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"Statistically, we can define surges as rare events - maybe voltage that is at least three standard deviations above the normal value."
Uhm... What is the unit of measure for time? A second? A minute?
Using normal distribution a value farther than three standard deviations has probability 0.3% (or 0.15% taking into account only positive deviations).
Should I deduce that surges occour every 333 (667) seconds or minutes on average?


Antonio: I'm just speculating there. I haven't been able to find a data source to know what is the right probability model for it. If we have empirical data, we just need to plot the periodic peaks of the voltage. The average of these peaks should be around 110V in the U.S. but it's not clear what the standard deviation is, or the shape of the distribution. Surges could be much more than 3 SD away - I just don't have any data to say one way or another.

In terms of sampling frequency, using shorter time units means that there are many more observations so it shouldn't matter.

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