|Solar Flare August 31, 2012 (creative commons licensed: Wikipedia)|
Electromagnetic Pulse (EMP) and Coronal Mass Ejection (CME's) events have concerned Public Safety and Security officials since the invention of the atomic bomb in World War II and the discovery of one of its side effects that are as damaging as the atomic blast itself, if detonated in space in the 1960's. CME's were first discovered and recorded by Richard Carrington in 1859. EMP of such magnitude would be extremely difficult to activate without prior detection. NNEMP's (non-nuclear) still require vast amounts of Chemical explosive. Further, such damage would only affected a single city, not entire counties, States or National scale, assuming it had sophisticated electronic navigation. There are very few nations that have cruise missile technology that could deliver such a payload due to chemical weight and shear size required. Nuclear EMP's are only valuable as a wide area weapon if detonated in at high altitude, which again, very few nations are capable of doing and all are known to western / NATO nations. Rogue nations are not known to have the capability to launch a device to the required 50 to 200 mile altitude (plus down range to target area, e.g. North Korea to a region over Central North America, approximately 7,500 miles) for a high altitude nuclear explosion (HANE) / High Altitude Electro-Magnetic Pulse (HEMP)
The EMP N-Club is very small, all are well known and fully understand Mutually Assured Destruction (MAD) doctrine, thus I find that possibility extremely remote. N-EMP's are still a highly complex device if it is to work as intended. You can't just build one of these things by going on-line at Willy E. Coyote's ACME store. Dirty half baked devices will not trigger a wide area EMP magnetic pulse. This is particularly true of any devices that are atmospherically detonated (as in the case of Hiroshima and Nagasaki) where the pulses were absorbed over a very small radius of less than 10 km's and those were more powerful than an improvised device is likely ever to be. Cruise missile technology is also a very complex platform. It is not just a matter of putting a gyro-based control unit with navigation platform tied into a GPS smart phone and enabling it to fly on automatic pilot.
There is a certain level of electronic safety from an EMP (and CME's) if known in advance (missile launch alert, etc. e.g. assuming North Korea had same performance as U.S. technology, the time to North America is approximately 30 - 35 minutes minimum, if not longer, + 30 to 45 minutes for fueling), which in turn is sufficient time to order - enable an emergency shut down of electrical grid prior to detonation. There are no documents that (nor should there be) that identify the length of time such an order would take to implement. But if a shutdown was ordered, a fair estimate from the time of a POTUS Executive order would be approximately 3 to 7 minutes or less and another 5 - 10 minutes to literately, pull the plug.
Today, we are more likely to see temporary disruption of services from our Sun caused by solar flares and geomagnetic storms that are more powerful (and more frequent) than any potential NNEMP event. Such events are known as CME's, or Coronal Mass Ejections.
Data and transaction records would be lost during that period as large scale data center shutdowns would exceed that time frame pending size and scale of the facility. Over the past 20 years, during times of electricity outages caused by blackouts, financial institutions have recorded very few records lost or cancelled, approximately less than 0.5%. Most Tier 1 banks and Government Treasury / financial institution Data Centers (but not your local ATM or bank branch) are CME / EMP hardened worldwide including most stock exchanges. This is only partially true, because if power is supplied by standard commercial service back up power generators, vulnerability may still exist. Experts agree, there is a clear and present danger to any section of the grid when CME's are exposed to high voltage (e.g. 600 volt) transformers operating. I am not aware of any testing on any of the types of transformers used in the North American Grid in a non-powered (charged) state and how magnetic pulses or Coronal Mass Ejections demonstrate levels of survival. But NASA has been working on the problem, creating an alert system to notify Grid Operators called Solar Shield.
It is true, not every electrically powered device, even when shut off is fully insulated from potential CME / EMP threat, the majority of equipment would be safe if they are turned off or not powered at time of an event. In my opinion, EMP events are highly unlikely while CME's do happen, sometimes with little advance warning. NASA published a YouTube video of a CME that narrowly missed earth in 2012.