Everyone experiences a power failure now and then. But there are two things most people don't know about power failures:

1. Some power failures are not accidental.
2. Power failures can damage your equipment.
3.  

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These Frequently Asked Questions (FAQ) covers all aspects of power failures:

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Questions about power failures:

• What is a power failure?

  A power failure is a loss of power to any electrical service for more than a few power line cycles.

• What are the various causes of power failures?

  There are many different causes of power failures:

  • Failure of power distribution equipment
  • Weather damage to power distribution equipment
  • A lightning strike damaging power distribution equipment
  • A lightning strike causing power line circuit breakers or fuses to blow
  • An overload or a short circuit blowing a power line circuit breaker or fuse
  • A traffic accident knocking down a power pole, breaking or shorting power lines
  • A tree or tree branch falling onto power lines, causing a short circuit
  • A wet tree branch drooping into power lines
  • Water getting into a buried power line
  • A faulty (cracked) insulator shorting when it is wet
  • A climbing animal (e.g. a squirrel) getting across the terminals of a pole transformer
  • Failure of a substation transformer
  • Failure of a pole transformer
  • Failure of the high tension lines from the power station
  • Power sharing grid failure
  • A vehicle that is too tall blunders into power lines.
  • A child's aluminized (silvery) Mylar balloon or kite touching two power lines
  • A high wind blowing two power lines together
  • Disaster (flood, wildfire, hurricane, tornado, earthquake, landslide, etc)
  • Failure of your electric meter
  • Something else breaking power lines or causing a short circuit
  • Power company workers turn off power to change out a bad component
  • Power company workers turn off power to switch load to a different power source
  • Power company workers turn off power because of a hazard (e.g. a child climbed a pole)
  • Some other unusual cause
• How can a power failure not be accidental?

  Any time the power is turned off by deliberate action:

  • Power company employees deliberately turn off the power when:
    1. Power equipment must be repaired.
    2. Loading of power lines must be changed,
    3. Power must be removed because of a hazard,
  • Other people damage power lines on purpose.
• Why doesn't the power company warn people before workers turn off power, so they have time to save computer files and shut down sensitive equipment?

  If the work can be scheduled a day or more in advance and the area is small, tags can be hung on doorknobs. A wider area outage might be put on radio and in the newspapers. But most such interruptions are urgent, caused by equipment failure or sudden shifts in loads. Usually there isn't time for a warning.

• Why does the power have to be out for several seconds when changing a distribution line to a different power source? Can't they throw a switch for an instant transfer?

  They have to throw two manually-operated switches up on power poles, using long fiberglass poles. Usually the two switches are many blocks apart, and the crews can't see each other.

  First they must open the switch that is supplying the power to the circuit. Then the crew that opened the switch must radio the other crew that it is alright to close the other switch. Since the switches might be on different phases, both switches must never be closed at the same time.

• Why does the power often go off and come on again twice before it stays off?

  This is an automatic system that tries to remove the cause of the power failure through a programmed process. Often the cause is a tree branch, which burns away. Also, a short circuit on one distribution line might blow the fuse or breaker on that line on one of the tries, allowing the other lines on that circuit to come back up.

  If the cause is removed from the main distribution line, the automatic power switch stays on. But if the overload or short persists, the automatic sequence tries only two times. Anything beyond two times usually requires a crew to repair what is wrong.

  Note that if you are on the distribution line with the fault, your power may come up at a lower than full voltage.

• Why is it that when the power comes back on, it sometimes goes off and on several times in the space of a few seconds?

  This is a case of the worker manually closing the breaker switch with the fiberglass pole, making the switch touch the contact, or failing to connect it up several times.

  This kind of failure can harm sensitive equipment.

• Why is it that the power sometimes partially fails by having a lower voltage without going out?

  This is usually due to an overload not large enough to trip a pole breaker, or a wet tree on a power line that doesn't draw enough current to trip a breaker.

  This kind of failure can harm sensitive equipment.

• What is three-phase power?

  It is much more efficient if a generator produces three different phases of AC power. The alternating current waveforms of the three phases are 120 degrees apart so they are equally spaced in time (see diagram for 120V/208V 3-phase). When a large industrial motor is connected to all three phases, the three windings automatically produce a rotating magnetic field, efficiently driving the rotor.

  Less wiring is needed to connect three-phase power. And single-phase services can be connected to any of the three phases and a neutral (grounded) wire, as seen below.

• How can electric motors rotate without three-phase power?

  A single-phase electric motor produces the rotating effect through other means. A brush motor uses the position of the brushes on the commutator to always produce rotation in the same direction. An induction motor usually has either secondary poles on the stator and a second winding with a capacitor, or has shorted loops of wire around secondary poles on the stator core (called shading rings). Both of these make a rotating magnetic field by changing the timing of the maximum pull from parts of the stator. This rotating magnetic field works much like the rotating magnetic field from three phases.

• Why do most power poles have four wires on a crossbar at the top, and three more power wires partway down the pole?

  The four wires at the top are the three phases of distribution lines (usually around 24,000 volts to ground) plus a grounded neutral wire for the three-phase set. The image at right is seen from below, as someone standing on the ground would see it.

  The three wires below are the 120/240 volt service lines (two hot lines and a neutral) for connecting service drops to buildings. Pole transformers (gray cylinder) feed the lower wires from one of the upper wires and the neutral. Note the fuse/switch connecting the distribution lines to the transformer.

  A ground wire in a metal conduit runs down the pole, connecting the distribution neutral and the service neutral to a ground stake buried at the bottom of the pole.

  Below the service power lines (separated by a safety space) are the lines for cable TV and telephone. There are splice boxes next to the pole for each service. Notice a set of service drops for power, cable, and telephone head off to a residence to the left of the power lines.

  All of the service drops are wires bundled with strong steel "messenger" wires, to hold them up and keep the weather from breaking them. They are attached to the power pole or to other messenger wires. The insulated hot and neutral lines in the service drop are twisted together with a messenger wire or are molded into a service drop cable assembly with a messenger wire.

• Why do some power poles have an extra wire above the three or four wires on the crossbar?

  This is a neutral wire strung to catch lightning and keep it away from the other lines. It is connected to ground stakes at the bottoms of the poles, as well as being bonded to all of the neutral wires. In many cases, the neutral is on top, and only the three phase wires are on the crossbars.

• Why do some power poles have an extra set of three more wires above the usual set of four wires?

  An additional set of power transmission wires at higher voltages is sometimes placed higher on the same set of poles. These carry power from one substation to another, usually at voltages higher than 100,000 volts.

  Note that the voltage on each wire is usually betrayed by the size of the insulators. Higher voltages need larger insulators and more separation from the poles and crossbars to keep power from forming a discharge arc in wet weather.

• Why do some power poles have only one or two wires at the tops of the poles?

  Only one phase serves the residences supplied by these poles, so there is only one distribution wire, plus its ground.

  Note that some poles have only the 120/240 volt service lines, and some carry nothing but cable TV or telephone lines. Also, some poles have only the distribution wires or only the transmission wires. Poles are installed as they are needed.

• Why do many businesses have three pole transformers? Isn't one enough?

  They have a three-phase service because they have loads that need all three phases. A transformer is needed for each phase.

• Why do power failures sometimes affect spotty parts of neighborhoods, but not the entire neighborhood?

  In this case, only one of the three phases is out. Any house still with power is connected to one of the other two phases that did not lose power.

• Why, after a close lightning strike, are my lights occasionally changing brightness?

  Lightning has damaged the neutral wire or jumper of the service entrance to your house. Call the power company as soon as possible. Some of your lights and appliances are getting too much voltage when they get brighter.

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Questions about damage from power failures:

• What can be damaged or disrupted by a power failure?

  Many different kinds of electrical equipment can be affected by power failures.

  There are three kinds of effects:

  1. Actual damage to electrical equipment
  2. Damage due to events caused by the lack of power
  3. Equipment forgets what it should be doing
• What kind of equipment can be actually damaged by a power failure?

  Many kinds of equipment can be damaged by a power failure, including:

  • Light bulbs
  • Energy saving CFL and LED light bulbs
  • Air conditioners
  • The contents of computer hard disks
  • Computer hard disks
  • Computer networking devices
  • Other computer devices
  • Computers
  • Television sets
  • Large electric motors
  • Three-phase equipment, if only one or two phases go out
  • Stereo music systems
  • Other electronic devices powered by power lines
  • Industrial equipment shut down in mid-process can jam
  • Anything with an embedded computer in it powered by power lines
  • Point of sale equipment
  • Traffic signals
  • Anything touching both the power ground and something else connected to the earth, but not the power ground.
• How can damage occur due to "events caused by the lack of power"?

  The lack of power can cause these kinds of damage:

  • Auto accidents due to dark traffic signals
  • Auto accidents due to lack of streetlights
  • People suddenly left in the dark in dangerous places
  • Food in refrigerators and freezers spoiling
  • Food in a stove that was doing a timed bake is left partially baked
  • Bread in a bread machine is unfinished
  • A factory assembly line shuts down in random order, causing objects to collide
  • Industrial processes are halted in mid process, possibly damaging equipment
  • Bank and other financial computers not responding, causing bank cards and checks to not be honored
  • Loss of computer files people were modifying when the power went off
  • Workers unable to work because there is no power
  • Businesses unable to complete transactions because their cash registers are dead
  • Looting by criminals due to failure of alarms
  • Materials being mixed (e.g. concrete) can set up in the mixer with the power off.
  • Flooding of a basement due to lack of power to a sump pump
  • Sewage spill due to lack of power to a sewage lift station pump
  • Failure of the water supply and any damage that might cause
• Which equipment forgets what it should be doing, and why?

  The lack of power causes these items to forget what they are supposed to do:

  • Electric clocks, their alarms, and often the choice of radio station the alarm uses
  • Timers turning things on and off forget everything, or are set to the wrong time
  • Power usage monitors forget everything
  • A computer forgets everything it was doing
  • Most home equipment forgets all of your settings.
  • Almost anything with a remote control has to be set up again after a power failure.
  • Television equipment must relearn all of the channels it can receive
  • Most new stereo equipment forgets everything. Most old equipment starts where it left off.
  • A stove doing a timed bake forgets everything. The cook can not know how much baking it already did.
  • A bread machine forgets where it is in the cycle, and can't be restarted at that point.
  • A Programmable heating/cooling system forgets its programming
  • Video games and arcade games forget what they were doing
  • Industrial processors forget where they were in a process
  • Industrial processes forgotten by the equipment and must be re-entered into the equipment memory
  • Some traffic signals forget their programming
  • Some traffic signals forget what time, date, and day of the week it is.
  • Financial transactions interrupted in progress can make money appear or disappear.
• How can a power failure damage a light bulb?

  There are several ways:

  • An overvoltage surge can burn out an incandescent lamp or shorten its life.
  • A failure or restore that causes the power to go on and off repeatedly shortens the life of a fluorescent lamp.
  • Voltage that is too low shortens the life of a fluorescent lamp.
  • A failure or restore that causes the power to go on and off repeatedly can destroy a CFL or LED power supply.
  • Voltage that is too high or too low can destroy the power supply in an LCD or LED.
• How can a power failure damage an air conditioner?

  These can damage an air conditioner:

  • A failure or restore that causes the power to go on and off repeatedly can vapor lock the compressor, shortening its life.
  • Voltage that is too low overheats both the compressor and fan motors, shortening their lives.
• What happens to a traffic signal when the power fails?

  This depends on the nature of the power failure and the traffic signal equipment:

  • The newest signals have LED signal lights and battery backup, and go through three stages:
    1. Stage 1 has normal signal operation. This continues until the battery is low
    2. Stage 2 has all red signals blinking. This continues until the battery is depleted.
    3. In stage 3, the signal is dark.
  • Older signals flash all red signals until the battery goes dead.
  • The oldest signals and signals without battery backup systems just go dark.
  • If the power goes on and off repeatedly in a short time, the traffic signal might not restart normally.
  • A power failure with an unusual voltage surge might damage the traffic signal or erase its programming.
  • The power failure can cause signal lamps or LEDs to fail.

  A dark traffic signal is supposed to be treated as an ALL-WAY STOP. But there are problems with this:

  • Not everyone knows this. Some people think the major street can go.
  • Some people on the main road don't see the signal, especially at night.
  • Traffic signals painted dark green, black, or brown cannot be seen at night.
• What can a power failure do to a computer?

  Usually the kind of power failure that does damage is the kind where the power goes on and off repeatedly. This causes the wrong voltages to be on some parts when the power comes on again.

  The power failure can damage the computer or its components:

  • The power failure can damage the switching power supply portion of the computer.
  • The power failure can damage any of the circuit boards.
  • The power failure can cause the flying head to scratch the hard disk, totally ruining the drive. It must be replaced.
  • Voltage differences could destroy a flash drive,
  • The power failure can damage any networking part of the computer by having a large voltage difference with the networking connection.
  • The power failure can damage peripherals (e.g. keyboard, mouse, printer) connected to the computer.

  The power failure can damage the information on the computer:

  • If the computer was writing to a file at the time the power failed, either the one file that was being written to is destroyed, or the directory of all files could be destroyed.
  • Voltage surges could erase a flash drive.
  • Settings related to how a computer connects to peripherals could be changed.

  The power failure can cause the computer to forget what it was doing:

  • The computer stops doing what it was supposed to be doing.
  • The file the user was working on is either lost or reverts to an earlier version.
  • A website goes down while the power is off.
  • A money transaction is interrupted.
• Can a power failure cause unusual problems in monetary areas?

  Yes, if a money transaction is interrupted in mid process:

  The page author has seen such a failure create $50 out of thin air.

  • The power failed at the local service point of a money-wiring service during a transaction. The local clerk gave the author back the $50 he was trying to wire, and told him to try later or at another branch.
  • When the author tried again later, the recipient sent the money back, saying he had already received it. The recipient said the money was sent at the time of the interrupted transaction.

  What is weird is that the books at both ends of the money-wiring service balanced, yet the recipient and the author both had what was supposed to be the same $50 at the same time. The computers seem to have created $50 out of thin air.

• How can a power failure cause damage to something touching both a power ground and an unrelated earth ground?

  Any kind of ground fault (short to ground) that causes a power failure can cause a large voltage difference between the power equipment ground and anything else connected to the earth, but not the power ground.

  • This voltage exists only during the actual high fault current that causes the power failure.
  • It can get as high as several thousand volts between the objects.
  • This effect has killed people who were touching both objects at the exact time the power failed.
  • This voltage does not exist under normal conditions and cannot be detected with test equipment.
  • An ohmmeter can detect a higher resistance between the two objects, which can show the possibility of this trouble.
  • This can be prevented by running a bonding wire between the two objects, so both are always at the same voltage.
  • Metal objects attached or driven into the ground (or attached to concrete on the ground) should be bonded to the power ground.

  See more about this at The Vital Importance of Bonding.

• What can happen to three-phase loads if one of the three phases fails, but the other two are still energized?

  The three-phase loads are usually motors or heating devices. The effects on this equipment can be:

  • A motor already running may continue running, but at reduced power.
  • A motor under load may stall, possibly damaging the motor if it does not start turning again.
  • A motor that is not running may not start turning, possibly damaging the motor.
  • The output of a heating device can be reduced.

  Often a cutoff device disconnects a motor from all phases, to protect it if any one of the phases loses power.

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