Oh, the Carrington Event again. This has been on HN several times. It ought to be on a list of common misconceptions.<p>- This has nothing to do with "EMP". That's a big but brief RF pulse with a rise time around 1ns. This is induced DC in long wires over a period of hours.<p>- The basic effect is that a current is induced in the earth's crust, resulting in a DC voltage between grounds at widely separated points. This is mostly a problem for long AC high-tension power lines using wye-connected transformers at each end which run in the same direction as mountains of igneous rock. The DC component can partially saturate the transformer, using up some of its capacity. If undetected, transformers can overheat, and worst case, burn out. This occurred on March 13, 1989 on some lines in the eastern US.<p>- PJM, which runs the northeastern US power grid, is now prepared for this. See PJM's training manual for this[1], starting at page 22. They get 1 to 6 days warning from the NOAA's Space Weather Prediction Center. They have monitoring for unwanted DC flows at multiple points in the power grid. When trouble appears, certain power lines have to have their current reduced.<p>- A few times a year, more at the top of the sunspot cycle, there are warnings of a potential problem. PJM last issued a warning on August 26, 2018. It didn't progress to an "alert", or actual action. There are people in a control room in Valley Forge, PA, and another backup location, watching this. They can reroute power around the trouble spots, call for extra generation output, and dump some loads if necessary. Load dumping starts with bulk buyers of "interruptible power" - aluminum smelters, Bitcoin mines, etc., which are willing to be the first turned off in exchange for a discount.<p>- It's not a problem for anything shorter than hundreds of miles. It's not a problem for high-voltage DC power lines, like the Pacific Intertie and the really long ones from western to eastern China. It has zero effect on fiber optics or small devices.<p>- When you encounter clueless reporters writing about the power grid, aim them at "PJM 101", which is a set of introductory training materials on how the power grid works, written for people who run it.<p>[1] <a href="https://pjm.com/-/media/training/nerc-certifications/gen-exam-materials/gof/20160104-conservative-operations.ashx?la=en" rel="nofollow">https://pjm.com/-/media/training/nerc-certifications/gen-exa...</a>