Automotive Battery Switch and relay wiring harness

02    Relays are a mechanical device that can connect or disconnect power to an accessory when it gets a low voltage ‘signal’ from a switch. Some people may ask why they should bother using a relay when you can just wire an accessory directly through a switch to its power source. There are two main reasons why relays are utilized in the automative wiring harness:

First, Using a relay keeps the higher voltage out of the passenger compartment of the vehicle and is just less taxing on your vehicle’s electrical system in general. If something were to fail and short, the chances of an interior fire are significantly reduced if a relay is utilized to allow the higher voltage switching occur in the engine compartment. This also reduces the load on the interior fuse panel by placing a smaller demand on it.

Second, It allows the use of less heavy gauge wire. The longer the wire, the higher the resistance. By using a relay near the item being switched, you use less of the heavier gauge wire.