What causes surges?

Surges are caused by lightning striking the electricity grid, electric motors starting up or shutting down, heavy equipment being switched and by (portable) generators during start up and shut down, especially when they run out of fuel.

Where you are on the electric grid also has an impact – a dense city grid has more controls; long distant country transmission lines (like single-earth return lines) are far more vulnerable to surges, brownouts, over-voltages and switching issues.