive parts) Examples: – Potential exposure of conductors Energized Exposed conductor t flow Curren – Leaking out of electricity to the case – No/ inadequate grounding – All users are at risk 4. What should/ can be done to minimize the risk of the dangers? Ensure proper insulation/ enclosure – For preventing direct contacts with live parts Ensure adequate isolation of current carrying parts from reach of people – When insulation is not feasible Ensure adequate safeguard for leakage of electricity – For preventing indirect contact with leak out currents Ensure proper shut down of power when performing electrical works Prevention of Electrocution Engineering Controls – Ensure adequate design and strength of (insulation and enclosing) material – Installation of safety devices Administrative/Management Controls – Procedures for safe operations – Arrangements for inspections, maintenance and repairs of systems and equipments Engineering Control Measures Safeguard against Direct Contact with Live Electrical Parts Adequate live conductors – Ensured by safe design and proper inspection and maintenance – Ensured by meeting appropriate safety standards – Stringent requirements in Electricity (Wiring) Regulation Adequate Isolation – Restrict access or contact by Enclosure/ Guarding/ Barrier insulation of Always purchase/use electrical equipment meeting appropriate safety standards Interlocking devices 連 鎖裝置 – Normally installed at access doors for dangerous parts. – Fool-proof device to ensure electricity supply is cut-off with the device is activated (when the door is opened) Hazardous Scenarios Isolation by Distance High voltage O/H Power Lines Common Safeguards Safeguards against Ground Fault Conditions Grounding/Earthing Automatic Protection – Residual Current Device (RCD) Leakage of Electricity Double Insulation – Safe design for electric hand tools Grounding/ Earthing How grounding works Indirect contact with electric current Current leakage Current leakage Touch Voltage = 220V Current passing thru’ body is high enough to cause serious injuries. 0.1 ohm Earth Wire Proper Wiring Earth wire 3-Pin Plug Important: Ensure that Earth Connection is Continuous Earth wire must be securely connected for providing earth protection 3-wires are required to provide earthing System Equipment Proper Wiring Cable must be firmly gripped to release strain from the conductor wires Connections at terminals bear all the pulling force during use – To ensure that the conductors would not be pulled out of the terminals Cable not firmly gripped Cable Grip Pulling force May lead to serious accident when the earth wire is pulled out Grounding of Conductive Surfaces Fatal Contact Loose earth wire accidentally touches the live terminal Flexible cable not firmly gripped Metallic conductive surfaces may be electrically charged accidentally – e.g. Work benches on which electrical equipments are placed Metal case electrically charged Conductive surface Double Insulation 雙重絕緣 Insulated surface Safety Standard for Portable Electrical Equipment Class I Protection by grounding Class II Protection by Double Insulation Automatic Protection Device Automatic disconnection of power when an electricity leak is detected Devices – Residual Current Device (RCD) – (Earth Leakage Circuit Breaker (ELCB) – Ground Fault Circuit Interrupter (GFCI) Residual Current Device Residual Current Device Schematic Diagram RCD Fuse Protection Test Button General Safety Practices Safe Work Practices and Procedures Only authorized, competent, and qualified (e.g. by training) persons are allowed to work on or around electrical equipment and/ or wiring. – Required by Electricity (Registration) Regulation General Safety Practices Purchase up-to-standard electrical equipment equipped with appropriate protective devices. General Safety Practices – 110 volts or lower – Suitable for lighting – Need a step-down device – Required by Electrical Products (Safety) Regulation General Safety Practices General Safety Practices Proper use of electrical equipment (not to interfere with protective devices) Use lower voltages Proper maintenance of system and equipment First Aid for Electrocuted Victims Emergency Procedures Emergency procedures in the event of an accident – Equipment emergency shutdown procedure, e.g. power cut-off switch can be easily reached. – Electric shock first aid procedure » The need for first-aid training Electric shock victim MUST be removed from contact with electricity by safe means before doing any necessary first aid treatment. First Aid for Electrocuted Victims CPR 心肺復甦 – Should be performed by a competent person Specific Safety Procedures Specific Safety Procedures Case Study (5.5 minutes) Electrical/Mechanical System Maintenance Lockout / Tagout Procedures Lockout/ Tagout Arrangement When Lockout/Tagout Procedure is Needed? Purpose: To avoid inadvertent release of energy (electrical and / or mechanical) causing serious harm to people working on the system by: – Effective isolation of power supply. – Clear notification. Power cut-off point is remote from the equipment Multiple power supply sources Work is prolonged Work involves multiple work teams Steps for Lockout/Tagout Procedure Plan the shut down of the system. Alert operator and other users of the shut down. Lockout the power supply to the system at the most appropriate point(s). Steps for Lockout/Tagout Procedure (cont) Have all teams/workers place their personal & individual padlocks on the lockout point. Put a warning tag at the lockout. Release all stored or residual energies (e.g. capacitors, loaded spring etc.) Test the circuit to confirm it is dead. Steps for Lockout/Tagout Procedure (cont) Each team/worker should remove only his own padlock upon completion of his part of work. Safeguard against Stored Energy Hazard of Stored Electrical Energy Electrical energy still be there after power cut out – Stored Up. Up Depending on amount of energy stored up -- can cause painful shocks or kill people Stored Energy Batteries can hurt – Can cause fire and explosion when shut-circuited. Capacitors Hazards: – Store up electrical energy – Electrical energy still remains after power cut out – Energy dissipates very slowly Precautions: – Hazards must be identified and proper safety procedures established, e.g. proper discharge and grounding Fire & Explosion Hazards Common Causes for Fire & Explosion Cable with insufficient size Effect of excessive current Common Causes for Fire & Explosion – Excessive heat due to » Poor/loose connections » Improper wiring » Substandard plugs/adaptors Common Causes for Fire & Explosion – Abused use of adaptors and extension socket boards Common Causes for Fire & Explosion – Poor electrical contacts » Plugs are loosely fitted to socket (poor electric contact) -- esp. for two pin plugs Over-current Protection Working in Hazardous Environment Use conductor of sufficient size – Specified in Wiring Regulations Fuse Protection Fuse Sparks generated by certain electrical tools and equipment, e.g.... motor, plugging and unplugging – Especially in hazardous atmosphere…... MCB Box Avoid Generation of Sparks in Hazardous Atmosphere Use spark proof / intrinsically safe equipment and installation in hazardous areas where there are: – flammable liquids; – combustible liquids operating at a temperature above their flash point; or – gases or combustible dusts that may be present in flammable, explosive and combustible concentrations. Legal Requirements Legislative Controls Legal Requirements Electricity Ordinance Cap. 406 – Electricity (Wiring) Regulation » Code of Practice – Electricity (Registration) Regulation – Electrical Products (Safety) Regulation – Electricity Supply Lines (Protection) Regulation – Enforced by EMSD Electricity (Registration) Regulation Requirements and procedures for registering: Electrical Workers Electrical Contractors Ensure competence of people working on electrical systems Registration Classes: – – – – – Grade A : Low voltage < 400A Grade B : Low voltage < 2500A Grade C : Low voltage of any capacity Grade R : Neo signs/ LV air con/ LV power generating facilities Grade H : High voltage installation Electricity (Wiring) Regulations Enacted in 1992 Code of Practice – The Golden Rules for the trade – Provide comprehensive practical guidelines to help REC/REW to do their jobs – Revised in 2003 – Downloadable free online Electrical Products (Safety) Regulation Ensure the safety standards of electrical products designed for domestic use, including plugs, sockets, adaptors and extension boards. Ensures safe equipments/appliances Electricity Supply Lines (Protection) Regulation Protect electricity supply lines from damage by construction works (e.g. road work) Risk assessments required before work starts. – Conducted by competent persons – Detection of cable locations Factories & Industrial Undertakings (Electricity) Reg. Complementary to Electricity (Wiring) Regulations. Safety requirements on electrical installations and use of electrical tools and equipment specifically applicable to all industrial undertakings. Enforced by Labour Department Relevant Standards for Electrical Equipment/Accessories Standards and Codes BS 415 -- Safety requirements for mains operated electronic and related apparatus for household and similar general use BS 4743 -- Specification for safety requirements for electronic measuring apparatus BS 3456 -- Specification for safety of household and similar electrical appliances BS 546 -- Two pole and earthing pin plugs, socketoutlets and socket-outlet adaptors BS 1363 -- Specification for 13A fused plugs and switched and unswitched socket-outlets