Terminal #85 = Power , 12 volt, switched, Fused at about 1 amp.
Terminal #86 = Ground or low, To hard a ground properly burnished area, and star washer, OR Can be used as switching control for a sender / Sensor device.
Terminal # 87 = Normally Open CONTACT. Power and larger wires from the current source (Battery) Goes here, Via a proper Fuse / Fuse link. Power is passed through these terminals ONLY when the coil is activated.
Terminal # 30 = Center-wiper OR movable Contact. Power for The End user Device (Fan, Coil, Ect..) and larger wires go here.
Terminal #87a = Normally CLOSED terminal..power from the Current source Battery From the Center-wiper) is passed straight through to the device (Fan, Coil, Ect..)Only when the relay is off..Can be used to switch between two circuits OR as an "OFF" indicator for the relay coil / Contact state..will go out when relay is activated..or shut down the circuit on it as the coil activates.
Use 87 for the high current terminal (Battery From the Fuse link/fuse) BECAUSE, If you have an open, unused, terminal (like 87A, normal on a fan type circuit) That terminal will have full battery power when the relay is off..You can drop a tool on it, or a ring or watchband on it (you shouldn't be wearing..) and it will have full battery power when off, Shorting out..
Using the above configuration, the only terminal that will have power in ANY state , is 87, until the relay is activated, then it will transfer to 30..UNLESS you plan on using 87A, then swap power to 30..