[Detrus]

I think the History Channel had a scaremongering show about coke can sized EMPs at 30,000 feet having the range to take out most of US electrical/computer systems.

If such EMPs did have the range, it would be scary indeed. Everything except hardened military equipment would go if a terrorist could sneak one in on a plane. Some records may be backed up in datacenters in other countries, that's the only protection for civilians.

But coke can EMPs at 30,000 feet don't really have the range. You'd need a specially designed nuclear warhead around 1 megaton at far higher altitude to take out a big chunk of the US.

I asked a related question on Quora http://www.quora.com/Are-datacenters-protected-against-EMP-a... lots of links in the answers