Using 87 vs 30 for 12V Fused Input or Output?

How come on some tech sites/forums, some installers say that 30 is supposed to be connected to constant 12v fused, but I learned (back when i was 14 years old) to use 87 for 12v constant fused (battery) - so am I doing it wrong? or does it not matter?? So can someone explain the difference ? I tried to google but all I'm finding are more and more relay diagrams!

If you are using it as a single pole single throw switch, it does not matter. 30 is the Common connection. 87A is the normally closed connection. 87 is the normally open connection. If using as a single pole double throw situation, you must power 30 and use 87 as output. Using as s double throw device means it can power 2 different things. One item can be powered when the relay is at rest. When you energize the coil of the relay it will then power the other device.

i am an idiot wrote: ...If using as a single pole double throw situation, you must power 30 and use 87 as output. Using as s double throw device means it can power 2 different things. One item can be powered when the relay is at rest. When you energize the coil of the relay it will then power the other device.

Not in all cases. It will depend on the application. For instance when wiring door lock actuators with a low current switch, 30 is your output, 87a is connected to ground, and 87 is connected to 12V+ on both SPDT relays. Another example is this 5 wire door lock setup below. the12volt • Support the12volt.com