Detecting on- and off-state of a micro switch
As I understand, you can't change the switch to a SPDT one, which would be the simplest.

Therefore, a possible solution (keeping the SPST switch you have now) is to add a resistor in parallel with the switch. Here is an example with a 5V supply and the currents you suggested in your example:







simulate this circuit &ndash; Schematic created using CircuitLab

Here, there are four possible states you can detect:


5mA when the switch is closed
1mA when the switch is opened
0mA when one of the long wire is broken (fault)
10mA when the long wires are shorted (fault)


Of course, you can't cover ALL kind of fault cases. For example, if the switch itself is broken (stays closed or opened all the time), you won't be able to detect this. Same if there is one faulty resistor. What you can detect here is when the long wires link is faulty.

Now, regarding the theory behind this: just Ohm's law.