In the event of a lightning strike or surge the surge arrestor conducts the charge bleeding it out of the circuit to ground.

PV systems are mounted in locations vulnerable to lightning. The cost of surge protectors are negligible compared to the cost of the equipment being protected.  Consider installing both a DC and an AC surge protector within your system. The DC surge protector can be mounted close to and electrically prior to the solar charge controller.  The AC surge protector can be mounted on the AC load or distribution panel. Both should be connected to the grounded service conductor, or service equipment grounding conductor or a separate grounding conductor.

The Midnite Solar surge suppressor is a solid state device. The MidNite Solar 115V Surge Protector Device (MNSPD) is a Type 2 device per UL1449 rev3 and has an outdoor rating of 4X for outdoor use. Protection for battery circuits.

The basic design of the DELTA silicon oxide varistor (SOV) arrestor is a pair of metal electrodes separated by a silicon oxide compound. Under normal conditions, the silicon oxide is a good insulator so that no line current flows between the electrodes to ground. When an excessively high voltage occurs on the electrodes, the high energy electrical flood ionizes the silicon oxide, changing it from a compound to silicon separate from oxygen. Since the silicon ion is a good conductor, the high energy current is conducted to ground. When the voltage falls toward normal, the silicon and oxygen recombine, forming silicon oxide and shutting off the conduction.