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:

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...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.

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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.



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