First, while EMP and CMP are different both can cause the same type of damage to electronic devices.
An EMP pulse would occur in just a few one ten thousandths of a second. That is what makes it so dangerous, it occurs so quickly that there is no warning. It is thousands of times more powerful than lightning strikes and occurs many thousands of times quicker. They will only occur as a weapon is detonated and there may or may not be a second detonation. Personally, I would wait at least twenty-four hours to break into my Faraday cages and retrieve any electronic I had stored. The reason for this is to allow time for a second weapon to be detonated.
CMP or Coronal Magnetic Pulse is cause by solar storms. Examples would be the Carrington Event in 1859 which lasted 2 days. Intense storms typically last 1-3 days and the surges are like a continuous series of waves. Today we should have from 24 to 92 hours of warning. That’s enough time for the utilities to shut things down in a controlled fashion, if they heed the warning.
The dangers of an EMP or CMP event are real. The electric grid is old, in some cases well over a hundred years in age, it also lacks essential backups to prevent outages. According to a 2014 report from the Library of Congress’ Congressional Research Service, “Despite their great size and internal complexity [high voltage] transformers can be readily disabled or destroyed.” Most of these transformers are no longer manufactured in the US. Many come from China.
A study by Penn State University found that, “This vulnerability of the electric power grid is inherent to its organization and therefore cannot be easily addressed without significant investment.”
In case of a major event of either type you would expect to have a large scale failure of the power grid that could last from 6 months to years. At the same time, many vehicles would fail and many smaller electronic items would fail. There is a lot of questions about radios and other small devices that were not plugged in, some people think they would survive. There seems to be a lot of confusion in the information that is available on this subject. I would error on the side of caution.
You can protect small electronic items by building a faraday cage. A faraday is defined as an enclosure formed by conductive material or by a mesh of such material. Such an enclosure blocks external static and non-static electric fields by channeling electricity through the mesh, providing constant voltage on all sides of the enclosure.
The following links provide additional information on EMP, CMP and improvising homemade faraday cages.
- An Excellent Article on the Dangers of EMP
- More on EMP Protection
- What is EMP (Electromagnetic pulse)
- LEDs and Faraday Cages
- Protecting Electronics with Faraday Cages
- More on Electromagnetic Pulse