Spark Gap Surge arresters

Spark Gap Arresters are the oldest form of Surge arresters.  The spark Gap lightning arrestors have two horned terminals with a distance between them.  One of the terminals is connected to the line while the other is connected to the earth.  
During normal current, the gap prevents leakage of current.  In the event of an surge, the voltage across the gap is sufficient to cause breakdown and start conducting.  This reduces the steepness of the surge. 

The arc which is formed during operation is extinguished as it rises across the airgap.

The downside of the spark gap arresters is that it is difficult to precisely set the trigger voltage.  Another difficulty with Spark Gap lightning arresters is to switch off the follow-on current after the voltage has returned to normal.