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To protect electronics from a massive EMP, first as everyone stated you need to place it in a Faraday cage second more importantly, you need to either un-plug it or place a surge-cutter between the faraday cage and the power source since a power surge will be carried within the electric power lines.
It does not matter if the equipment is powered on or off since a massive induction current will burn the circuit especially anything using ICs since they are very sensitive to large currency compared to vacuum tubes.
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