How Fire Alarm Monitoring Works

How Fire Alarm Monitoring Works

Fire alarm monitoring is a two-step signal chain between your fire alarm panel and the fire department. Under NFPA 72, the entire sequence, panel to monitoring station to fire dispatch, must complete within 180 seconds. Two 90-second clocks, running back-to-back: the first ends when the central station receives the signal, the second ends when the operator notifies fire dispatch. According to the U.S. Fire Administration, approximately 110,000 nonresidential fires occur in the US annually. In an unoccupied building without monitoring, delay from ignition to fire department notification can exceed 17 minutes. With monitoring, that window compresses to under 3 minutes.

What NFPA 72 Actually Requires: Three Types of Supervising Stations

NFPA 72 defines three distinct supervising station categories, and they have meaningfully different obligations. Central station service is the highest standard, defined in UL 827, with staffing minimums, redundant infrastructure, response time requirements, and mandatory runner service. Proprietary and remote supervising stations have fewer formal obligations. Most commercial buildings have central station service, but the certificate of occupancy doesn't specify which type you have.

Runner service is the piece of central station service most building owners don't know is required. Under NFPA 72, when an alarm signal occurs and the system doesn't restore, a runner must arrive at the building within 2 hours. For unresolved trouble signals, the window is 4 hours. If your monitoring company has never mentioned this obligation, ask them directly whether they provide it.

Not all monitoring companies are UL 827-listed central stations. Some operate as remote supervising stations with fewer obligations. Ask your monitoring company for a UL certificate of monitoring, a property-specific document that expires annually and confirms compliant central station service. If they can't produce one, you don't have what most insurers and AHJs expect.

Alarm, Supervisory, and Trouble: Three Signals With Three Response Paths

Your fire alarm system does not send one kind of signal to the monitoring station. It sends three, and the response obligations are different for each. An ALARM signal indicates a confirmed fire condition: smoke or heat detection, a manual pull station, a waterflow switch on an active sprinkler. The central station dispatches fire department immediately. A SUPERVISORY signal indicates a fault in a life-safety subsystem, a sprinkler valve closed, a fire pump not running, a tank low on water. The building has reduced fire protection capability, but it is not on fire. The operator notifies the account contact and the AHJ if required. A TROUBLE signal indicates the fire alarm system itself has a problem, a comm path loss, a low battery, a circuit fault. The operator notifies the account contact so the issue can be resolved before it becomes an impairment.

Building owners frequently assume any signal triggers a fire dispatch. It does not. Trouble and supervisory signals require contact, documentation, and follow-up, but not 911. This is also why the runner-response windows are different by signal type: 2 hours for an unresolved alarm, 4 hours for an unresolved trouble. Understanding which signal your system is sending shapes both your expectations and the questions you can ask your monitoring company when something is reported.

The Two 90-Second Rules Most Building Owners Don't Know

When your fire alarm panel sends a signal, two sequential 90-second clocks begin. The first runs from alarm initiation: the central station must receive, display, and record the signal within 90 seconds. The second clock starts only when the first ends. Once the signal is received and recorded, the operator has another 90 seconds to retransmit to the local fire department. Maximum 180 seconds total from detector activation to fire dispatch notification, the clocks run back-to-back, not in parallel.

In practice, cellular and IP transmission happens in seconds. The binding constraint is the operator response clock. When central stations are short-staffed or handling high signal volumes, that 90-second window gets tight. Some monitoring companies with high subscriber loads and thin staffing run slower-than-code response times in practice. UL 827 audits are periodic, not continuous, compliance between audits depends on the company's internal discipline.

NFPA 72-2016 added limited allowance for call-before-dispatch verification on fire signals, but the 90-second retransmission window still applies. If an operator can't reach someone with authority to cancel within that window, they must dispatch. Most central stations default to immediate dispatch for any fire alarm signal and call the building simultaneously or immediately after.

POTS Retirement: Why Copper-Based Fire Alarms Need to Migrate

POTS stands for Plain Old Telephone Service, the copper landline that most fire alarm communicators used for decades. A DACT (digital alarm communicator transmitter) would dial two phone numbers and transmit the signal over copper. That infrastructure is being retired. The FCC reduced required customer notice periods from 180 days to 90 days in early 2025, and AT&T began retiring copper in affected service areas starting June 2026.

When a carrier retires the copper serving your building, the DACT loses its dial tone. In most cases the panel detects line failure and reports a trouble signal, and the central station reaches out to the account contact to flag the comm loss. The issue is not invisibility, it is that the building is operating on a signal path scheduled for shutdown. The fix is a communicator upgrade ahead of the carrier's retirement date, not after. The 3G cellular sunset in 2022 created the same scenario for buildings with 3G communicators, and POTS retirement does it at much larger scale.

If your fire alarm communicator is still running on a phone line, or on a 3G or older cellular module, plan the upgrade now rather than waiting for a notice. A licensed fire alarm contractor can test the current communication path and confirm what technology is actually in use.

Cellular Fire Alarm Communicators: The Current Standard

A cellular communicator connects to your FACP's telephone output ports, intercepts the dialer signal, and retransmits over cellular to the central station's receiver. It works with virtually any panel brand without reprogramming the FACP. The current commercial standard is 5G LTE-M, designed for IoT and alarm transmission, with better building penetration than standard LTE and carrier support expected well into the 2030s.

The systems we install through our fire alarm services typically include the Napco StarLink SLE-MAX2-FIRE: dual-SIM with both Verizon and AT&T in the same unit, and dual-path with cellular plus IP. If one carrier has an outage, it fails over automatically. If the IP path fails, cellular handles the signal. It's the most resilient communicator architecture available in a single unit.

A single-path cellular communicator meets NFPA 72 as the sole path if supervised at 60-minute intervals. Dual-path with different technology types is required by some jurisdictions and most commercial insurers above the NFPA 72 baseline. Post-2013 NFPA 72 editions require dual-path to use different technology types: cellular plus IP satisfies this requirement, two cellular paths from two different carriers don't.

What Happens in the 4-to-6-Minute Window Before Fire Department Arrival

NFPA 1710 sets response time benchmarks for fire departments: first engine on scene within 4 minutes for 90% of incidents, full effective force within 6 minutes. That's the window between alarm activation and suppression capability arriving at your building. Monitoring gets the clock started. The building systems fill the gap while fire departments are in transit.

When the alarm activates, several systems engage simultaneously: sprinkler heads open where fire reaches the fusible link, HVAC systems shut down or isolate to prevent smoke spread through ductwork, elevators recall to the lobby, and stairwell pressurization maintains egress in high-rise buildings. These building systems do the actual work during that 4-to-6-minute window.

The case for monitoring in unoccupied buildings is straightforward. Without monitoring, no one may call 911 for 17 minutes or more in an empty building overnight. With monitoring, that call happens in under 3 minutes. The difference is more than 14 minutes in an environment where fire damage potential roughly doubles every passing minute.

What UL 827 Certification Requires and Why It Matters

UL 827 is the listing standard for central station alarm services. It requires documented staffing ratios, redundant communication paths, backup power, written signal response procedures for each account, and periodic unannounced audits. An unlisted monitoring service has none of those requirements enforced by a third party. Insurance companies frequently require UL 827-listed monitoring for commercial property coverage, confirm which type you have.

A UL certificate of monitoring is property-specific and expires annually. It names the alarm company and the protected premises. This is the document your insurer or AHJ will request during an inspection or after a loss. If your monitoring company provides certificate documentation, it will reference UL 827 or indicate the specific listing. If it doesn't, ask why.

The annual fire alarm inspection is also where monitoring continuity gets verified. The inspector will test signal transmission to the central station as part of acceptance testing. Any communication path failures should be documented in the inspection report and resolved before the inspection closes out.