You and De are probably more versed in this than I.

EMP has burned up telegraph lines, but I imagine today's lines are more shielded, judging by successful transits by squirrels, lol. But the shielding doesn't seem sufficient? Hawaii was impacted by far-away Pacific testing, but they survived and I don't recall seeing anything on lines burned up. Those were traditional nukes.

Today, there are nukes designed to maximize the EMP-effect -- low yield, high energy. I imagine there are degrees of strength of the EMP effect but I don't know the science behind it.

What the push is for is for hardening the key transformers at the core of the grids. They are huge, weigh 100-500 tons, are special order, mostly from outside the US, and take 6 months to 5 years, to build.

Given food disappears off the shelf in a day or two in an emergency and if transportation really does get clobbered, a 90% casualty rate seems possible, presuming other countries don't have enough food to feed 300 million Americans or ability to deliver it.

Therein lies the dispersion benefit of globalization. While our manufacturing might be down, including the machines that build the machines, chips, etc., many parts should be available from around the world.

Passages from two articles below. I would be curious what you think on this info.

1. EMP

James Woolsey and Peter Pry, 2015

Nuclear EMP attack is part of the military doctrines, plans and exercises of all of these nations for a revolutionary new way of warfare that focuses on attacking electric grids and civilian critical infrastructures--what they call Total Information Warfare or No Contact Wars, and what some western analysts call Cybergeddon or Blackout Wars.

The nuclear EMP threat is as real as North Korea's KSM-3 satellite, that regularly orbits over the U.S. on the optimum trajectory and altitude to evade our National Missile Defenses and, if the KSM-3 were a nuclear warhead, to place an EMP field over all 48 contiguous United States.

The EMP threat is as real as non-nuclear radiofrequency weapons that have already been used by terrorists and criminals in Europe and Asia, and no doubt will sooner or later be used here against America. ...

Yet a recent Wall Street Journal article (May 1, 2015) on NORAD moving back into Cheyenne Mountain and spending $700 million to further harden the mountain against a nuclear EMP attack from North Korea, received hundreds of comments from shocked readers, half of whom still think that EMP is science fiction.

We know that EMP is not science fiction but an existential threat that would have catastrophic consequences for our society because of high-altitude nuclear tests by the U.S. and Russia during the early Cold War, decades of underground nuclear testing, and over 50 years of tests using EMP simulators.  For example, in 1961 and 1962, the USSR conducted several EMP tests in Kazakhstan above its own territory, deliberately destroying the electric grid and other critical infrastructures over an area larger than Western Europe.  The Congressional EMP Commission based its threat assessment partially on using EMP simulators to test modern electronics--which the Commission found are over one million times more vulnerable than the electronics of the 1960s. ...

The iconic EMP attack detonates a single warhead about 300 kilometers high over the center of the U.S., generating an EMP field over all 48 contiguous United States.

However, any warhead detonated 30 kilometers high anywhere over the eastern half of the U.S. would collapse the Eastern Grid.  The Eastern Grid generates 75 percent of U.S. electricity and supports most of the national population.  Such an attack could be made by a short-range Scud missile launched off a freighter, by a jet fighter or small private jet doing a zoom climb, or even by a meteorological balloon.

According to a February 2015 article by President Ronald Reagan's national security brain trust--Dr. William Graham who was Reagan's Science Advisor and ran NASA, Ambassador Henry Cooper who was Director of the Strategic Defense Initiative, and Fritz Ermarth who was Chairman of the National Intelligence Council--North Korea and Iran have both practiced the iconic nuclear EMP attack against the United States.  Both nations have orbited satellites on south polar trajectories that evade U.S. early warning radars and National Missile Defenses.  North Korea and Iran have both orbited satellites at altitudes that, if the satellites were nuclear warheads, would place an EMP field over all 48 contiguous United States. ...

North Korea and Iran have also apparently practiced making a nuclear EMP attack using a short-range missile launched off a freighter.  Such an attack could be conducted anonymously to escape U.S. retaliation--thus defeating nuclear deterrence. ...

Natural EMP. ... In 1989, the Hydro-Quebec Storm blacked-out half of Canada for a day causing economic losses amounting to billions of dollars.
However, we are most concerned about the rare solar super-storm, like the 1921 Railroad Storm, which happened before American civilization became dependent for survival upon electricity and the electric grid.  The National Academy of Sciences estimates that if the Railroad Storm were to recur today, there would be a nationwide blackout with recovery requiring 4-10 years, if recovery is possible at all.

The most powerful geomagnetic storm on record is the 1859 Carrington Event.  Estimates are that Carrington was about 10 times more powerful than the 1921 Railroad Storm and 100 times more powerful than the 1989 Hydro-Quebec Storm.  The Carrington Event was a worldwide phenomenon, causing forest fires from flaring telegraph lines, burning telegraph stations, and destroying the just laid intercontinental telegraph cable at the bottom of the Atlantic Ocean.

If a solar super-storm like the Carrington Event recurred today, it would collapse electric grids and life-sustaining critical infrastructures worldwide, putting at risk the lives of billions.
NASA in July 2014 reported that two years earlier, on July 23, 2012 , the Earth narrowly escaped another Carrington Event.  A Carrington-class coronal mass ejection crossed the path of the Earth, missing our planet by just three days.  NASA assesses that the resulting geomagnetic storm would have had catastrophic consequences worldwide. ...

Radio-Frequency Weapons. ... Reportedly, according to the Wall Street Journal, a study by the U.S. Federal Energy Regulatory Commission warns that a terrorist attack that destroys  just 9 key transformer substations could cause a nationwide blackout lasting 18 months. ...

Nuclear EMP is worse than natural EMP and the EMP from RFWs because it combines several threats in one.  Nuclear EMP has a long-wavelength component like a geomagnetic super-storm, a short-wavelength component like Radio-Frequency Weapons, a mid-wavelength component like lightning--and is potentially more powerful and can do deeper damage than all three. 
Thus, protecting the electric grid and other critical infrastructures from nuclear EMP attack will also protect against a Carrington Event and RFWs.  Moreover, protecting against nuclear EMP will also protect the grid and other critical infrastructures from the worst over-voltages that may be generated by severe weather, physical sabotage, or cyber attacks.

http://www.familysecuritymatters.org/publications/detail/heading-toward-an-emp-catastrophe

2. Transformers

Large power transformers are essential critical infrastructure to the electric grid, and are huge, weighing up to 820,000 pounds.  If large power transformers are destroyed by a geomagnetic disturbance (GMD) electromagnetic pulse (EMP), cyber-attack, sabotage, severe weather, floods, or simply old age, parts or all of the electric grid could be down in a region for 6 months to 2 years.
This is because the USA imports 85% of them, there is competition with other nations for limited production and raw materials such as special grade electrical steel, a high cost ranging from $2.5 to $10 million dollars (including transport and installation), and they are custom built, with long lead times to design, bid, manufacture, and deliver, with components depending on long foreign production and supply chains.  The United States large power transformers are aging faster than they’re being replaced, and even more are needed for new intermittent renewable generation, which has the potential to damage them if not integrated carefully into the existing electric grid.  There are possibly tens of thousands of LPT’s in America, mostly built between 1954 and 1978, so an increasing percentage of these aging LPT’s will need to be replaced within the next few decades. 

Although prices vary by manufacturer and by size, an LPT can cost millions of dollars and weigh between approximately 100 and 400 tons (or between 200,000 and 800,000 pounds). The procurement and manufacturing of LPTs is a complex process that includes pre-qualification of manufacturers, a competitive bidding process, the purchase of raw materials, and special modes of transportation due to its size and weight.

http://energyskeptic.com/2015/power-transformers-that-take-up-to-2-years-to-build/