From the report by Medatech Corporation prepared for Oak Ridge National Laboratory:
The Early-Time (E1) High-Altitude Electromagnetic Pulse (HEMP) and Its Impact on the U.S. Power Grid
http://large.stanford.edu/courses/2019/ph241/rogers1/docs/meta-r-320.pdf
2.14 E1 HEMP Effects on Systems E1 HEMP peak field levels can be very high, and certainly there could be some devices within a system that have some vulnerability to such high levels. However, typically vulnerabilities involve the intermediate step of having voltages and currents generated on a conductor, and then that signal getting to the fragile device. The conductor might be deep within the system, such as wiring in the internal circuits, however a major concern is external cabling attached to the system, for two reasons. First, outside wiring can be very long, which tends to increase coupled signal levels, while internal wires are limited by the enclosure size. Secondly, the system enclosure and support structure, especially if metallic, attenuates electromagnetic fields and leads to lower coupling for internal wires. With a peak field of 50 kV/m, even a short “antenna” 10 cm (4 inches) long can mean a voltage of about 5000 volts, and it could be much higher for longer lines. Power lines, of course, can be very long. Other possible long lines include communication lines within a facility, such as network lines and phone lines. For power substations there are also the various sensing and activation lines used for the relaying process that maintains power reliability and tries to lessen harm to the power system from faults. With the advance of modern systems, and miniaturization of components, the normal operating voltages of systems tends to be a few volts, and so HEMP levels of thousands of volts or more cannot be good for the system. Also, the operating frequencies of systems, such as computers and various types of controllers, are such that an E1 HEMP pulse would cover many clock cycles. Thus the fact that E1 HEMP type pulses can have effects, as has been found in vulnerability tests, is not surprising.
I was referring to radiation.