Fire Alarm Pull Stations: Closing a Circuit, Not Shorting It
Pull stations close a circuit (not short it). Fire alarm panels distinguish alarm (closed), normal (supervised resistance), and trouble (open circuit) using end-of-line resistors.
A fire alarm pull station works by closing (completing) an electrical circuit, not shorting it — an important distinction. Normal state: The circuit is open (disconnected). No current flows. Activated state: Pulling the handle brings two contacts together, closing the circuit. Current flows through the fire alarm loop. The fire alarm control panel monitors the loop for: - Open circuit = trouble/fault (wire break) - Closed circuit = alarm (device activated) - Specific resistance values = normal supervision When someone manually touches exposed wires on a pull station missing its handle, they are closing the circuit — doing exactly what the handle mechanism would do. This is NOT a short circuit. Short circuit vs closed circuit: - Closed circuit: Current flows through the intended path at the designed resistance - Short circuit: Current bypasses the intended path through an unintended low-resistance connection, drawing excessive current Fire alarm systems use supervised circuits with end-of-line resistors specifically so the panel can distinguish between normal (supervised resistance), alarm (closed/low resistance), and trouble (open/broken wire).