For glass, that's true in normal temperatures/voltages, but when it gets hot/melts it starts being a conductor instead. Same if the voltage is high enough, see

For glass, that's true in normal temperatures/voltages, but when it gets hot/melts it starts being a conductor instead. Same if the voltage is high enough, see https://en.wikipedia.org/wiki/Dielectric_strength#Break_down_field_strength https://en.wikipedia.org/wiki/Dielectric_strength#Break_down... for a table and description about that effect.