Control Panels, Circuits, and Wiring Methods
Panel anatomy
A fire alarm control panel contains the power supply, the CPU/control logic, the SLC or conventional zone cards, NAC outputs, relay outputs, the communicator (DACT or IP/cellular), batteries, and terminations for remote annunciators. The panel must detect and annunciate trouble conditions including open circuits, short circuits, ground faults, and loss of primary or secondary power. Addressable devices carry individual addresses; conventional zones report as a group.
Circuit classes
NFPA 72 defines pathway classes (A, B, C, D, E, F, N, X). The two you will be tested on: Class B is a two-wire circuit that annunciates a trouble on an open or ground fault but cannot continue to operate past a single break. Class A is a four-wire (out-and-back) circuit that continues to function with a single open - the panel reports a trouble while all devices still operate. Class X adds survivability, so the circuit keeps working during a fire condition. Where the building code requires circuit integrity or survivability (high-rise, certain occupancies), Class A, Class X, or a listed circuit-protection system is mandatory.
Wiring methods
NEC Article 760 splits fire alarm wiring into power-limited (PLFA) and non-power-limited (NPLFA) circuits, with cable types like FPL, FPLR (riser), and FPLP (plenum) selected by location. PLFA circuits must generally maintain separation from power and Class 1 circuits (commonly 2 in., or a barrier/raceway) so a fault cannot couple power into the alarm circuit. Conductors should be identified at terminals, and fire alarm circuits must not be shared with other systems.
Worked example
A designer runs a single two-wire SLC out to a wing of a high-rise and back to nothing. If the building code requires survivability, Class B is not acceptable - the run must be Class A or Class X with the required protection.
Traps
Calling a four-wire circuit "Class B"; using plenum cable in a riser; mixing power and fire alarm conductors in one raceway.