Is it permissible to use a “normally closed” switch to monitor a “trouble” or “supervisory” condition on a fire alarm system?
Certainly. Many devices use a normally closed switch: cover tampers on listed sprinkler flow and pressure switches, damper position indicating switches, and valve tamper cords (in some sprinkler rooms), to name a few. In every case, however, the circuit is supervised by an end-of-line resistor which has some very specific installation requirements. Some older fire alarm panels are incapable of providing a latching supervisory signal which you will have to document if any changes or additions are made.
