Chapter 3: Electrical Knowledge Flashcards
What are the four basic properties or units of electricity?
- electrical potential or force (voltage)
- current
- resistance
- power
How are the basic units of electricity expressed?
- volts (voltage)
- amperes (current)
- ohms (resistance)
- watts (power)
volt
a unit of electrical force that equates to pressure; roughly analogous to psi (pounds per square inch) in a water pipe or hose; one volt is required to push one ampere of current across one ohm of resistance
Voltage =
Current X Resistance
How is voltage expressed?
in 1,000-volt units or kilovolts (kV) [a 345,000-volt transmission line would be identified as a 345-kV line]
What are most residential use voltages?
generally limited to 120 volts for lights, outlets, and so on, and 240 volts for air conditioners, electric heaters, and kitchen ranges; ordinary household voltages can be fatal!
How many volts do primary distribution lines generally carry?
between 2.4 and 34.5 kV
How many volts do transmission lines carry?
as high as 765 kV
ampere
amp; a measure of electrical current or movement through a conductor
electrical current
the flow rate of electricity in a wire measured in amps similar to gallons per minute when measuring water flow
What is the amperage of a circuit?
it varies; depends on the demand on the given circuit; as more lights and appliances are turned on in your house, the amperage or current flow is increased
How many amps do most households have?
most households are limited to about 60 to 200 amps
How many amps do most circuits have?
individual circuits are limited to between 15 and 30 amps by circuit breakers or fuses, which protect the system from excessive current flow
ohm
a standard unit of resistance to the flow of electric current; equal to the resistance of a circuit in which a force of one volt will maintain a current of one amp
What are insulators made of?
materials with high resistance and thus are poor conductors
watt
a measure of power
How is wattage expressed?
in kilowatts (kW), which is the equivalent of 1,000 watts; 1 kW is approximately 1-1/3 horsepower
Ohm’s Law
describes the relationship between voltage, current, and resistance; Volts = Amps X Resistance or V = I X R
Volts =
Amps X Resistance; or V = I X R; where V = voltage measured in volts, I = current measured in amperes, and R = resistance measured in ohms
What is the relationship between wattage or power (P), voltage, and current?
Power = Volts X Current, or P = V X I
Power =
Volts X Current; or P = V X I
conductivity
the capacity of material to transmit electricity
electrical potential
the force or voltage of electricity roughly analogous to pounds per square inch in a water pipe or hose; the condition that allows electricity to flow from one object to another
When a potential difference or “potential” exists, where will electricity flow?
electricity will flow from the point of higher voltage toward that of lower voltage along a conductive pathway if it exists
electrical resistance
determines how much energy is required to move electricity along a conductive pathway; measured in ohms
What does high electrical resistance mean?
materials with high electrical resistance require more energy and cause more heating of the conductor than do materials of low electrical resistance
What are examples of materials used as electrical wiring?
aluminum and copper; because they have very low electrical resistance
What does low electrical resistance mean?
very conductive
What are examples of materials used as insulators?
porcelain, epoxy, and fiberglass; because they resist the flow of electricity
What does high electrical resistance mean?
non-conductive
What are insulators intended to do?
insulators are intended to prevent the flow of current
What happens if the potential difference is high enough relative to the size of the insulator?
the current can arc or flash over the insulator; in this situation the air actually provides the conductive pathway
Why do higher voltage lines require larger insulators or longer insulator strings and greater distances between phases?
because the air can provide a conductive pathway and flashover can occur
How do environmental conditions play a role in the likelihood of a flashover?
more flashover potential occurs during periods of high humidity; flashover is also common when the humid air contains a high amount of particulate matter, as might be experienced during a brushfire or dust storm
powerhouse
the generating plant that is the source of most power or energy distributed throughout the United States
Describe a typical electrical system layout
- generating station (13,800 volts generated)
- transmission substation (voltage increases to 345,000 volts)
- industrial customer or to another transmission substation
- first voltage reduction (69,000 volt distribution)
- substation - power center industrial plant
- second voltage reduction (13,800 volt distribution)
- commercial or industrial customer
- distribution transformer
- residential customer
What are the most common sources of energy?
fossil fuels, water or wind turbines, and nuclear reactors
What voltage is most electric energy generated at?
a range of 13,200 to 24,000 volts; then it is raised to transmission levels in a substation located in the generating station
transmission lines
conductors used to transmit electricity from the generating station to the distribution network that usually carry voltages in excess of 100,000 volts; carry the extra-high-voltage electric energy from the generating plant
What is the voltage of transmission lines?
voltages range from 100,000 to 765,000 volts
transmission substation
decrease the voltage from transmission to subtransmission voltages
subtransmission lines
transmit the electricity from the transmission substations, through metropolitan areas, to distribution substations located in the area of the load to be served
What is the voltage of subtransmission lines?
voltages range from 22,000 to 161,000 volts
distribution substation
reduces the voltage from subtransmission voltages to primary distribution voltages
distribution lines
electricity lines that carry the power that is to be delivered to customers
primary lines
distribution lines that carry voltage ranging from 2,400 volts to 34,500 volts
What is the voltage of primary distribution lines?
voltages ranging from 2,400 to 34,500 volts (2.4 kV to 34.5 kV)
Who receives electricity from primary distribution lines?
commercial or industrial customers and residential customers
What level voltage are 22,000 and 34,500 volts?
can be either subtransmission or distribution
What is the difference between distribution and subtransmission lines?
distribution lines serve the end user while subtransmission lines terminate at a distribution substation
distribution transformer
device located on poles that reduce the voltage supplied from the primary circuits to other parts of the circuit
transformer
a device that either raises or lowers voltage between circuits; can increase or decrease voltage depending on direction of current flow
secondary lines
circuits that carry lower voltages from distribution transformers to the point of use such as streetlights or residential properties; originate at the distribution transformer and extend along rear-lot lines, alleys, or streets
secondary service wires
lower-voltage lines, generally 120 to 480 volts, run between the transformer and point of use
services
power lines that extend from the secondary or distribution transformer to a customer’s property
What is the voltage of secondary lines?
generally 120 to 480 volts
Where do secondary lines typically run?
along alleys, along rear-lot lines and streets, and past residences
service drops
service wires that run from the secondary wires or distribution transformer to a residence
What is the distribution system?
the distribution system connects the transmission system with the end user; consists of distribution substations, distribution feeder circuits, distribution equipment, and secondary and service lines
feeder circuits
circuits that feed electricity to other parts of the distribution network
Where are distribution substations located?
relatively close to the end user
distribution feeder circuis
originate at the terminals of a circuit breaker or a circuit re-closer in the distribution substation; often referred to as distribution main-feeder or express-feeder circuits; serve as the source to primary or branch circuits
protective equipment
safety devices that protect the distribution system; designed to shield the system from faults, short circuits, and current surges by de-energizing the system close to the source of failure; protects electrical equipment and minimizes the number of customers impacted by the resulting interruption
faults
interruptions to the electricity supply; a lower resistance-to-ground path created by a tree, person, or other obstacle coming into contact with a phase (one line of a circuit)
short circuits
an interruption in an electric current in which the electricity finds a different path to ground
current surges
a momentary excessive increase in the amount of electricity flowing through a wire
What is the primary protective equipment for the distribution system?
the circuit breaker in the substation
Where is the circuit breaker located?
in the substation
How are circuit breakers activated?
they are activated by protective relays that respond to overcurrent conditions such as faults or short circuits on a circuit or line
What does an open circuit breaker do?
de-energizes the entire distribution feeder circuit and will affect many customers
What are examples of protective devices?
circuit breakers, fuses, and re-closers
fuse
device on the conductors that respond to sudden increases in voltage by cutting the power supply; protective device that protects laterals connected to feeders; installed on circuits to isolate branch primary circuits and equipment, protecting the distribution feeder circuit; ideally selected so they are large enough in size to not limit normal load capacity but still protect the circuit during overcurrent conditions; interrupts the current whenever there is a short circuit or an overload
What does a blown fuse do?
de-energizes the circuit and interrupts service to its end users; eliminates the need for the substation circuit breaker to open and minimizes the number of customers affected; the cutout falls open, creating a break in the line, which de-energizes the circuit from that point forward until another fuse is installed
lightning arrestors
devices placed on poles and towers designed to attract lightning and take it to the ground; pole-mounted devices that are connected to the phase conductors of the circuit designed to redirect electrical surges to the ground instead of allowing them to continue down the circuit; channels lightning, or the excessive current it causes, to the ground by a wire that leads directly to a copper rod at the base of the pole
What is the purpose of lightning arrestors?
installed to minimize customer service interruptions; redirect electrical surges to the ground instead of allowing them to continue down the circuit
primary circuits
circuits that carry the highest voltage to other parts of the distribution network; normally one, two, or three-phase lines that connect to the main-feeder distribution circuits through fused cutouts
cutouts
the part of the fuse that falls open, creating a break in the line when an overload or short circuit occurs
What voltage are residential customers normally provided?
120 or 240 volts
What voltage do commercial, light industrial consumers, and large industrial consumers usually provided?
voltages ranging from 120 to 480
back feed
a process whereby electricity is fed back into downed lines, usually from a home generator that can re-energize the lines; the voltage can be raised if it passes through a transformer; flow of electricity in an unintended direction
How does back feed normally occur?
when a customer is using a generator during an outage and a line is down; the back-fed voltage could be “stepped up” or increased by a transformer and pose a significant hazard to anyone thinking the lines are de-energized
How are secondary lines normally mounted?
often three-wire single-phase circuits, mounted vertically below the primaries or in a bundled cable
How are services generally consisted?
generally triplex cables consisting of two coated aluminum conductors and a bare aluminum steel-reinforced neutral wire that provides physical support for the conductors; older construction is three separated conductors in an “open wire” configuration
What are distribution poles made of?
most distribution poles are wooden, but other materials such as aluminum, steel, fiberglass, and concrete are also used
What are high visibility warning signs for?
used by utilities on poles to caution unauthorized individuals about the dangers above/on a pole that provides access to high voltage equipment
Copper v. Aluminum for electric wires
Copper is a better conductor but heavier and more expensive
Aluminum is lighter and less expensive but has a tendency to stretch
ACSR
Aluminum Conductor Steel-Core Reinforced; aluminum wire with a steel-reinforcing core; steel core is used to add strength since aluminum has a tendency to stretch
What is the most common type of wire used today?
aluminum wire with a steel-reinforcing core (ACSR); copper was the predominant material used for conductors in the past
weatherproofing
cover over most wires that does not insulate the wire; deteriorates and hangs down on older construction
tree wire
heavily coated wire; provides a level of resistance that is greater than bare or weatherproof-covered wire; resistance helps protect the circuit from cross-phase faults through tree branches
How should all wires be considered?
all overhead wires should be considered uninsulated; even if tree wire is covered, it should still be considered uninsulated; all overhead and underground electrical conductors and all communication wires and cables should be considered energized with potentially fatal voltages
phase
an energized conductor or wire in a circuit
energized conductor
conductors through which electricity is flowing
How many phases do single-phase and three-phase circuits have?
single-phase circuit has only one wire conducting electricity; three-phase circuit has three energized phases or conductors
neutral wire
ground wire; generally accompanies the primary distribution phase conductors; might or might not be mounted on insulators; provides additional lightning protection
Where is the distribution neutral wire located?
typically beneath the s or at the very top of the pole above the conductors; it can also be at the same level as the phase conductors; can be hard to tell apart from other phases
phase-to-phase
the potential between live conductors or between live and neutral conductors
phase-to-neutral
the potential between live conductors and the neutral wire
Wye construction
construction of most multiphase lines consisting of three phases and a ground neutral wire
Describe voltage differential among phases
phases have polarity creating a voltage differential among phases; voltage difference between any two phases is higher (approximately 1.73 times) than the voltage difference between a phase and the ground or neutral wire
Delta construction
construction type that has three phases but no ground or neutral wire
What construction type is most common?
Wye construction is more common; Delta circuit is occasionally used
guy wires
steel or wire ropes used to support a pole; supporting wires that are not intended to conduct electricity; generally very strong, galvanized steel wires that brace the pole to provide additional strength
Where are guys often used?
where a line turns or dead-ends in areas of high winds or in other situations where higher than normal stress is anticipated
guy covers
objects installed near the ground on guy wires to increase their visibility and reduce the hazard of someone accidentally walking or driving into them; high visibility cover
Are guy wires conductive?
yes; although not intended to conduct electricity, guy wires are conductive and could become accidentally energized by direct or indirect contact with an energized wire
Where are distribution transformers located?
most often mounted on a distribution pole; commonly mounted on concrete pads on the ground too
phase to ground
the potential between live conductors and the ground
re-closers
fuses that open in response to a momentary surge in voltage and close again if the surge subsides in a short time; designed to reset or re-close the circuit after a preprogrammed period of time
What are re-closers intended to do?
intended to prevent extended service interruptions resulting from instantaneous or otherwise temporary short circuits
voltage regulator
a device used to keep the voltage within conductors at a constant level within defined tolerances; used whenever it is necessary to eliminate objectionable variations in the voltage level of a supply circuit; acts like a transformer, either increasing or decreasing the circuit voltage as needed
capacitors
devices that can store energy and are used to either boost voltage or provide power factor correction; a group (usually one bank per phase) mounted on electric power lines
Are telephone and cable TV wires part of the distribution system?
No; they are commonly found attached to the distribution system, but they are not part of it
Are telephone and CATV wires intended to carry current?
these wires are intended to carry either no current, or at most, a very low-voltage current; they have the potential, however, to carry higher voltages if accidentally energized by higher-voltage lines; normally carry only minimal electric current or none at all, but have the potential to carry very high voltages if they are accidentally energized through contact with higher-voltage lines
How does electricity travel?
electricity will travel along all paths but takes the shortest path or the one of least resistance to the ground
phase-to-phase fault
a fault when current flow occurs between two energized conductors
phase-to-neutral fault
a fault when current flow occurs between an energized conductor and a neutral wire
step potential
voltage between the feet of the person standing near an energized, grounded object
Who enforces various regulations to prevent injuries related to the operation and maintenance of electrical power generation, transmission, and distribution lines and equipment?
The U.S. Department of Labor Occupational Safety and Health Administration (OSHA) and state partners
29 CFR 1910.269 Electric Power Generation, Transmission, and Distribution
federal regulation that covers employee safety and certification, prejob briefings, electric lockout and tag-out procedures, and proper handling and storage of tools and equipment
Who has established safe working standards to further protect employees and prevent injuries?
state agencies, federal agencies, and organizations such as the American National Standards Institute (ANSI)
Can OSHA compliance officers reference ANSI standards even though they are not law?
Yes, ANSI standards can have the weight of law and OSHA compliance officers can reference them in citing a safety violation in tree care operations; if an employer does not comply with recognized industry standards the employer could be considered negligent in a court of law
What does ANSI Z133.1-2000 require that every tree worker must be instructed of?
- when direct contact is made
- when indirect contact is made
- how indirect contact is made
- how electric shock occurs
- minimum approach (working) distances
direct contact
when any part of the body touches or contacts an energized conductor or other energized electrical fixture or apparatus
indirect contact
when any part of the body touches any object in contact with an energized electrical conductor or other energized fixture or apparatus
How can indirect contact be made?
through conductive tools, tree branches, trucks, equipment, or other conductive objects, or as a result of communication wires and cables, fences, or guy wires being accidentally energized
How does electric shock occur?
occurs when a tree worker, by either direct or indirect contact with an energized conductor, energized tree limb, tool, equipment, or other object provides a path for the flow of electricity to a grounded object or to the ground itself; simultaneous contact with two energized conductors will also cause electric shock that may result in serious or fatal injury
qualified line-clearance arborists
utility arborists who have been trained to specified standards for line-clearance work; per ANSI Z133.1-2000, someone specifically trained and qualified to work safely at a minimum specified approach distance to electrical conductors
minimum approach distances from energized conductors for persons other than qualified line-clearance arborists and trainees
- 0-50.0 kV - 10 ft
- 1-72.5 kV - 10 ft 9 in
- 6-121.0 kV - 12 ft 4 in
- 0-145.0 kV - 13 ft 2 in
- 0-169.0 kV - 14 ft
- 0-242.0 kV - 16ft 5 in
- 0-362 kV - 20 ft 5 in
- 0-550.0 kV - 26 ft 8 in
- 0-800 kV - 35 ft
minimum working distances from energized conductors located between sea level and 5,000 ft for qualified line-clearance arborists and trainees
0.05-1.0 kV - avoid contact
1.1-15.0 kV - 2 ft 4 in
15.1-36.0 kV - 2 ft 9 in
36.1-46.0 kV - 3 ft
46.1-72.5 kV - 3 ft 9 in
72.6-121.0 kV - 4 ft 6 in
138.0-145.0 kV - 5 ft 2 in
161.0-169.0 kV - 6 ft
230.0-242.0 kV - 7 ft 11 in
345.0-362 kV - 13 ft 2 in
500.0-550.0 kV - 19 ft
765.0-800 kV - 27 ft 4 in
varies by nominal voltage and are adjusted by an elevation factor
ANSI 2000
minimum approach distances from energized conductors for persons other than qualified line-clearance arborists and trainees and minimum working distances from energized conductors located between sea level and 5,000 ft for qualified line-clearance arborists and trainees
What does the National Electrical Safety Code require?
the NESC requires wires carrying various voltages be placed in such a way as to ensure a minimum distance above ground is maintained; the higher the voltage, the greater the ground-to-conductor clearance required; requires that the highest-voltage wire be on top
ground-to-conductor clearance
the critical distance that must be maintained between energized (live) conductors and the ground
approach distances
the minimum distances that must be maintained between conductors and qualified line-clearance personnel and their tools; minimum working distance from energized conductors and arborists
What is the standard recognized minimum working distance?
the existence of an electrical hazard exists when a worker, tool, or conductive object is within 10 feet of an energized overhead conductor rated 50 kV, phase-to-phase or less; it increases at a rate of 4 inches for every 10 kV of additional voltage
When can non-qualified electrical tree workers work within the minimum separation distance?
only when the lines are de-energized and when they are grounded
How does elevation impact minimum working distances?
minimum working distances increase with higher elevation
failure modes
the two distinct causes of electrical outages
What are the two distinct failure modes of tree caused power outages?
mechanical and electrical
mechanical failures
involve tree-caused physical damage to conductors, electrical equipment, and/or supporting structures
electrical failure
occurs when a tree or branch comes into contact with energized conductors, causing a short circuit; event often opens the substation feeder breaker or blows a branch line fuse, causing an outage; when a tree or branch causes a fault (electrical short circuit) by creating an unexpected phase-to-phase, phase-to-neutral, or phase-to-ground current flow
What failure mode is most common?
mechanical failure is the most common cause of tree-related service interruptions; trees or tree parts falling on electrical facilities often cause mechanical failures
What poses the greatest risk of physically damaging the system?
trees overhanging the conductors or trees growing adjacent to the facilities
How do electrical failures occur?
when a tree part contacts the wires or electrical equipment and creates a new pathway for the electrical current to flow; also can occur when the tree pushes two wires together
What characteristics influence the likelihood of an electric fault causing a tree-related service interruption?
- voltage gradient (amount of voltage per foot of distance - kV per foot - determined by the voltage and the spacing of the lines; as it increases the risk of a tree-caused outage also increases)
- species of the tree and the diameter of the tree part creating the fault (larger-diameter branches pose greater risks)
overcurrent protection strategy of most utilities
- a set of fuses or re-closers protecting the substation; usually located on the three-phase primary distribution line relatively close to the substation
- series of fuses or re-closers along the three-phase primary distribution lines
- fuses or re-closers for each single-phase lateral or tap, which extends off the three-phase primary distribution line
- series of protective devices along the single-phase tap lines to the end of the line
natural order of importance for sections of the circuit
- feeder from the substation to the first protection device is the most critical because it feeds the entire circuit
- main three-phase line, from the first protection device to the end of the circuit, supplies all of the single-phase taps and therefore affects the second largest number of end users
- single-phase cutouts or re-closers are located where each single-phase lateral or tap extends off the three-phase primary distribution line and affect the third largest group of customers
- cutouts or re-closers along the single-phase tap affect a smaller number of customers and are of lesser importance
How does putting lines underground impact trees and tree health?
excavation associated with underground utilities can have a substantial, if not fatal, impact on tree health mainly due to the roots and degradation of the soil in which they are growing
What can happen when the roots of a tree are damaged?
- reduced tree growth (mechanism for supplying the aboveground portions of the tree with water and nutrients)
- cause death of branches or branch tips
- initiate a general decline in tree health
- expose the tree to disease or insect infestation
- reduce the stability of the tree and increase its susceptibility to windthrow (mechanical support)
- kill the tree
Where are the vast majority of roots located?
the vast majority of roots of most species are located within the top 18 inches of the soil; typically spread out from the base of the tree to a distance up to two times the tree height
tree protection zone
the area around the base of the tree that should be left undisturbed during trenching and tunneling operations
How to determine the size of the tree protection zone?
adjusted for the size of the tree (trunk diameter) independent of canopy information; optimal TPZ is based on species tolerance to disturbance (good, moderate, or poor) and tree age (young, mature, or overmature)
Guidelines for optimal tree protection zones for trees (Matheny and Clark 1998)
Species tolerance + tree age = distance from trunk in feet per inch of trunk diameter good + young = 0.5 good + mature = 0.75 good + overmature = 1.0 moderate + young = 0.75 moderate + mature = 1.0 moderate + overmature = 1.25 poor + young = 1.0 poor + mature = 1.25 poor + overmature = 1.5 value above x DBH = radius of TPZ from the trunk
How to calculate the optimal tree protection zone
- evaluate the species tolerance of the tree (good, moderate, or poor)
- identify the tree age (young - less than 20% of life expectancy, mature - 20-80% of life expectancy, overmature - greater than 80% of life expectancy)
- multiply the value (distance from trunk in feet per inch of trunk diameter) by the tree’s diameter (DBH) to determine the optimal radius (in feet) of the tree protection zone from the trunk
Where should trenches for underground service be in relation to trees?
should avoid trees by as much distance as possible and should always avoid the tree protection zone
trenching
digging to install utilities of concern due to root damage
backfill
filling the trench with material after excavation and placement of the utility line(s)
Where to store soil removed during trenching?
should be stored near the side of the trench that is away from the tree; minimize likelihood of disturbing root system with equipment used to backfill the trench or inadvertently leaving soil behind
When should trenches be backfilled?
as quickly as possible; small roots can dry out in 10-15 minutes if exposed to warm, dry air
What soil to use when backfilling?
use the soil removed from the trench
backfilling reminders
- don’t overcompact; the firmness of the backfill should be consistent with the original conditions; overcompacting will eliminate proper aeration of the soil (only a small layer of extra soil over the roots can change soil aeration and deprive the roots of necessary oxygen)
- don’t use the trench to dispose of leftover materials, concrete, gravel, sand, fluids, or trash
- do water the backfill to keep roots moist
How to minimize the damage to severed roots?
hand digging is preferable to machine trenching; exposed roots should be neatly cut with a sharp saw; cuts should be made parallel to the trench wall; if roots 2 inches or larger are torn during excavation they should be cleanly cut at an undamaged portion (roots that are torn or crushed by the digging operation are not likely to recover; sharply cut roots are more likely to produce new roots, helping the tree to recover from the damage)
tunneling
alternate means to trenching for installation of underground utilities
What is preferred in tunneling v. trenching?
tunneling is preferred over trenching; if done properly, tunneling will do almost no harm to the tree
When/where to tunnel?
trenching should stop at the tree protection zone and tunneling should begin at the edge; tunneling should begin as soon as the roots 1 inch in diameter are encountered by trenching operation; offsetting the tunnel from the center of the tree is preferred; 2 feet is the minimum depth for tunneling to minimize root damage, but recommended depth is greater (trees <8” - 3 ft; larger trees - 4 ft)
What should an emergency management plan contain?
- Priorities (safety; restoration of electricity)
- Notification and communication procedures (key contacts: utility, contractors, government agencies, press; centralized emergency coordination; chain of command)
- job-site procedures (equipment; access; response to avoidable tree-related emergencies; response to unavoidable tree-related emergencies)
- completion of work (ticket management; brush and debris management)
- documentation procedures (emergency call-out log; verification of completed work)
