One of the Doomsday scenarios that continues to fascinate is the possibility of the nation's electric grid being laid low by an electromagnetic pulse (EMP) or other attack.
In a House hearing last May, experts described the dire consequences that might result from a high-altitude nuclear detonation by a rogue power – North Korea was prominently mentioned, naturally. For example, Dr. Peter Pry from the Task Force on National & Homeland Security testified about an "existential threat", with a 90% fatality rate through "starvation, disease, and societal collapse."
For a more nuanced analysis, check out these articles written by a then-Harvard astrophysicist, which discusses the three types of electromagnetic pulses, E1, E2 and E3, caused by a nuclear burst at 25 – 35km altitude.
Each type has a different effect on electrical systems. An E1 pulse, the fastest, has a rise time in the nano-second range. It only lasts for a microsecond, but during that time can induce fields of up to 30kV/m at ground level. E1 affects primarily integrated circuits, and can damage relays, computer controls, and communications.
An E3 pulse is the lowest amplitude and has the lowest frequency, less than 1Hz. It can last up to 1,000 seconds, though, and poses the highest risk to the electrical grid since it can induce large currents even in buried cables. An E3 pulse can destroy HV and EHV transformers due to internal heating.
Unsurprisingly, publically-available real-world data is limited. One unclassified study of US Cold War test describes power outages in Hawaii after the Starfish series of high-altitude tests in 1962. Although no unambiguous electronic failures were noted, the sensitivity of current-day electronics is up to one million times greater than in the 1960s.
That hasn't stopped Hollywood. Systemic electrical grid failure is a popular plot element in post-apocalyptic TV shows. Take Revolution, in which weaponized "nanites" spread beyond their original target and permanently disable all electricity, not just the grid.
Following the event, the US splits up into six republics, whose members go about hacking each other up with knives, swords, spears, and so on. Hey, it's a TV show – write the science consultant, if you can find one. Or become one yourself. For the inside scoop on that option, go here.
But I digress. The relative amount of the three EMP l\pulses is heavily dependent on yield, altitude, and other factors. Even apart from the technical difficulty, one drawback for a potential aggressor is that nuclear-powered EMP attacks are likely to leave identifiable clues such as a distinctive IR signature from the launch vehicle, inviting a massive retaliatory strike.
Terrorists are far more likely to employ easier, cheaper, and more readily-available methods such as explosives.
There's no doubt, though, that a large-scale failure of the electric grid would have serious, even fatal, consequences. The most likely cause of such a failure isn't to be found on Earth at all – it's a large solar flare followed by an extreme geomagnetic storm.