12/20/2012
The Hong Kong University of Science and Technology
Health, Safety and Environment Office
Electrical Safety
Dr. C. M. LI
Senior HSE Manager
Electrical Safety
Electrical Hazards
-
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
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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
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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
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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
------------1000 ohms
= 220 mA (a very dangerous current)
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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!
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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.
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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
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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.
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Safety Devices
Fuses and circuit-breakers are normally employed as
protecting devices for ground fault, overloading or short
in the wiring system. Their proper functioning
automatically open or break the circuit in the event
excessive current flow.
circuit
circuit
would
of an
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.
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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.
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Safe Use of Electricity
Safe Use of Plugs:
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.
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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
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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.
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