A light switch will be at its most vulnerable when it is opening or closing. When the light is switched on there will be almost no resistance in the switch, and although the current is now flowing, the power dissipated in the switch will be quite low.

 

Conversely when the light is switched off then the switch will have an almost infinite resistance, and there is no current flowing at all, so the power dissipated will be zero. The danger to the switch is therefore during the time it is switched on or off, and the resistance changes from almost zero, to nearly infinite.

 

It is clear that during this time the light switch must have a finite resistance, and as current will be starting to flow, then power will be dissipated in the switch. The longer the switch remains in this state (partly on, partly off) the more it will heat up, and the more likely it will become damaged. Therefore switches are always designed with some sort of quick-break technology (eg over center spring) so that they switch from fully on to fully off (and back again) as quickly as possible.

 

An additional constraint on the design is that if the switch is supplying an inductive load, then breaking the contact will develop a large voltage across the switch, resulting in an electric arc. This both prolongs the time of the off-on transition, and worsens the heating effect.

 

The arc also causes corrosion of the switch contacts, and the contact area becomes fouled with corrosion by-products. The life of the switch is therefore often specified as the number of cycles of switching (disconnection) it can endure at the specified current.