ELECTRICAL SAFETY 1. Why electricity is dangerous?

advertisement
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
Download