EMP (electromagnetic pulse) can hit without a warning or with it and catch anyone by surprise. It is likely there won't be much time to prepare and it won't be the best time to look for knowledge on how to prepare when it may be happening.

EMP can be as a natural doom coming from the Sun through massive CME, or caused by weapons, nuclear explosions, EMP bombs and others. Their effects can also greatly differ. Some types of EMP effect long wires, electrical grid (E3), others directly electronic equipment (E1, E2). Numerous doom scenarios have EMP on the list.

[link to en.wikipedia.org]

With electric grid down and possibly (but not necessarily) no electronic equipment, survival may become a real issue. Survival equipment like flashlight, radio or important equipment that is unprotected can be lost.

EMP protection may be a bit more complicated than simple Faraday cage.

[link to www.ets-lindgren.com]

Any connected wire, metal structure, is it grounding or pipe, will act as an antenna making things a lot worse. These may start sparks and fires as a result. Underground cannot provide protection from EMP unless hundreds of yards underground are considered. Radio waves can travel through soil, and it is not a "big obstacle" for high power EMP.

A lot of shelters have generators, lights, ventilation that are unprotected and are likely to go out in case of EMP. This part is often omitted and only few survivalists take into account possible outcomes.