basic wire probing, behavior

Is there a link here that explains and/or demonstrates the basics of wire testing/probing? For example, with a DMM's black probe connected to ground, Let's say you come across a door wire that registers with power (12volt). However, when the door is opened, the wire drops to zero volts and back to 12v when the door is closed. Question is what do you consider this wire to be? Positive or Negative trigger. My feeling would be a negative trigger. What concerns me is if a alarm system has a negative input for the door trigger, what happens when the door is closed and 12v is going through this wire to the alarm main module? Or is this all taken cared of by the brain module. Would diode isolating be necessary so 12v surge does not push through? It would just seem to have a good explanation on how wires behave ans what to look expect. Been looking around but can not find any...

Thats a neg trigger. The alarm genrally doesnt care if there is 12 volts applied to that circuit, all it wants to see is a (-) ground to complete it's [he alarms] circuit to make it go off...Tire Proz Stillwater Mn

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