Depending on what equipment you need protected from voltage surge, you can employ one of three devices. One is stock; the other two are modified. ""Better"" (right in photo): Metal-oxide varistors for surge protection. ""Best"" (left in photo): Metal-oxide varistors, RF chokes, and capacitors for surge protection and filtering. Both surge protectors use MOV protection between hot and neutral lines and between both legs and ground for full isolation from transients.