Chapter 6 Flashcards
Electricity:
The flow of negatively charged particles called electrons through an electrically conductive material
Atom Structure
- nucleaus
- positively charged center (protons)
- neutral particles (neutrons)
- The electrons
- energy bands of orbiting negatively charged particles.
- Each ring of electrons contains a particular quantity of negative charges.
The path of electric current includes
- The source of electrical power,
- A conductor e.g. a metal wire to act as the path,
- A device (جهاز)to use the current (load), and
- A path( route) to the ground.
Zero potential (voltage)
- The earth (ground) is considered to have zero potential.
- A ground is a direct electrical connection to the earth, or a connection to a particular point in an electrical circuit.
Voltage [V]:
•The potential difference between two points in a circuit is called the voltage difference. It is measured in volts[V].
Current [I]:
•Electrical current is produced by the flow of electrons through a conductor. It is measured in amperes[A].
Resistance [R]:
•The impedance (try to stop) to the flow of electrons. The higher the resistance, the lower the current. It is measured in ohms[_O_].
Frequency [F]:
- The number of cycles per second. Most industrial and domestic users of electricity are supplied by alternating current [AC]. In the United States standard AC circuits cycle 60 times per second. It is measured in hertz[Hz].
- In many other countries 50 hertz
Ohm’s Law:
•describes the relationship between the voltage, current, and resistance (V=I R), (I=V/R), or (R=V/I).
Where:
V = voltage
I = current
R = resistance
Power [P]:
The heat produced by the circuit elements due to the flow of current through them. It is measured in watts[W].
P = V I = I2R
AC
Alternating current
an electric current in which the flow of electric charge periodically reverses direction
DC
direct current
•the flow of electric charge is only in one direction.
Most industrial and domestic use of electricity is supplied by
•alternating current [AC].
Because voltage alternates in AC current, an effective current for AC circuits is computed which is slightly less than peak (highest)current during a cycle
Effective current AC =
Peak current AC * 0.707
Effective voltage AC =
Peak voltage AC * 0.707
Potential Difference:
The potential difference between two points in a circuit is measured by voltage. The higher the voltage, the more electrical current will flow between the positive and negative points
Lightning
Lightning is static charges from clouds following the path of least resistance to the earth, producing very high voltage and current. Lightning tends to strike the tallest object on the earth below the clouds
Grounding
- Typically an electrical circuit wiring has a hot wire [red or black insulation] carrying current, a neutral wire [white], and a ground wire [green or bare].
- The ground wire is connected to the ground.
Open ground
- happens when the ground wire is connected improperly(may be connected to neutral wire instead of the ground).
- Usually the equipment with this wiring will operate normally.
If a short occurs in
the equipment circuitry (wiring) without proper grounding, anyone touching that equipment might be severely shocked and die.
Lightening rod
protects the building by diverting lightning to the ground via a conduction pathway
Lightning Hazard Control Measures
- Place lightning rods so that the upper end is higher than nearby structures.
- Avoid standing in high places or near tall objects. Be aware that trees in an open field may be the tallest object nearby.
- Do not work with flammable liquids or gases during electric storms.
- Ensure proper grounding of all electrical equipment.
- If you are inside a car, stay inside it.
- If in a small boat, lie down in the bottom of the boat.
- If in a metal building, stay in the building and do not touch the walls of the building.
- Wear rubber clothing if outdoors. Why?
- Avoid using a telephone during an electrical storm.
- Do not use electrical equipment during the storm.
- Avoid standing near open doors or windows where lightning may enter the building directly.
A short circuit
•the circuit load has been bypassed (not fully loaded)
Short circuits can be another source of electrical hazard if
•a human is the conductor to the ground, thereby bypassing the load, which may in most cases ends by killing him/her.
Techniques used to avoid electrical hazards
- Ionizers,
- Radioactive neutralizers,
- Antistatic Material, and
- Humidification
- Fuses
- Circuit breakers
Ionizers
•electrostatic neutralizers ionize the air surrounding a charged surface to provide a conductive path for the flow of electrons.
Radioactive neutralizers
•include a radioactive element that emits positive particles to neutralize collected negative electrical charges. Workers need to be safely isolated from the radioactive particle emitter.
Antistatic material
•have been used effectively to reduce electrical static hazards.
Humidification
is a method of reducing electrical static. Raising the humidity above 65% reduces charge accumulation
Electrical system grounding is achieved when
- one conductor of the circuit is connected to the earth
- Power surges and voltage changes are attenuated (weakened) and usually eliminated with proper system grounding.
Bonding
is used to connect two pieces of equipment by a conductor. Bonding can reduce the potential differences between the equipment and thus reduce the possibility of sparking
Grounding
•provides a conducting path between the equipment and the earth.
Bonding and grounding together are used for
entire electrical systems
Testing the electricity
- circuit tester
- receptacle wiring tester
- continuity tester
circuit tester
- A circuit tester is an inexpensive piece of test equipment with two wire leads capped by probes and connected to a small bulb.
- This simple tester can ensure that power has been turned off before electrical maintenance begins.
- When one lead makes contact with a hot wire and the other lead connects to the ground conductor, the bulb lights if there is power in the circuit
receptacle wiring tester
A receptacle wiring tester is a device with two standard plug probes for insertion into an ordinary 110 V (220 V in UAE) outlet and a probe for the ground. Indicator lights show an improperly wired receptacle [outlet].
is a device used to verify that an AC wall outlet is wired properly
continuity tester
- A continuity tester may be used to determine whether a conductor has a break in the circuit.
- Continuity is checked on circuits that are disconnected from the power source.
Freeze current
The current that freezes a person to the conductor; where the person cannot voluntarily release his or her grasp (10mA —> 50mA).
mA = miliamper
Let-go current:
The highest current level at which a person in contact with the conductor can release the grasp of the conductor (6 mA —> 9 mA).
Connections mistakes and hazards
- jumping the ground wire to the neutral wire.
- Reversed Polarity
Jumping the Ground Wire Hazard
- One common mistake is to jump the ground wire to the neutral wire.
- Equipment usually operates in a customary way, but the hazard occurs when low voltages are generated on exposed parts of the equipment, such as the housing. If the neutral circuit becomes corroded or loose, the voltage on the ground wire increases to a dangerous level.
Reversed Polarity
- With reversed polarity, the hot and neutral wires have been reversed.
- A worker who is not aware that the [hot] and [neutral] wires have been reversed could be injured or cause further confusion by connecting the circuit to other apparatus.
Warning devices
are less effective than designed-in safety precautions.
•It is better to design safety into equipment and system than to rely on human behavior such as reading and following the label.
Warning devices to alert personnel about detected hazards may include
- lights,
- colored indicators,
- on/off blinkers,
- audible signals, or
- labels.
Designed-in safety precautions such as
interlocks automatically break the circuit when an unsafe condition is detected. Elevator doors typically have interlocks to ensure that the elevator does not move when the doors are open
Electrocution:
(killing by electric shock)accounts for approximately 6% of all workplace deaths in the United States every year.
Establishing an effective electrical safety program
National Institute of Safety and Health [NIOSH] recommends the following:
•Develop and implement a comprehensive safety program and when necessary review or modify existing programs to address(cure, solve) thoroughly the area of electrical safety in the workplace.
Establishing an effective electrical safety program
•Ensure compliance with existing OSHA regulations.
- Provide all workers with adequate training in the identification and control of the hazards associated with electrical energy in the workplace.
- Provide additional specialized electrical safety training to those working with or around exposed components of electrical circuits.
- Develop and implement (apply)procedures to control hazardous electrical energy that include lockout and tag-out procedures.
- Ensure that workers follow these procedures.
- Provide testing or detection equipment for those who work directly with electrical energy that ensures their safety during performance of their assigned tasks.
- Ensure compliance (obeying, observing) with the National Electrical Code and the National Electrical Safety Code.
- Conduct safety meetings regularly.
- Conduct scheduled and unscheduled inspections at work sites.
