I used it to connect a 12V battery box with USB ports to charge mobile phones during a power outage. The 12 volt supply (plus from the battery) goes to terminal 1, the outer silver terminal. The 12V power output (positive) to an accessory, load, fuse block or busbar is applied to pin 2, the center silver terminal. Ground (negative) goes to terminal 3, the outer brass terminal. When terminal 3 is connected to ground, the blue "Power On" LED lights up. You don't need to connect pin 3 to ground if you don't want the "Power On" LED to be on. For example, my USB ports light up when power is applied, so I can use that light as a power indicator instead of connecting pin 3 to ground. If you are bothered by the blue light, do not connect terminal 3 to ground. All your loads and accessories are grounded directly to the negative battery terminal and do not need to be connected to the switch ground.
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