Donnie B. is correct. An EMP would either permenently fry any electronic device or have no noticable effect. Lightning produces an EMP-like effect, but most of you have probably used cell phones, TV's, or computers during an electrical storm with no noticable effect.

Power systems are designed to handle faults. So a small EMP device may cause a fault, but since the pulse is short lived, the relays would automatically flip back on in about second. My guess is you can either have a large EMP which would fry the fault detection circuits and cause a long term blackout and mass hysteria, or you can have small EMP which do little more than cause the lights to flicker.