The fire alarm system is the brain of a hospital's emergency response, but it acts almost entirely through other building systems — HVAC, doors, elevators, medical gas, electrical, and security. This Article is about the wiring between systems: the interlock matrix that defines which input causes which output, and the written Sequence of Operations (SOO) that makes that logic explicit, testable, and auditable. The neighboring Articles cover how smoke control and stairwell pressurization are designed, and how fire-protection resilience and emergency power are built; this one covers how all of those systems talk to each other on an alarm and how the integrated behavior is documented and proven.
In a modern hospital, no single system "handles a fire." A smoke detector in a corridor trips the fire alarm control unit (FACU), which in turn closes smoke dampers, shuts down an air handler, releases magnetically held smoke-barrier doors, recalls elevators, unlocks egress doors, and tells the smoke-control system in an atrium to start exhausting — all within seconds, in a defined order, with confirmation feedback. The fire alarm panel is rarely the system that does the protecting; it is the system that commands the protecting.
The interlock layer is the set of input-to-output relationships that cross system boundaries. Because those relationships span trades — fire alarm (Division 28), HVAC and controls (Division 23), conveying systems (Division 14), electrical (Division 26), plumbing/medical gas (Division 22), and electronic safety/security (Division 28) — they are the single most common source of:
The Sequence of Operations (also called the fire alarm SOO, the cause-and-effect matrix, or the input/output matrix) is the controlling document that captures every one of these relationships in one place. It is the contract the design intends, the program the installers build to, the script the integrated test follows, and the record the AHJ and accreditation surveyor check against. Treating it as a coordination artifact — owned, baselined, and version-controlled from design through activation — is the single highest-leverage practice in this domain.
The interlock behavior is not optional or stylistic; the major relationships are mandated. The SOO must reconcile the requirements of several codes that each govern a slice of the matrix:
| Code / standard | What it governs in the interlock layer |
|---|---|
| NFPA 72 (National Fire Alarm & Signaling Code) | The fire alarm system itself, its inputs/outputs, supervision of interconnected circuits, the survivability of notification and control pathways, monitoring of integrity of connections to other systems, and the documentation/record-of-completion that the SOO feeds. |
| NFPA 101 (Life Safety Code) & IBC/IFC | Egress-door release/unlock on alarm, electromagnetically locked and delayed-egress door behavior, smoke-barrier and corridor door release of hold-opens, elevator-lobby and area-of-refuge requirements, and the defend-in-place strategy the whole matrix serves. |
| NFPA 99 (Health Care Facilities Code) | Medical-gas zone-valve and alarm relationships, behavior of systems serving critical-care spaces, and the risk-category framework that drives how aggressive the response must be. |
| NFPA 90A | Air-handling-system shutdown and smoke-damper closure on detection, and the duct-detector relationships that drive HVAC response. |
| NFPA 92 (Standards for Smoke Control Systems) | The activation, control, and response of dedicated and non-dedicated smoke-control systems, including atrium exhaust and zoned smoke control, and the integration of those sequences with the FACU. |
| NFPA 105 | Smoke-door and smoke-damper performance and their release on detection. |
| ASME A17.1 / A17.3 (Safety Code for Elevators) | Phase I emergency recall, Phase II firefighter operation, shunt-trip of elevator power ahead of sprinkler discharge in the machine room/hoistway, and the detector zones that drive recall direction. |
| NEC / NFPA 70, Article 517 | The electrical interconnection, circuit integrity, and the essential-electrical-system relationships that keep the interlock layer powered during a fire. |
| NFPA 110 | The emergency/standby power that must carry smoke-control fans, fire pumps, and FACU loads — the power side of the interlock. |
| NFPA 80 / NFPA 110-type listings for hold-opens | Listed release of fire and smoke door hold-open devices on alarm. |
| NFPA 4 (Integrated Fire Protection and Life Safety System Testing) | The requirement to integrated-test the whole matrix — not just each system in isolation — using a test plan derived from the SOO. (Covered in depth by the integrated-testing sibling Article; the SOO is its primary input.) |
The AHJ — the local fire marshal or building official — and the accreditation organization (Joint Commission, DNV, or another CMS-deeming body, under the CMS Conditions of Participation and the Life Safety chapter) both inspect the result of these relationships. CMS adopts NFPA 101 and NFPA 99 by reference, which makes the interlock behavior a Condition-of-Participation matter, not merely a building-code matter.
The core engineering artifact is a matrix that lists every fire-alarm initiating input down one axis and every controlled output across the other, with the intended action at each intersection. A useful matrix distinguishes the initiating zone (where the event occurs) because most responses are zone-specific rather than building-wide.
Typical initiating inputs: