Under most conditions, sparks occur between contacts of switches, circuit breakers, etc. during the split instant between fully closed and fully open positions (these sparks are really refered to as arcs or arcing). This instant is the point when the full pressure of the internal spring-loaded mechanism that is pushing or pulling contacts together or apart; is not yet felt. This pressure is usually felt only in the fully closed or fully open positions.
The brightness and duration of the arc is directly tied to the amount of power is being switched and the condition of the contacts themselves (contacts become pitted and dirty over time and arcing becomes more noticeable). Many larger switches and circuit breakers employ specially engineered arc flash suppression techniques and materials as the intense heat from the arc is destructive to ordinary insulated switch materials. Ordinarily, the arcing should not be visible to the naked eye.
Here\'s a video of an incredible arc at a power switching station when the switch is opened.
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