all nukes emit emp. as the u.s. discovered in 1962 starfish prime, when as ken states, the west coast and hawaii experienced disruptions to electric devices. johnston atoll is 750 miles south west of hawaii.
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today's nukes specifically designed to deliver emp would devastate the power grid and fry motherboards. even if all circuits were switched off, the gamma and x rays would still infiltrate unprotected devices.
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an emp nuke does not need the accuracy of conventional nukes to be effective. an emp nuke lit up 400 miles above the u.s. would knock out all electronic devices, vehicles and power systems in the u.s. 
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in an emp strike all aircraft crash, all pacemakers fail, all deliveries of utilities, food and medical supplies stop. it would take years to repair and restart power and water supplies. millions die, cities dissolve in chaos.
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yes, a conventional nuke would cause local damage. an emp nuke would send the u.s. into the dark ages.  ken is rightly concerned about emp nukes.