Under normal operation, the lightning arrester is off the line i.e. it conducts no current to earth or the gap is non-conducting. On the occurrence of overvoltage, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground. In this way, the excess charge on the line due to the surge is harmlessly conducted through the arrester to the ground instead of being sent back over the line.