During some situations a transmission line(live one) running across a river suddenly falls into the river then

Question

how long will its effect will be felt(i.e.how long should one stay without getting electroculated)

Answer 1

stay where? in the water? get out immediately if you are still alive.

But usually a breaker somewhere in the line will open and shut down the voltage, but you don't want to count on that.

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Answer 2

You don't want to be in the water near the line while the line is live.  There is no safe time.

There are multi-breakers that will take the line off-line within seconds.  These usually re-store power a few seconds later.  This usually happens 3 times before the multi-breaker takes the line off-line permanently