First of all the surge protector fits like a glove in my Siemens breaker box. Blinking green light means it is active. Please note this device model only fits Siemens brand service boxes and because it protects both phase lines it takes up 2 spaces. Make sure to turn of the main breaker( mine is on top marked 100 Amps) then wear rubber gloves and shoes. Saftey is always first and lots of YouTube videos to help. Not something to mess with if unsure.