10.4 Secondary Power Battery Calculations & Supervising Station Monitoring

10.4 Secondary Power Battery Calculations & Supervising Station Monitoring

A fire alarm system cannot safeguard life and property if electrical failure disables its control units during a disaster. Fires frequently ignite during severe weather events, municipal power grid outages, or electrical distribution failures within a building. Consequently, NFPA 72 Chapter 10 establishes rigorous redundancy criteria for primary and secondary power supplies, and Chapter 26 dictates how fire signals are transmitted to off-site emergency dispatch centers. Under NFPA 1030 JPR 8.3.16, the fire plan examiner must evaluate secondary battery load calculations, audit ampere-hour formulas, enforce the mandatory safety factor, and verify supervising station communication transmission pathways.

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Primary and Secondary Power Supplies (NFPA 72 Chapter 10)

Every fire alarm system must be served by at least two independent, reliable power supplies: a primary power source and a secondary (standby) power source.

Primary Power Supply (the applicable NFPA 72 provisions)

The primary power supply must be an unswitched, dedicated branch circuit supplied by a commercial electric utility or a continuously supervised on-site prime-mover generator. To prevent accidental power interruption, NFPA 72 mandates:

1. Dedicated Branch Circuit: The circuit must serve the fire alarm control unit and its associated power boosters exclusively; no other lighting, receptacle, or appliance loads are permitted.
2. Mechanical Circuit Breaker Lock-On: Protect the supply disconnect against unauthorized operation by the means permitted by the applicable provisions, and identify it and its location. Do not assume one mechanical lock arrangement is the only permitted method.
3. Red Marking and Labeling: The circuit breaker handle or enclosure must be clearly identified in red and labeled "FIRE ALARM CIRCUIT."
4. Location Documentation: The exact location of the dedicated branch circuit breaker (electrical room number, panelboard ID, and circuit number) must be permanently marked inside the FACU enclosure.

Secondary Power Supply (the applicable NFPA 72 provisions)

When primary commercial utility power fails, the system must automatically transfer to secondary power within $10\text{ seconds}$ without loss of signal data or false alarm generation. Secondary power can be supplied by:

1. Storage Batteries: Valve-regulated lead-acid (VRLA) or listed nickel-cadmium/lithium-ion batteries supported by an automatic battery charger.
2. Automatic Emergency Engine-Generator: An emergency generator complying with NFPA 110 (Standard for Emergency and Standby Power Systems) (Type 10, Class 24, Level 1). When a generator serves as secondary power, the applicable NFPA 72 provisions mandates that storage batteries must still be provided with sufficient capacity to operate the system for not less than $4\text{ hours}$ of standby, bridging the gap during generator startup, transfer switch cycling, or fuel maintenance.

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Secondary Power Operational Durations

Under the applicable NFPA 72 provisions, secondary storage battery supplies must satisfy strict operational runtimes:

1. Standard Protected Premises Systems (Non-EVACS)

• Standby Duration: Must operate the entire system under normal non-alarm quiescent (supervisory) conditions for not less than $24\text{ hours}$.
• Alarm Duration: At the end of the 24-hour standby period, the secondary power supply must be capable of operating all connected notification appliances and emergency control functions at full maximum load for not less than $5\text{ minutes}$.

2. Emergency Voice/Alarm Communication Systems (EVACS)

• Standby Duration: Must operate the system under normal supervisory conditions for not less than $24\text{ hours}$.
• Alarm Duration: At the end of the 24-hour standby period, the secondary power supply must be capable of operating all audio amplifiers, speakers, strobes, and emergency voice channels under full evacuation alarm load for not less than $15\text{ minutes}$ with any additional duration required by the actual system and approved criteria.

Other System Types

Supervising-station, household, generator-backed and other arrangements have their own provisions. Identify the actual equipment and permitted supply combination before selecting durations. Do not transfer one protected-premises battery formula indiscriminately to every receiver or communication system.

The Secondary Battery Capacity Calculation & The 1.25 Safety Factor

A critical task in fire alarm plan review is performing an independent mathematical audit of the designer's battery ampere-hour (Ah) calculation.

The Mathematical Formula

Total battery capacity ($C$) in ampere-hours is determined by combining the quiescent standby demand and the dynamic alarm demand, then applying a mandatory $1.25$ aging multiplier:

$C = 1.25 \times \left[ \left(I_{\text{standby}} \times T_{\text{standby}}\right) + \left(I_{\text{alarm}} \times T_{\text{alarm}}\right) \right]$

Where:

• $C$ = Required minimum battery capacity in ampere-hours ($\text{Ah}$).
• $I_{\text{standby}}$ = Total supervisory quiescent current draw of all FACU modules, LCD annunciators, smoke detectors, monitor modules, and dialers under non-alarm conditions (amperes, $\text{A}$).
• $T_{\text{standby}}$ = Required standby time, equal to $24\text{ hours}$.
• $I_{\text{alarm}}$ = Total active current draw of the FACU, illuminated annunciators, energized relays, auxiliary control modules, and all connected notification appliance circuits (horns, strobes, speakers) under full alarm condition (amperes, $\text{A}$).
• $T_{\text{alarm}}$ = Required alarm duration converted to hours:
  • For standard systems: $5\text{ minutes} = \frac{5}{60} = \mathbf{0.0833\text{ hours}}$.
  • For EVACS systems: $15\text{ minutes} = \frac{15}{60} = \mathbf{0.25\text{ hours}}$.
• $1.25$ Multiplier: 25% increase for aging (or aging margin) codified in the applicable NFPA 72 provisions.

Engineering Rationale for the 1.25 Multiplier

Lead-acid batteries experience continuous electro-chemical degradation throughout their operating life. Grid plate oxidation, electrolyte stratification, internal plate sulfation, and elevated mechanical room temperatures steadily reduce plate surface area. IEEE and battery manufacturer testing demonstrates that when a lead-acid battery reaches the end of its typical 3-to-5 year operating lifespan, its storage capacity has deteriorated to roughly $80\%$ of its original nameplate rating ($0.80$).

Dividing the calculated load by $0.80$ is mathematically identical to multiplying by $1.25$ ($1 / 0.80 = 1.25$). The 1.25 aging factor corresponds to dividing by 0.80 remaining capacity. Also verify temperature, discharge-rate, listing and manufacturer criteria; this arithmetic does not guarantee every battery’s performance.

Supervising Station Systems (NFPA 72 Chapter 26)

When a protected premises fire alarm trips, relying on civilian occupants to telephone 911 produces fatal delays. NFPA 72 Chapter 26 governs supervising station systems, which monitor protected premises continuously and dispatch emergency responders automatically upon signal receipt.

The Three Core Supervising Station Types

1. Central Station Service (the applicable NFPA 72 provisions):
   • An independent, third-party monitoring agency listed by an approved testing laboratory (e.g., UL or FM).
   • Operates under a strict, mandatory turnkey service contract that covers equipment installation, 24/7 monitoring, retransmission to the 911 dispatch center (PSAP), routine inspection, annual testing, and maintenance.
   • Mandatory Runner Service: Central stations must maintain qualified runners (technicians or security officers) capable of responding directly to the protected premises:
     • Verify the applicable runner-response conditions and time limits in the actual central-station provisions.
2. Remote Supervising Station Systems (the applicable NFPA 72 provisions):
   • Transmits alarm, supervisory, and trouble signals directly to a remote monitoring location, such as a municipal fire department communications center (PSAP / 911 dispatch) or a commercial monitoring station.
   • Unlike central station service, remote supervising stations do not provide contracted runner response; upon receiving an alarm, operators immediately notify the fire department.
3. Proprietary Supervising Station Systems (the applicable NFPA 72 provisions):
   • Owned and operated directly by the property owner to protect contiguous or non-contiguous properties under the same management (e.g., major university campuses, large hospital networks, military bases, massive corporate headquarters).
   • Must be housed in a secure, fire-rated operations room staffed continuously 24 hours a day, 365 days a year by trained, dedicated proprietary operators.

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Transmission Technologies & Pathway Supervision (the applicable NFPA 72 provisions)

Historically, fire alarm transmission relied on Digital Alarm Communicator Transmitters (DACTs) connected to copper Public Switched Telephone Network (PSTN / POTS) phone lines. As telecommunications providers phase out analog copper infrastructure in favor of fiber-optic and cellular networks, NFPA 72 has transitioned to modern, performance-based communication pathways.

Communication Pathway Review

Identify the technology, listed equipment, required communication performance and the exact applicable Chapter 26 provisions. Check signal transmission, pathway supervision, loss annunciation, redundant operation and restoration. A transmitter test interval and the maximum time to recognize and report failure are different requirements; they must not be conflated.

Do not apply a generic one-hour/24-hour/one-hour matrix to every single- and multiple-path technology. Verify the actual permitted arrangement and timing, including independence of redundant paths. Two services sharing one router, power supply or physical cable can have a common failure even when marketed as two paths.

For a supplied test criterion of failure annunciation within 60 minutes, a recorded failure at 09:00 and annunciation at 10:12 exceeds the criterion by 12 minutes. That arithmetic example does not establish the applicable code interval; the reviewer must first verify it for the actual technology. The acceptance test should document the failure introduced, observed indications and behavior of the remaining path.

Central station service includes responsibilities beyond simply receiving signals. Verify the required listing, service elements, contracts or certificates and runner response under the actual provisions. Runner response for equipment service is distinct from immediate emergency signal handling; a multi-hour runner allowance is not permission to delay fire department notification. Identify whether the arrangement is central, proprietary or remote supervising-station service and verify its actual requirements.