Timing of lights switching off when the power source is disconnected Not that I don't know the norms, but I have a doubt. Say, in the middle of a very large and well-lit city, is the power station powering all of the lights. If I were to view from space (or at least from high enough to see the entire city in one go) the city, and the power house was to cut off the supply at once, in what trend/manner would the lights go off — the outermost first, followed by the inner lights or the the ones that are closest to the power house would go off first and the farther ones following them, or it would all go dark at once?
P.S - It is a typical movie scene but what's the physics behind it? I mean, a reasonable explanation could be given involving potential difference, etc. for all of the scenarios, but what I want to know is which one happens to be right?
 A: The timing of events between the cut of the power station and the cut of the lights is all about the electrical power infrastructure. Along any given wire, the effect of breaking the circuit travels at close to the speed of light down the wire, so is a very small time: just a few microseconds to cross a city some kilometres wide. The main timing considerations here are therefore related to other things: the decay-time of transformers in the transformer stations, the response of any given light to having its current cut, and things like that.
Transformer circuits 'don't like' having their current cut; they tend to respond with a voltage surge when the current changes abruptly. Therefore transformer substations have various protections to cope with this possibility, including capacitance. I don't know the details but I suspect the response overall is still pretty quick, of the order of milliseconds (maybe someone else will post a more informed answer where this time is known).
Finally, electric lights also themselves go off and on pretty quickly.
At the fast timescale that all this is happening, the first lights to go off are not those geographically closest to the power station but those with the fastest transformer stations. Therefore I think the order across the city will show districts switching off in rapid succession, but the order will not have much relation to the distance from the power station. However, all this happens so fast that our human eyes will not track it; the change will appear instantaneous.
After writing the above I found this you tube video:
https://www.youtube.com/watch?v=Ew5LIQC9aF4
It looks like the paragraph above is mostly right but misses one aspect.
In the video you can notice that entire districts flicker simultaneously to our eyes, which is what I thought. However, I notice that different light circuits in a given area take longer to go off. There is clearly a longer time constant somewhere, provided by something like a signficant capacitance, that can briefly maintain the current in circuits that are less demanding of current.
