Thing about a radio antenna. What a radio antenna is basically is a wire either sticking up in the air or running horizontal to the ground. ( like in the classic TV antenna).
The distribution wires of the power grid are also wires running horizontal to the ground.
Because of this they can work as antennas. So a solar flare can create additional currents in the wires. Currents that are not supposed to be there. It doesn’t matter of the power is on or off its going to get currents that are not intended.
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