In spite of NASA's refusal to accept that the pressures resulting from deep impact's impact on the comet being well beyond those found on the sun, they've posted no warnings.
They assume that the comet is a rubble pile. As if there isn't any potential of a comet impacting an meteorite, and producing a solid core.
When a impact pressure produces a pressure in excess of that found only on the sun, then the potential of a nuclear reaction involving non-fissle isotopes becomes possible.
Unplug your computers transient protector from the wall but, do not unplug your computer from the transient protector.
If the comet is a rubble pile, then only 4.5 tons of tnt will equal it, and there will be no EMP pulse. If not, unguessable. I've done the math, and if they hit a hard spot of sufficient mass, then the pressures will exceed those found on the sun, where raw hydrogen is in fusion and fision.
A nuclear reaction will generate an EMP pulse. But, that far away, and the antenna that would recieve the full potential of that power isn't a small box filled with components, like your computer. But, a birds nest of wire like the power grid that leads to your house from the power station.