In order to protect the pipe insulation and the insulating flanges against damage caused by surge voltages, isolating spark gaps are used. In the event of a surge voltage, the isolating spark gap becomes low resistant and discharges the voltage. Lightning protection equipotential bonding is thus ensured.
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current".