The 721 heavy duty selector switch provides four possible operation settings, Off, Battery 1, Both and Battery 2 giving the choice of charging a selected battery or isolating current draw to a single battery. The switch can be recess mounted in a 85mm hole or surface mounted. Screw holes 5mm roundhead. Removable rear cover insulates the rear terminals against short circuit and removable side plates provide access for heavy duty battery cables. Please note this system will not protect electronics from harmful engine starting spikes.