Robert, An alarm system hey, When I worked for Westinghouse alarm company about 40 years ago we would use from 15 to 20 Ma. for our contact wetting current in loops. When using switches, you can't rely on anything less to break the oxidization of NO contacts. The system I designed for my home uses a cunstant current source for the NC loop as well as current source's the all NO switches inseries with the LED's powered by 12 volts. I have never had a problem with it. Using constant current allows for some loop resistance variation over time as switches and wiring connections age. No false alarms...... Also you DON'T have to worry about shielding the wiring due to electrical noise.