A non-conductive lining is essential; cage contents must not come in conductive contact with the metal cage material itself.
Most consumer electronics come wrapped in a nice non-conductive case.
Personally, I keep my essential electronics in a 1/4" thick steel safe, lined with fire resistant materials. Even this may not offer enough protection:
EMP Attacks—What the U.S. Must Do Now
Excerpt:
In many respects, an EMP attack is a unique and unprecedented threat to the United States. EMP is a high-intensity burst of electromagnetic energy caused by the rapid acceleration of charged particles. EMP is most often created from gamma rays emitted during a nuclear explosion. At altitudes between 40 to 400 kilometers, these gamma rays produce high-energy free electrons that give rise to an oscillating electric current that destroys electronic equipment.[1]
The direct effects are due to electromagnetic “shocking” of electronics and the stressing of electrical systems. Indirect effects include the cascading damage that occurs because of these shocked, damaged, and destroyed electronics and electronic systems that are embedded in critical infrastructure. These indirect effects can be even more severe than the direct effects. For example, surges might simultaneously cause electrical fires and incapacitate traffic control and emergency dispatch systems. In turn, responders will be unable to respond to resulting mass fires.
An electromagnetic pulse consists of three components: E1 is a free-field energy pulse that occurs in a fraction of a second. The generated “electromagnetic shock” then damages, disrupts, and destroys electronics and electronic systems in a near simultaneous time frame over a very large area. Faraday cage protection and other mechanisms designed to defend against lighting strikes will not withstand this assault. Only specialized technology integrated into equipment can harden it against EMP. If the electromagnetic distortion is large enough, the E1 shock will even destroy lightly EMP-shielded equipment in addition to most consumer electronics.[2] Devices that incorporate antennas by nature accept electronic signals and cannot be shielded against E1, meaning trillions of dollars worth of electronics will fail after an EMP assault, regardless of protective measures. E1 is also particularly worrisome because it destroys Supervisory Control and Data Acquisition components that are critical to many of our national infrastructures.[3]
E2 covers essentially the same area as E1 but is more geographically widespread and has lower amplitude than E1. The E2 component has similar effects as lightning. In general, it would not be a critical threat to infrastructure, since most systems have built-in protection against occasional lightning strikes. The E2 threat compounds that of the E1 component since it strikes a fraction of a second after the E1 has very likely damaged or destroyed the protective devices that would have prevented E2 damage. The syncretistic effects mean that E2 typically inflicts more damage than E1 since it bypasses traditional protective measures, vastly amplifying the damage inflicted by EMP.[4]
E3 is a longer duration pulse, lasting up to one minute. It disrupts long electricity transmission lines and subsequently causes damage to the electrical supply and distribution systems connected to these lines. This E3 element of EMP is not a freely propagating wave, but is a result of the electromagnetic distortion in the earth’s atmosphere. In this regard, E3 is similar to a massive geomagnetic storm, and is particularly damaging to long-line infrastructure, such as electrical cables and transformers. A moderate blast of E3 reportedly could directly affect up to 70 percent of the U.S. power grid.[5]
The timing of the three components is an important part of the equation in relation to the damage that EMP generates. The damage from each strike amplifies the damage caused by each succeeding strike. The combination of the three components can cause irreversible damage to many electronic systems. With the combined damage from earlier E1 and E2 blasts, E3 has the potential to destroy the nation’s electrical grid and thus inflict catastrophic damage on the United States.[6]
In practice, the precise EMP effects vary depending on many factors. One of the most important variables is altitude. The most effective altitude is above the visible horizon. If detonation is too low, most of the electro-magnetic force from the EMP will be driven into the ground, creating deadly nuclear fallout that deprives the weapon of its non-casualty appeal. Damage is inversely related to the target’s distance from the epicenter of detonation. In general, the further from the epicenter, the weaker the EMP effects. Yield is another factor to consider. The higher the yield, the greater the effect. Even so, since the effects travel through electric lines and waterways, and have secondary spill-over impacts on other infrastructure, it is difficult to predict the possible extent of damage from a large-scale EMP attack.[7]
https://www.heritage.org/defense/report/emp-attacks-what-the-us-must-do-now