I had a power outage and I thought about the utility poles near the street. How do electricians know which power line is down considering that there are many miles of them? For example, if there's a car that crashed into the pole and brought the power line down, how does the electrician (or power utility) know where the break is located?

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    \$\begingroup\$ I don't have enough hard details for an answer, but in reality the power grid is not just a pile of AC transmission lines and transformers. At many locations on the grid (such as at some shut-off points, transformers, meters, etc) there are smart devices that communicate using various protocols and can both indicate state, and take certain actions under remote control (such as opening/closing an interrupter) \$\endgroup\$
    – nanofarad
    Commented May 17, 2016 at 21:17
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    \$\begingroup\$ The power is broken where the complaint calls end. \$\endgroup\$ Commented May 17, 2016 at 21:17
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    \$\begingroup\$ One possibility is tdr \$\endgroup\$
    – PlasmaHH
    Commented May 17, 2016 at 21:19
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    \$\begingroup\$ In many areas, at least here in Florida, all substations have a cross-linked grid of fiber-optic cable that pass along system status in both directions. They go to 'router' substations that pass any 'fault' information back to a central control center, even if automatic responses are built in. \$\endgroup\$
    – user105652
    Commented May 17, 2016 at 22:06
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    \$\begingroup\$ In the case of a car crash, the police will inform the power utility directly, assuming they get notified immediately. \$\endgroup\$
    – Dave Tweed
    Commented May 17, 2016 at 23:09

6 Answers 6


Customer service power lines are typically segmented into blocks or rings, so that damage to part of the grid does not disable a huge geographic region. A substation may feed several separate customer grids near it from a large regional power line.

Each customer grid has its own circuit breakers at the substation, which are usually outfitted with automatic reclosers. During a power failure, you may notice the power go off, come back on, go off, come back on, and then go off a third time and stay off. This is due to automatic reclosing, which attempts to immediately restore power. Some line failures such as a falling dead tree branch are brief, and power can be restored immediately after the tree branch has fallen past the line.

If the fault can not be cleared, the faulting line segment is usually immediately reported by the recloser or substation to a regional electric grid control center. It is often completely unnecessary to call in a widespread power failure during a storm, as the control center knows immediately when a line segment has lost power.

For example, automatic reclosing on Youtube:

Smart Grid - Self Healing - NOJA Power https://www.youtube.com/watch?v=rVTIwr1Rk2c


There is no way to instantly pinpoint the location of a fault on a line segment that cannot be automatically restored. These faults are typically located by linemen getting into a utility truck and driving around the neighborhood inspecting every part of the line segment. They have detailed maps to guide them. Multiple utility trucks may be assigned to inspect the down segment. If you see a electric utility truck slowly driving down the street during an outage, it is because they are visually inspecting the lines.

When there is an outage at night, utility trucks are equipped with high power lighting. The utility trucks will drive slowly along roads with one person aiming the spotlight, and another person looking at the illuminated lines and equipment to check for damage.

This is part of the reason why an outage that can't be automatically reclosed may take several hours to be restored, due to the linemen needing to drive around and inspect all the lines to check for broken wires, damaged insulators, heavy ice weighing down lines, or objects contacting the lines.

And this is also why the customer's local power lines are segmented into small areas, because the larger the customer line segment, the more time it takes to drive around during an outage to locate the fault. The driving to look for line problems takes even more time in adverse weather conditions such as a blizzard or freezing rain.

For example, on Youtube:

How Power Gets Restored - Puget Sound Energy https://www.youtube.com/watch?v=gLikTRjrmnc


Once the power line fault is discovered, the linemen will take additional safety measures to ground and/or short-circuit the power lines, so that in the event power is restored before their work is completed, the safety grounds will immediately re-trip the line breakers and prevent injury to the linemen.

Safety grounding also protects linemen from home power systems which could be miswired to backfeed power into the inactive line. A small home generator is capable of backfeeding into a down power line and energizing it at its 12,000 volt (or higher) operating voltage. Safety grounding and shorting of the down power line assures these miswired home generation systems will result in blown fuses or burned out home generators, rather than a dead lineman.

For example, on Youtube:

Protective Grounding https://www.youtube.com/watch?v=T0vZDl2kFI8


For the very large transmission lines that feed a region (the poles are much taller than local grids), these are often cross-country and may not be near roads. Access to these lines for inspection and repair can be done by ground vehicles, but access is difficult in the winter or when there has been rain and the ground is soft and muddy.

Inspection and repair work for these very large transmission lines is instead often done by helicopter, and due to the helicopter not being in contact with the ground, they can approach a live power line, make contact with it, and work on the line to do maintenance or repairs while it still is fully powered.

For example, on Youtube:

Helicopter Maintenance on energized 765,000 Volt Line https://www.youtube.com/watch?v=x94BH9TUiHM

'Hot-Washing' the Insulators of a 500,000 Volt Power Line! https://www.youtube.com/watch?v=lcjhjna9jZE

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    \$\begingroup\$ So what happens if a large transmission line gets taken out? I know they keep trees away from those so it's less likely to happen in the first place, but just out of curiosity \$\endgroup\$
    – Random832
    Commented May 18, 2016 at 2:53
  • \$\begingroup\$ @Random832: Then you have major incident like en.wikipedia.org/wiki/Northeast_blackout_of_2003 \$\endgroup\$
    – mharr
    Commented May 18, 2016 at 18:12
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    \$\begingroup\$ But of course, for the rest of us figuring out exactly WHERE the problem lies still depends on guys driving around in trucks, looking for the downed lines/branch-across-the-wires/fried squirrel that's causing the issue. (Don't laugh - we once had a power outage caused by a squirrel who put back feet on one insulator/wire and front feet on the other insulator/wire and...ZZZZZ-OTTTT! (darkness). Guy in the truck came along, got out his extensible pole, knocked the remains off the insulators, went back to the substation, and...LIGHT! :-) \$\endgroup\$ Commented May 18, 2016 at 22:04
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    \$\begingroup\$ @mharr There were multiple factors that led to that blackout, notably lack of visibility to a large part of the system. Transmission lines going down don't usually result in widespread blackouts, especially these days. \$\endgroup\$ Commented May 19, 2016 at 20:45
  • \$\begingroup\$ A tornado caused a 3.5 day blackout for this area a few years ago. This whole county (and more) normally gets power from just one power plant. The tornado came close to that power plant, and took down all the high-voltage power lines to this county. It took 3.5 days to restore just one of the several paths that would allow the power to reach this county again, and longer to restore power to some areas with more direct damage from that day's tornadoes. \$\endgroup\$
    – user6030
    Commented May 19, 2016 at 22:12

By the way, Dale's answer is quite good, and I mean only to append to it.

They have a system of signaling over power lines. It's not a particularly high bit rate, but it gets the job done. Many smart meters can receive or send that way. Often they can remotely turn off your power if you call them and ask them to. They can also get the meter reading that way. Of course they have other monitors on poletops, some with cellular, radio or wired connections who relay for stations downstream of them.

If a poletop or substation breaker or recloser trips, it can often phone home and say so.

Needless to say, it's not hard at all to interrogate the network and get a map. Of course as you know from PC board layout, a segment which is short on the schematic can be quite long and even branched on the PCB. So this may reveal a segment that is miles long and has to be hand inspected, possibly along railroad tracks, backyards or cornfields - power lines don't always follow roads.

How did they do it in the old days? Customers called in. They got a flashlight to read the phone number off the emergency contact page of the phone book, picked up the phone, got a dial tone, and called the power company and reported it.

How did they get a dial tone with no AC power? Wireless phones which need a powered base station weren't common. U-verse didn't exist. The phone was powered through the phone line itself, from massive submarine sized batteries in the central office, backed with burly Caterpillar generators. I've been through 5-7 day outages where phone reliability was never in question.

  • \$\begingroup\$ So how'd the phone company deal with their outages? \$\endgroup\$
    – Random832
    Commented May 18, 2016 at 2:55
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    \$\begingroup\$ @Random832 Diesel generators. \$\endgroup\$ Commented May 18, 2016 at 7:54
  • \$\begingroup\$ @MichaelHampton I meant, if a phone line was cut, how was the outage reported. \$\endgroup\$
    – Random832
    Commented May 18, 2016 at 14:01
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    \$\begingroup\$ @Random832 From a neighbor's house, or from a pay phone. \$\endgroup\$ Commented May 18, 2016 at 14:03
  • \$\begingroup\$ @Random832, it looks like Graham's answer pertains to your phone line question. @ MichaelHampton, that's great... \$\endgroup\$
    – JPhi1618
    Commented May 18, 2016 at 14:17

Time-domain reflectometry is your friend here. Electrical conductance isn't as simple as it looks. If you put a pulse on the broken line, you'll get a reflection of that pulse coming back from wherever the line is broken. Measure the time between first and reflected pulses, divide by the propagation speed in that cable, and you have your distance.

  • \$\begingroup\$ Does this also work for other line faults, such as short circuits? \$\endgroup\$
    – Mark
    Commented May 19, 2016 at 3:18
  • \$\begingroup\$ @Mark: it can, when the circumstances are right \$\endgroup\$
    – PlasmaHH
    Commented May 19, 2016 at 9:21
  • \$\begingroup\$ @Mark From the point of view of a power line, what you think of as a "short circuit" is just a change in impedance. \$\endgroup\$
    – Graham
    Commented May 19, 2016 at 11:28

Most smart meters transmit in the 902-928 mhz band at low power and transmit to the next house then that meter then transmits to the next house in the chain until it encounters a data receiver that then uploads it to the power company via cell phone link. Each power meter has it own ID# and if no data is received the power company exactly where the outage is complete with street addresses of the customer.

  • \$\begingroup\$ You're assuming smart meters have been installed; here in Queensland (Australia), they basically don't exist (there was a small trial recently, and the other day I saw the meter reader read the neighbour's meter). Currently, I believe Victoria (think Melbourne) is the only area in AU with a significant roll-out of smart meters. In Brisbane, many homes still have the old analog meters with their multi-dial 'displays' and spinning wheel. \$\endgroup\$
    – Calrion
    Commented May 18, 2016 at 4:16
  • \$\begingroup\$ I can only answer for Canada. We had the old analog ones that you could mess with about a year and a half ago. \$\endgroup\$
    – Old_Fossil
    Commented May 18, 2016 at 4:24
  • \$\begingroup\$ And I know they have the powerline based system too, because I was able to get a remote power drop shut off remotely - it was too far from other boxes for what you describe to work. \$\endgroup\$ Commented May 19, 2016 at 17:00
  • \$\begingroup\$ @Harper: Are you in a rural or urban area? This info I quoted was from power company sources. In this instance it was Hydro Quebec and Ontario 1. \$\endgroup\$
    – Old_Fossil
    Commented May 19, 2016 at 21:24
  • \$\begingroup\$ In that case, quite rural. It's clear the various power companies are using a mishmash of technologies. \$\endgroup\$ Commented May 19, 2016 at 21:42

Electrical Grid control rooms have the ability to pinpoint to within 3 meters where power goes out on modern electrical grids. It's not just electricity that flows through the lines. It's status, and control signals as well. It's similar to being a large computer network. We can control the power to certain sub-stations, de-energize a transformer (thus turning off power to a neighborhood, house, apartment, etc) and with the new Smart Meters here in BC, we can turn on, and off the power right at your house which is super handy in emergencies. Say fire, or flood. The 1st responders won't go into a place that is still energized, and risk themselves getting hurt or killed, so when that call comes in, we can just turn the power off with a couple clicks of a mouse.


Every city has its circuit breakers. every grid has circuit breakers. Every sight has its circuit breakers. All these breakers are basically PLC. These all PLC are connected to NMS (Network manangement sytem) These NMS works on SNMP model. when a site goes down. Circuit breaker is tripped and it will show alarm on NMS. So electricians know the exact place of network failure. I think so understand how these circuit breakers work.


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