Electrical by xiaoyounan


									                                    Things to talk about in this session:

                                    1.   Why electricity is dangerous?
                                         What kinds of dangers can be
ELECTRICAL SAFETY                   2.
                                         encountered with electricity?
                                    3.   Under what situations would
                                         electricity be dangerous?
                                    4.   What should/can be done to
                                         minimize the risk of the dangers?

                                   Electricity can be dangerous because:
1. Why electricity is dangerous?    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

                                                                   As a form of Energy

                                                                  Electricity can:
                                                                   – Cause fire
                                                                      » Electric faults (short
                                                                      » Overheating (resistance in
                                                                      » 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
Fire caused by electric faults in an electric fan killed a       – Cell damage – by electrical energy
young man in a domestic dwelling.
                                                                 – Burns – by heating effect of electric current

    Types of burn                                            Effects of Electrocution
                                                             Degree of Harmful Effect:
   – Contact Burn                                             Magnitude of current
   (associates with electrocution)                               – Voltage
                                                                 – Body resistance
                                                              Duration of contact
                                                              The current pathway through the body
Effects of Current Thru Body                     Expected Current thru’ body
        Current                 Effects
                                                 Current passing through the body depends on:
1 mA                    Faint sensation
                                                  Voltage applied
1 – 8 mA                Clear sensation
                                                  Resistance of body
10 mA                   Loss of muscle control       – moisture of skin
100 mA                  Paralysis/ Suffocation       – other factors (e.g. size, weight etc.)
                                                    Resistance of other insulators
100 – 200 mA            Fibrillation
> 200 mA                Severe Burn

Body Resistance                                  Current through body

Varies depending on many factors:                Worst Condition (wet):
(mainly on the wetness of the body)              220V / 300 ohms = 733 mA
 500 kilo-ohms (dry skin)
 300 ohms (wet skin)
                                                 Best Condition (dry):
                                                 220V / 500,000 ohms = 0.44 mA

                                                 Don’t take the chance!
    Secondary Hazards                               Arc Flash
                                                  Energy released due to the
   Falls                                          development of an electric arc
    – jerk reaction                               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
          Accidental shorting of the circuit
                                               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                                     An electrician accidentally
“Arc flash” hazards                                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:                           Electrocution Risk
                                                    Electricity is safe when
  When there is significant risk for people         it is properly shielded
   to get into contact with electric current.        from contact by human
                                                     body, by means of:
    – Electrocution                                  – Insulation
  When “excessive” heat is generated by             – Enclosure
   the electric current                             Under certain (faults)
                                                     conditions, people can
    – Fire                                           get into contact with
  When electrical works are not carried             electric current, directly
                                                     or indirectly.
   out according to safe work procedure

                                                     Direct Contacts
                                                – People get into contact
                                                  with current carrying
                                                  parts (conductors), due
How can people get in contact                     to:
                                                   » Need to open up
with electric current?                                enclosure, e.g. for
                                                      repair and
                                                      maintenance works
     What are those fault conditions?                    Power source not
                                                          properly shut
                                                         Maintenance
                                                          people are more at
 Direct Contacts
– People get into contact with current carrying parts         Examples:
  due to defective/damaged enclosure or insulation            – Damaged casing/cable/plugs etc. where
  materials                                                     live conductors are exposed.

                                                             Direct Contacts
  – Inadequate insulation (Fray wire)
                                                           Due to Unsafe design of equipment
  – Exposed conductors
                                                        – Exposed conductors

                                                        Exposed Conductors

                                                       Indirect Contacts
Examples:                                              (with energized conductive parts)
  – Potential exposure of conductors                  – Leaking out of electricity to the case
                                                      – No/ inadequate grounding
                                                      – All users are at risk
                   Exposed conductor

                                             t flow
                                                      Ensure proper insulation/ enclosure
                                                       – For preventing direct contacts with live parts
4. What should/ can be done to                        Ensure adequate isolation of current carrying
minimize the risk of the dangers?                      parts from reach of people
                                                       – When insulation is not feasible
                                                      Ensure adequate safeguard for leakage of
                                                       – For preventing indirect contact with leak out
                                                      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         Engineering Control Measures
    – Installation of safety devices
   Administrative/Management Controls
    – Procedures for safe operations
    – Arrangements for inspections, maintenance
      and repairs of systems and equipments
                                                 Adequate            insulation of
Safeguard against Direct Contact                    live conductors
                                                        – Ensured by safe design and proper
with Live Electrical Parts                                inspection and maintenance
                                                        – Ensured by meeting appropriate
                                                          safety standards
                                                        – Stringent requirements in
                                                          Electricity (Wiring) Regulation
                                                   Always purchase/use electrical
                                                    equipment meeting appropriate
                                                    safety standards

  Adequate      Isolation                             Interlocking devices 連
   – Restrict access or contact by Enclosure/           鎖裝置
     Guarding/ Barrier                                   – Normally installed at
                                                           access doors for dangerous
                                                         – Fool-proof device to
                                                           ensure electricity supply is
                                                           cut-off with the device is
                                                           activated (when the door is
                                          Hazardous Scenarios
 Isolation by Distance
     High voltage O/H Power Lines

                                    Common Safeguards
Safeguards against Ground Fault
                                     Grounding/Earthing
                                     Automatic Protection
       Leakage of Electricity           – Residual Current Device (RCD)
                                       Double Insulation
                                        – Safe design for electric hand tools
Grounding/ Earthing
                                                                    How grounding works
  Indirect contact with electric current

                          Current                                                                        Current
                          leakage                                                                        leakage
                             Touch Voltage = 220V
                                    Current passing thru’ body
                                    is high enough to cause                        0.1 ohm
                                    serious injuries.

                                                                                 Earth Wire

                                                                     Proper Wiring
                                                                 Earth wire
                                                                              3-Pin Plug               Earth wire must be
                                                                                                        securely connected for
                                                                                                        providing earth
                 Important:                                                                             protection

 Ensure that Earth Connection is                                                              3-wires are required to provide earthing


                                                                                              System               Equipment
    Proper Wiring

   Cable must be firmly                                                                                Connections at
                                                                                                     terminals bear all the
    gripped to release strain                                                                         pulling force during
    from the conductor wires
    – 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
                                                                 Metallic conductive surfaces may be
                                                                  electrically charged accidentally
                                                                   – e.g. Work benches on which electrical equipments
           Loose earth wire                                          are placed
           touches the live
           terminal           Metal case
          Flexible cable      charged
          not firmly
                                        Conductive surface

  Double Insulation
                                                                Insulated surface

       Safety Standard
                                             Class I                  Class II
Portable Electrical Equipment
                                Protection by grounding      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
                                  – Ground Fault Circuit Interrupter
Residual Current Device       Residual Current Device
Schematic Diagram


                                  Fuse Protection

                                                                              Test Button

                              General Safety Practices
                                 Only authorized, competent, and qualified (e.g. by
                                  training) persons are allowed to work on or around
                                  electrical equipment and/ or wiring.
        Safe Work Practices        – Required by Electricity (Registration) Regulation
     General Safety Practices                                    General Safety Practices
   Purchase up-to-standard electrical                         Use lower voltages
    equipment equipped with appropriate                         – 110 volts or lower
    protective devices.                                         – Suitable for lighting
    – Required by Electrical Products (Safety) Regulation       – Need a step-down device

     General Safety Practices                                    General Safety Practices
                                                               Proper maintenance of system and
        Proper use of electrical equipment (not to             equipment
         interfere with protective devices)
     Emergency Procedures                                      First Aid for Electrocuted Victims
   Emergency procedures in the event of an                 Electric shock
    accident                                                 victim MUST be
                                                             removed from
    – Equipment emergency shutdown procedure,
                                                             contact with
      e.g. power cut-off switch can be easily reached.       electricity by safe
    – Electric shock first aid procedure                     means before doing
         » The need for first-aid training                   any necessary first
                                                             aid treatment.

     First Aid for Electrocuted
        CPR 心肺復甦
          – Should be performed
            by a competent                                   Specific Safety Procedures
Specific Safety Procedures                   Case Study
                                                       (5.5 minutes)
   Electrical/Mechanical System

 Lockout / Tagout Procedures

Lockout/ Tagout Arrangement                  When Lockout/Tagout Procedure is
Purpose:                                      Power cut-off point is remote from the
 To avoid inadvertent release of energy       equipment
  (electrical and / or mechanical) causing    Multiple power supply sources
  serious harm to people working on the       Work is prolonged
  system by:                                  Work involves multiple work teams
    – Effective isolation of power supply.
    – Clear notification.
    Steps for Lockout/Tagout Procedure           Steps for Lockout/Tagout Procedure
   Plan the shut down of the system.           Have all teams/workers place
   Alert operator and other users of the        their personal & individual
    shut down.                                   padlocks on the lockout point.
   Lockout the power supply to the system      Put a warning tag at the
    at the most appropriate point(s).            lockout.
                                                Release all stored or residual
                                                 energies (e.g. capacitors, loaded
                                                 spring etc.)
                                                Test the circuit to confirm it is

    Steps for Lockout/Tagout Procedure
       Each team/worker
        should remove only his
        own padlock upon                                    Safeguard against
        completion of his part
        of work.                                              Stored Energy
     Hazard of Stored Electrical Energy          Capacitors

       Electrical energy still be there after   – Store up electrical energy
                                                 – Electrical energy still
        power cut out – Stored Up.Up               remains after power cut out
       Depending on amount of energy stored     – Energy dissipates very
        up -- can cause painful shocks or kill   Precautions:
        people                                   – Hazards must be identified
                                                   and proper safety
                                                   procedures established, e.g.
                                                   proper discharge and

      Stored Energy
    Batteries can hurt
    – Can cause fire and
      explosion when
                                                 Explosion Hazards
Common Causes for Fire & Explosion    Common Causes for Fire & Explosion
   Cable with insufficient size       – Abused use of adaptors and
       Effect of excessive current       extension socket boards

Common Causes for Fire & Explosion    Common Causes for Fire & Explosion
    – Excessive heat due to             – Poor electrical contacts
       » Poor/loose connections           » Plugs are loosely fitted to socket (poor electric
                                            contact) -- esp. for two pin plugs
       » Improper wiring
       » Substandard plugs/adaptors
Over-current Protection                                      Working in Hazardous Environment
                                                          Sparks generated by
   Use conductor of                                       certain electrical tools
    sufficient size                                        and equipment, e.g....
    – Specified in Wiring                                  motor, plugging and
      Regulations                                          unplugging –
                                                          Especially in
   Fuse Protection                                        hazardous

        MCB Box

Avoid Generation of Sparks in
Hazardous Atmosphere
   Use spark proof / intrinsically safe
    equipment and installation in
    hazardous areas where there are:
                                                                    Legal Requirements
    – flammable liquids;
    – combustible liquids operating at a
                                                                         Legislative Controls
      temperature above their flash point; or
    – gases or combustible dusts that may be
      present in flammable, explosive and
      combustible concentrations.
   Legal Requirements                                                    Electricity (Wiring) Regulations

       Electricity Ordinance Cap. 406
                                                                            Enacted in 1992
         – Electricity (Wiring) Regulation                                  Code of Practice
            » Code of Practice                                               – The Golden Rules for
         – Electricity (Registration) Regulation                               the trade
                                                                             – Provide
         – Electrical Products (Safety) Regulation                             comprehensive
         – Electricity Supply Lines (Protection)                               practical guidelines
                                                                               to help REC/REW to
           Regulation                                                          do their jobs
         – Enforced by EMSD                                                  – Revised in 2003
                                                                             – Downloadable free

   Electricity (Registration) Regulation                                 Electrical Products (Safety) Regulation
Requirements and procedures for registering:
 Electrical Workers                                                      Ensure the safety standards of electrical
 Electrical Contractors                                                   products designed for domestic use,
 Ensure competence of people working on electrical
                                                                           including plugs, sockets, adaptors and
 Registration Classes:
                                                                           extension boards.
   –   Grade A : Low voltage < 400A                                       Ensures safe equipments/appliances
   –   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 Supply Lines (Protection)   Factories & Industrial Undertakings
    Regulation                              (Electricity) Reg.
   Protect electricity
    supply lines from
                                              Complementary to Electricity (Wiring)
    damage by construction                    Regulations.
    works (e.g. road work)                   Safety requirements on electrical
   Risk assessments                          installations and use of electrical tools
    required before work                      and equipment specifically applicable
    starts.                                   to all industrial undertakings.
    – Conducted by competent
      persons                                Enforced by Labour Department
    – Detection of cable

                                            Relevant Standards for Electrical
                                               BS 415 -- Safety requirements for mains operated
                                                electronic and related apparatus for household and
        Standards and Codes                     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, socket-
                                                outlets and socket-outlet adaptors
                                               BS 1363 -- Specification for 13A fused plugs and
                                                switched and unswitched socket-outlets

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