Cadmium plated isolator with terminal nuts and lock-washers. 3 terminal isolator for metal frame ground systems, 4 terminals has insulated ground terminal for use with fiberglass or other non-conductive materials. Isolates starting battery, provides secondary battery current for auxiliary power and full time charging of both batteries. 4 terminals has insulated ground terminal | Tekonsha 4 Terminal 12V Breakaway Battery Solenoid