A modern voice-alarm system is not a sounder circuit. The voice-alarm control and indicating equipment (VACIE) supervises loudspeaker lines, stores evacuation messages, routes audio to zones and drives 100 V line amplifiers, and EN 54-16:2008 is the European standard that defines its mandatory and optional functions. For most of its life the rack is remarkably quiet electrically; during a real evacuation it must deliver intelligible speech at high audio power for many minutes. Designing the nickel-metal hydride backup that bridges a mains failure therefore starts, as always in this series, with an honest load profile rather than a nameplate wattage.
EN 54-16:2008, Fire detection and fire alarm systems - Part 16: Voice alarm control and indicating equipment, treats the VACIE as a life-safety device. A compliant chain includes an emergency microphone, a fire-alarm interface, message generation, signal routing, amplification and supervised voice-alarm outputs. Industry guidance from EN 54 certification bodies and manufacturers such as TOA, Panasonic and LDA Audio Tech is consistent: the power supply that feeds a VACIE, whether internal or external, must itself satisfy EN 54-4.
Two functions shape the electrical duty. First, mandatory loudspeaker-line monitoring continuously watches each 100 V branch for open- and short-circuit faults, which keeps the controller and its monitoring relays alive at a small but permanent current. Second, when the fire interface triggers, the amplifier bank reproduces a stored message or a live microphone announcement across selected zones, and that is when current climbs steeply.
In the quiescent regime the VACIE, its DSP/router card, line-supervision electronics and control-panel display draw a modest, near-constant DC current - in a small to mid-size 19-inch rack typically a few hundred milliamperes on the 24 V rail, with brief blips during a scheduled weekly test tone or a routine amplifier self-check. This regime lasts for weeks and is what the EN 54-4 standby calculation is built on.
In the alarm regime the power amplifiers track the voice waveform. Music and speech have a high crest factor: the average power is far below the peak, and an evacuation message repeated with silences between cycles has a duty factor well below a continuous tone. The first animated figure traces exactly this transition - flat supervision, a short test blip, then an envelope that follows the spoken message rather than a square wave.
A naïve calculation multiplies the amplifier's rated output by the evacuation duration and over-sizes the battery dramatically. A 240 W amplifier fed from 24 V implies 10 A at full continuous output, but a repeated spoken message with pauses might run at a quarter of that on average once amplifier efficiency and the speech duty factor are included. The second figure compares evacuation minutes available from one fixed NiMH pack across duty factors of 100%, 50%, 35% and 25%; the difference between a continuous-tone assumption and a realistic speech envelope is the difference between a pack that fits the rack and one that does not.
The designer must still size for the worst credible case - simultaneous all-zone paging, a live emergency-microphone announcement that can be held continuously, and the option in EN 54-16 of redundant amplifiers where the specification demands that a single amplifier fault never silences evacuation.
EN 54-4 power-supply equipment carries the same endurance logic used by fire panels: a defined quiescent standby period followed by an alarm period, with the battery required to supply both. For voice alarm the common design contract is 24 hours of quiescent operation plus at least 30 minutes of alarm/evacuation duty; unattended or higher-grade installations extend the standby term. After a discharge the charger must restore the battery to 80% within 24 hours and to full charge within a further 48 hours.
Crucially, the changeover from mains to battery must be seamless enough that an in-progress evacuation message is not clipped and must never itself generate a fault or silence the output. That requirement favours a chemistry that is permanently connected and float-ready rather than one that depends on a mechanical transfer or a long wake-up.
Nickel-metal hydride is an aqueous, sealed chemistry with a flat 1.2 V discharge plateau and low internal resistance, so a ten-cell string presents a stable 12 V nominal rail and a twenty-cell string a 24 V rail that hold voltage through the speech burst. Unlike primary lithium it can be float-charged for years and recharged after every real event; unlike VRLA it tolerates the warm ceiling-void and equipment-rack temperatures that accelerate lead-grid corrosion and water loss.
A correctly designed permanent trickle at or below C/20 keeps the pack at full readiness without the sustained overcharge that dries out the separator. Because the VACIE is almost always at mains and only occasionally discharged, the NiMH pack lives in the regime - long float, rare, shallow cycles - where its calendar stability and low self-discharge are most valuable.
The engineering conversation for a PAVA backup pack should open with five numbers: the 24 V quiescent current and its duration, the amplifier rated power and efficiency, the realistic speech duty factor and the longest continuous announcement, the target standby term, and the worst enclosure temperature. With those, the energy budget, string count, cell size and charge regime follow directly.
The companion papers in this three-part set do exactly that: Paper B walks through chemistry choice and the step-by-step Ah calculation, and Paper C maps the result onto the EN 54-16/EN 54-4/EN 54-24 type-test and evidence trail. Together they turn a nameplate amplifier rating into a defensible, certifiable NiMH backup design.
Weijiang Power manufactures industrial-grade nickel-metal hydride cells and custom backup packs for voice-alarm control equipment, 100 V line amplifiers and life-safety PA systems. Tell our engineers your EN 54-4 standby target, amplifier rating and enclosure temperature and we will size a sealed, low-self-discharge NiMH pack with the tabs, fuses, NTC and connectors your assembly needs. Request a data pack and factory test report on the products page.
