Surge Arrestor

A surge arrestor is a small pod with some wires coming out of it that tries to be completely invisible to the cirquit that you wire it into until the moment comes that it sees an 'overvoltage' on the line. Then it quickly changes it's character and becomes a dead short getting rid of whatever voltage is threatening your equipment. Normally you place them in lines going into your system coming from the 'outside' where your lines are subject to all kinds of electrical pollution.

Surge Arrestors are used to protect the sensitive innards of your system (Batteries, Inverter, charge controller and whatever else you've got) from voltages that should not normally be present in your system (such as lightning strike induced surges).

Surge arrestors exist in two main kinds, spark gaps where a small gap between two electrodes allows a spark to jump between the conductor that is being protected and the ground when the voltage goes above the 'gap breakdown' voltage, it then needs to go all the way down to a much lower voltage before the spark will extinguish and then normal operation will continue. The other kind is semiconductor based, it functions in much the same way on the outside, but the inside is based on a semiconductor called a 'transzorb', which is basically nothing but a very large thyristor (for the electronic buffs amongst you).

Spark gaps are on the way out, transzorbs are the coming thing, but both will do the job.