physics//electromagnetism//electric field//dielectric breakdown

Dielectric breakdown is the sudden failure of an insulating material when the electric field across it exceeds what it can hold, turning it into a conductor for an instant, and it sets the minimum spacing and insulation thickness in every high-voltage design. A spark from a fingertip, a lightning stroke and a failed cable insulation are the same event at different scales.

Dielectric breakdown is the sudden failure of an insulating material when the electric field across it exceeds what it can hold, turning it into a conductor for an instant, and it sets the minimum spacing and insulation thickness in every high-voltage design. A spark from a fingertip, a lightning stroke and a failed cable insulation are the same event at different scales.

The mechanism in a gas is an avalanche. A few free electrons are always present; in a strong enough field they gain enough speed between collisions to knock electrons off the molecules they hit, which is ionization. Each collision frees more electrons, which free more, and within nanoseconds a channel of ions and electrons opens through the gas and the stored charge rushes down it, heating the channel until it glows. For dry air at normal pressure the threshold is about 3 kV per millimetre.

Breakdown is decided by the field, not by the voltage alone.

A few thousand volts on a hand do nothing at a metre and jump a few millimetres to a doorknob; a storm cloud at hundreds of millions of volts jumps kilometres, and both stop at the same field strength.

The number scales the design. Overhead lines are spaced metres apart and hung on long insulator strings because of it; a connector rated for 1 kV needs millimetres of creepage between its pins.

Solids are stronger and less forgiving. Good plastics and ceramics hold tens of kilovolts per millimetre, but once a solid insulator breaks down it is usually burnt through for good, while air heals the moment the spark ends.

Lightning is the extreme case: charge separated inside a storm cloud builds a field over the air below until a channel forms and carries tens of kiloamperes to the ground.