Here is a brief summary of Starfish Prime:
Starfish Prime was a high-altitude nuclear test conducted by the United States on July 9, 1962.
Launched by a rocket containing a thermonuclear warhead the explosion took place 250 miles above a point 19 miles southwest of Johnston Island in the Pacific Ocean. It produced a yield equivalent to 1.4 megatons of TNT.
Starfish Prime caused an electromagnetic pulse (EMP) which was far larger than expected, so much larger that it drove much of the instrumentation off scale, causing great difficulty in getting accurate measurements. The Starfish Prime electromagnetic pulse also caused electrical damage in Hawaii, about 898 miles away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms and damaging a telephone company microwave link. The EMP damage to the microwave link shut down telephone calls from Kauai to the other Hawaiian islands.