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?
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
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.
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.
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.
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.