The electric fences were used to defend the Federal Military's bases such as the military assistance base on Roku San. They are placed completely around the perimeters of bases and each segment is physically connected, unlike the pillars. The current created through them was constant and very powerful, capable of killing any Arachnid that came into contact with it.
On Roku San during the Second Bug War the base's main electric systems went down causing the perimeter fences to lose power. In this situation Arachnid Warrior Bugs were easily capable of ripping large segments of the fences apart in short amounts of time.