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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.

The Australian standard allows for -6% to +10% of nominal voltage, in our case 240V. Because most electronic devices use switched mode power supplies they will easily cope with that and a lot more, allowing devices to be used both with 110V and 240V. Variations in voltage are more likely due to location and changes in total demand. As the current flow increases, there must be an increase in the voltage drop along the lines.

The surges would be due to lightning and rarely problems with the power system. Those would generally be much greater than the nominal voltage. Hundreds or thousands of volts greater. Direct lightning strikes would produce huge spikes and can't be solved with a surge protector. Its internals vaporise. Close strikes produce voltage spikes of varying levels on the wires.

Ken: Thanks for the comment. I was hoping there are some readers who might know about this topic more. I don't know what the right distribution is to model the voltage data since I don't have any datasets. But it's an interesting application to use for teaching if I can find some data.

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