[RayChuang88]

Only one thing: you need a megaton-yield nuclear weapon for EMP to be effective. And alas, that's beyond the technology of North Korea or Iran--a 20 kT nuclear device exploding in Earth orbit has the effect of a solar storm. After what happened to the Quebec power grid in 1989 when a major solar storm knocked out part of the grid, they've put in more sophisticated safety systems to protect the grid from the effects of a solar storm, so a low-yield nuclear weapon will have relatively little effects on the ground below.

According to what we know about the Dominic Starfish Prime test, it used a W49 nuclear warhead (essentially a modified B28 nuclear bomb) with a yield of 1.4 MT. As such, the explosion at 250 miles above the ground had pretty awesome effects, as witnessed by the substantial electrical interference in Hawaii. It makes me wonder during the Cold War, the Russians had planned to use the older SS-9 (R-36) missiles armed with a circa 20 MT nuclear warhead to detonate about 200-300 miles above the ground for enormous EMP effects.

[vanilla swirl]

If you read the link I posted;
http://www.futurescience.com/emp.html
you will see that Starfish Prime was 1.44 megaton but a smaller nuclear fission weapon requiring far less expertise would be much more effective. Also take into account the fact that WHERE the event occurs relating to the earth’s magnetic field makes a difference as well. The northern hemisphere (USA) has more electromagnetic “lines of force” available for the device to interact with than the south pacific.