50,000 V/m is the unclassified field strength. Using simulation codes (from memory) a 1kT calculates out to around 40,000 V/m.
The impact to the power grid is projected to be to communications, controls, relays, generation plants. Below 100kV damage to infrastructure is possible. Above it’s not likely for a variety of reasons.
An EMP will not decay as you might expect, but a localized emitter would decay.
Damage predictions are statistical. Not everything will be destroyed but many things are expected to be damaged, destroyed, or upset.
Thank you...