Ever since Tesla and Edison crossed their ‘AC / DC sabres’, flipping a switch of a power circuit introduced a shocking risk: arcing. Arcing, both inventors learned later, is caused when the potential difference (measured in volts) between poles causes the air between them to heat up so extremely that it turns into a plasma. In other ominous words: the insulator becomes a conductor.

When switching the power on, this added conductivity is not the biggest deal. You want electrons to move from one pole to the other, so the airgap is bridged and the arc will cease to exist the moment the circuit is closed. When switching the power off, you don’t want electrons to keep moving, let alone give you unexpected potentially dangerous fireworks.

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