DrClaude said:
Why is it that when a power line is downed, there is no circuit breaker somewhere going off? Is it that, even with a direct connection to the ground, the current is still within the usual range?
Yes, that's correct, at least some of the time. A short circuit has an impedance, and the lines between the source and the short also have an impedance. Therefore there is a range of magnitudes of short circuit currents, some of which overlap with the ranges of normal current.
Also bear in mind that with 3 phase circuits, there are several ways to make a short. Line-to-line short, line-to-ground, double-line-to-ground, three-lines-to-ground, and line-to-line-to-line shorts. Each type causes different levels of short circuit current. The relays look at the magnitude of currents, and at the unbalances in the 3 phases, and at time.
We call it the "art and science" of protective relaying, because both words apply to actual practice.
Time also comes into play. There are cascades of protective relays, breakers, and fuses. Hopefully, the relay closest to the short opens. If it fails to open, another relay more distant backs that up, but only after a time delay to give the first one time to act and be detected. If the 2nd level fails there is a 3rd level after a longer delay and so on. If there is a short near your house, the first level hopefully interrups only your street. The 2nd level might shut off the neighborhood, and so on. More delay and wider impact at each level.
So, when you see videos of short circuits arcing, it is common to see it continue for a long time, but not an infinite time. The first level should stop it in a fraction of a second. The highest level might take 30-60 seconds to act.
The most frequent types of short involve a single line to a tree branch or a squirrel. Those are usually self-healing as the tip of the branch burns away or the squirrel falls. We call that fault clearing. The impact on customers is less if the fault clears itself in a few seconds, as compared to sitting in the dark waiting for a line crew and a truck to show up and reclose the breakers manually. Therefore, we take some risks. Some short circuits are allowed to persist hoping that they will self clear. The breakers trip after a time delay when the logic decides that it will not self clear.
There is also logic that attempts to re-route the power to customers, by opening some breakers and closing others. Sometimes, that works, but sometimes it just results in feeding the short circuit arc from the opposite direction.
We also use automatic reclosers. that may close the breaker once again several seconds later, in the hope that the fault self-cleared. If it did not, the recloser restarts the arc. Automatic reclosers may try, 5 or even 10 times before giving up, so the arcs at the fault location may be seen to start and stop repeatedly over several minutes.
You may see the net result of these various schemes at home. The lights dim, then go out, then they may flash on for a moment, flash again, and finally come back steadily after 30 seconds or a minute. You see the net result of several layers of protective strategies and self-clearing behavior of the fault. Customers a few streets down may experience a different sequence than you.
I can't resist a plug for my personal hero. Much of the basic methods and strategies for protective relaying were invented by Thomas Edison as an integral part of the Edison Electric Illuminating Company. Edison did much more than invent the light bulb. He conceived the whole industry and power grid including all necessary attachments and accessories all the way down to insulators and brackets to hold the wires, operating procedures, even billing methods. He delivered a turnkey electric utility on the first instance in NYC. Then he delivered a complete second instance for the Paris Exposition of 1889.
Thanks
@DrClaude for giving me the excuse to expound.
or there are point to point systems that could use the power lines themselves - too accessible, perhaps?
