Transcript 5.Safe Work Practices Preventing Electrical Hazards

Principle of Engineering
Heating effect and magnetic effect of current.
Electrostatic hazards and electrical safety
Electricity Session 4 (2 hours)
Magnetism Force
• North and South Poles of Magnet磁石,磁鐵;
磁體
• Attracts & oppose other magnets
• Opposite Poles Attract
• Like Poles repel
• Attracts certain metals such as iron,
nickel, and cobalt.
• Exploration: hands on experimentation
Magnetic Field Pattern
• Magnetic field
pattern can be seen
using iron filings
• a magnetic field – a
region in space where
each point influenced
is influenced
magnetically.
Mapping the Magnetic
Field
• Place a rare-earth
magnet on square
paper
• Exploration: Place the
compass on different
regions of the square
paper and record
direction of needle in
terms of arrows.
Magnetic Field Pattern of
Attracting/Repelling Magnets
Types of Magnets
• Permanent Magnet
• Electromagnet
– Coil wound on iron core
– Field strength α current i
– Field strength α no. of turns N
• Exploration: To test the effect
of core, current and number of
turns on Electromagnetic field
strength by winding a coil
with/without core and use it to
attract / repel a hanging
permanent magnet (taped under
a table or workbench)
Magnetic Effect of
Current
• Magnetism and
current are related
• Exploration: Run a
wire (should have a
straight portion at
least 8 inches long)
through a square
paper and plot the
magnetic field
pattern
Force on Current in a
Magnetic Field
• Circular magnetic field by current
interacts with external magnetic
field  force
• Electrical  mechanical energy
conversion
• Exploration: use a straight thin
wire and pass a 0.5-1 A current
through it. Put a magnet near it
and experience the attraction and
repulsion. Verify the right hand
MOTOR rule. What is the effect
of a larger current?
Application
• Wire loop in
magnetic field 
Motor
• More turns  coil
 stronger force
Disassemble a
speaker to see
how it works
Current Induced by Motion
in Magnetic Field
• Electromagnetic induction 感應電磁
• Motion produces current
• Mechanical  electrical energy conversion
Direction of
motion of
wire
S
N
Direction
of induced
current
Electromagnetic Induction
Application: Generator
• Exploration: connect a generator (which is
really a toy motor), preferably in a gear box,
to another motor. Rotate the generator to
drive the other motor to move. Alternatively
use the generator to light up an LED.
Electromagnetic Induction:
Moving Coil in Magnetic Field
• Moving Coil in magnetic field generates
currrent
• See flash animation in
http://www.bbc.co.uk/schools/gcsebitesi
ze/physics/electricity/electromagneticind
uctionrev2.shtml
• Demonstration (TY only): show to the
students the rotating magnetic wheel
project that can be borrowed from C218
Electromagnetic induction
application: flashlight
• Flashlight without
battery: the “shake
light”
• Magnet shaken in &
out of coil/solenoid
Electrostatic hazards
• Many people ask about shocks experienced
when they touch the door, filing cabinet,
lift, or other metal object
• Daily Life experiences:
– Move aluminum can with balloon charged up by
rubbing balloon with cloth
– Plastic comb and hair
– Plastic bag strips rubbed together repelling
– Rubbed plastic ruler and paper/aluminum foil
Electrostatic hazards
See Structure of Matter first
• Matter 物質 composed of
Molecules 分子
• Molecules composed of
Atoms 原子
• Structure of Atoms:
electrons (- charge) 電子,
nucleus: protons (+
charge) 質子, neutrons
中子
Electrostatic hazards
• Static electricity 靜電 is generated
whenever two materials are in contact with
each other.
• All materials are made of electrical
charges in the material atoms. In the
universe there are equal amounts of
negative electrical charge (electrons) and
positive charge (protons). These generally
try to stay in balance of equal amounts at
every location.
Electrostatic hazards
• However, when two materials are in contact,
some of the charges redistribute by moving
from one material to the other. This leaves
an excess of positive charge on one material,
and an equal negative charge on the other.
• When the materials move apart, each takes
it's charge with it. One material becomes
charged positively, and the other negatively.
Material becomes charged
positively, and the negatively
Rub a plastic sheet  the
sheet becomes positively
charged
Rub a rubber sheet  the
sheet becomes negatively
charged
Electrostatic hazards
• If the materials are able to conduct electricity
away the charges will dissipate and eventually
recombine.
• In this case, static electricity effects may be
too small to be noticed.
• However, if the charges are separated faster
than the material can dissipate them, the
amount of electrostatic charge builds up.
• Eventually a high voltage, and the effects of
static electricity, may be noticed.
Electrostatic hazards
• If you experience static shocks while
working in an area where flammable
atmospheres (solvent vapours or dust
clouds) might be present, seek advice
immediately. There may be a fire or
explosion risk.
Electrostatic hazards
• Electrostatic charging has frequently
caused:
• Fires and explosions
• Disruption of production lines
• Degradation of products
• Equipment malfunction, computer
downtime
• Electrostatic shocks to
personnel
Electrostatic hazards
• Static charge build-up is enhanced when
the air is dry. So, static problems and
effects are often noticed in dry air
conditions.
• I get shocks when I'm sitting, or get up
from the chair - and I haven't walked
anywhere! Why?
Electrostatic hazards
• When you sit in a chair the contact between
your clothes and the chair can generate a lot of
electrostatic charge on your clothes. While you
stay in contact with the chair your body voltage
stays low. If you lean forward so you back
moves away from the chair back, or if you get
up out of the chair, then you take the
electrostatic charge with you. Your body voltage
can rise very rapidly to a high voltage as the
charge is separated from it's counter charge on
the chair.
Electrostatic hazards
• Are static shocks a health risk?
Electrostatic hazards
• Fortunately there is little risk attached
to such electrostatic discharges. In most
cases they are just a common
nuisance. The biggest risk is that a shock
could cause you to have an accidental
injury. For example, you might withdraw
your arm suddenly and hit it against
something.
Frictional Charges
Rub a plastic sheet  the
sheet becomes positively
charged
Rub a rubber sheet  the
sheet becomes negatively
charged
Frictional charges
What if two balloons were
rubbed and placed together?
+
What if two rulers were
rubbed and placed together?
- - - - -
What if a balloon and a
ruler were rubbed and
placed together?
+
Van de Graaff generator
• Provide a large and continuous supply of
charge
• A Charge Pump
• A Charge Separator
How does
it work!?
Principle of Van de Graaff
generator
Electrical Safety
Electrical Shocks Occur
-> People injury or dead
執波而觸電男童送院時面呈紫黑，口吐白沫。
Electrical Safety
• The effects of electric shock
depend upon the type of circuit,
its voltage, resistance, current,
pathway through the body, and
duration of the contact.
Electrical Safety
• Effects of Electric Current in the Human Body
Current Reaction
• 1 Mill ampere - Perception level. Just a faint tingle.
• 5 Milliamperes -Slight shock felt; not painful but disturbing.
Average individual can let go. However, strong involuntary
reactions to shocks in this range can lead to injuries.
• 6-25 Milliamperes (women) - Painful shock, muscular control
is lost.
• 9-30 Milliamperes (men) -This is called the freezing current
or "let-go" range.
• 50-150 Milliamperes - Extreme pain, respiratory arrest,
severe muscular contractions. * Individual cannot let go.
Death is possible.
• 1,000-4,300 Milliamperes - Ventricular fibrillation. (The
rhythmic pumping action of the heart ceases.) Muscular
contraction and nerve damage occur.
• Death is most likely.10,000 – Milliamperes Cardiac arrest,
severe burns and probable death.
Preventing Electrical
Hazards
1.
2.
3.
4.
5.
6.
Insulation
Guarding
Grounding
Circuit Protection Devices
Safe Work Practices
Training
Preventing Electrical
Hazards
1. Insulation
One way to safeguard individuals from
electrically energized wires and parts is through
insulation. An insulator is any material with high
resistance to electric current. Insulators—such
as glass, mica, rubber, and plastic—are put on
conductors to prevent shock, fires, and short
circuits.
Preventing Electrical
Hazards
2.Guarding
Live parts of electric equipment operating at 50 volts or more
must be guarded against accidental contact. Guarding of live parts
may be accomplished by:
•
•
•
•
location in a room, vault, or similar enclosure
use of permanent, substantial partitions or screens
location on a suitable balcony, gallery, or platform elevated
elevation of 8 feet (2.44 meters) or more above the floor.
Entrances to rooms and other guarded locations containing
exposed live parts must be marked with conspicuous warning signs
forbidding unqualified persons to enter.
Preventing Electrical
Hazards
3.Grounding
The term "ground" refers to a conductive body, usually the earth,
and means a conductive connection, whether intentional or
accidental, by which an electric circuit or equipment is connected
to earth or the ground plane.
By "grounding" a tool or electrical system, a low-resistance path to
the earth is intentionally created. When properly done, this path
offers sufficiently low resistance and has sufficient current
carrying capacity to prevent the buildup of voltages that may
result in a personnel hazard.
This does not guarantee that no one will receive a shock, be injured,
or be killed
Preventing Electrical
Hazards
4.Circuit Protection Devices
Circuit protection devices (fuses, circuit breakers,
and ground-fault circuit interrupters) are designed to
automatically limit or shut off the flow of electricity
in the event of a ground-fault, overload, or short
circuit in the wiring system.
Fuses and circuit-breakers are over-current devices
that are placed in circuits to monitor the amount of
current that the circuit will carry. They automatically
open or break the circuit when the amount of current
flow becomes excessive and therefore unsafe.
Preventing Electrical
Hazards
4.Circuit Protection Devices
Fuses are designed to melt when too much current
flows through them. Circuit breakers, on the other
hand, are designed to trip open the circuit by electromechanical means.
Fuses and circuit breakers are intended primarily for
the protection of conductors and equipment.
Preventing Electrical
Hazards
5.Safe Work Practices
Employees and others working with electric
equipment need to use safe work practices.
These include: deenergizing electric equipment
before inspecting or making repairs, using electric
tools that are in good repair, using good judgment
when working near energized lines, and using
appropriate protective equipment.
Preventing Electrical
Hazards
6. Training
To ensure that they use safe work practices,
employees must be aware of the electrical hazards to
which they will be ex-posed. Employees must be
trained in safety-related work practices as well as any
other procedures necessary for safety from
electrical hazards.