Does an In-Building Radio Coverage System Need Battery Backup, and How Many Hours Must It Run Through a Mains Failure?

Yes — but only part of the system does. The distribution network is entirely passive: antennas, coax, couplers and taps carry signal with no power at all and are unaffected by a mains failure. What stops is the active head-end — the on-site repeater or bi-directional amplifier (BDA) feeding it — and when that stops the whole building goes silent. Singapore sets no mandatory backup duration for a private two-way radio system, so I design to 4 hours as the floor and 12 hours where the system is life-safety critical.

Which parts of the system actually need power?

This distinction gets lost more often than it should, and it changes the size of the problem. A passive DAS — the antennas on each floor, the half-inch and seven-eighths-inch feeder, the directional couplers tapped off the riser trunk — draws nothing. Cut the mains and all of it still works. The single point of failure is the head-end: one repeater or BDA, usually in an ELV room or rooftop plant room.

So you are backing up one cabinet rather than a building's worth of equipment — but that one power supply carries 100% of your coverage, which is the practical consequence of the architecture described in our explainer on what a distributed antenna system actually is. Active DAS designs, which distribute powered remote units, are the exception; most Singapore installations are passive.

What does Singapore actually require?

Plainly: no Singapore code sets a backup duration for a private two-way radio DAS, and owners are often told otherwise. IMDA's Code of Practice for Info-communication Facilities in Buildings (COPIF) governs the space and riser provisioning a developer must make available for telecommunication systems; it says nothing about secondary power duration. The SCDF Fire Code 2023, Chapter 5 sets secondary power supply requirements for fire safety installations and refers battery capacity out to SS 645 for fire alarm systems and SS 563 for emergency lighting — but a private two-way radio system is not a fire safety installation under that chapter unless it was specifically designed in as one.

The closest explicit figure SCDF publishes for emergency communications backup sits in the Code of Practice for Fire Precautions in Rapid Transit Systems (CPFPRTS) 2022, Clause 8.2: 4 hours of backup battery capacity for the emergency voice communication system, halvable to 2 hours where a dual feeder power supply is provided. That applies to rapid transit, not to a commercial tower — but it is the authority's own benchmark for keeping emergency voice comms alive, and it is the number I anchor Singapore designs to.

Overseas practice sets a higher bar. Under NFPA 1225 and IFC 510, US emergency responder radio systems require at least 12 hours of secondary power, with some authorities having jurisdiction demanding 24. If your specification was written by an international consultant, expect 12 hours in it.

How many hours should I specify?

• 4 hours — the practical Singapore floor, aligned to CPFPRTS 2022 Clause 8.2

• 2 hours — acceptable only where a genuine dual feeder supply backs the head-end

• 12 hours — NFPA 1225 / IFC 510 minimum; specify for hospitals, data centres and US-standard tenders

• 24 hours — some overseas AHJs; rare here, seen in defence and critical infrastructure specs

• 80% — usable depth of discharge for VRLA batteries; never size to 100%

• 25% — end-of-life ageing derating to add on top

How do you size the battery bank?

Work from current draw. For a 40 W UHF DMR repeater on 13.8 V, transmit current is typically around 11 A, receive 2 A and standby 1.5 A — verify against the actual datasheet, as these vary by model.

The usual 5/5/90 duty cycle is wrong here. During an evacuation the traffic is near-continuous, so I size on 20/20/60, giving an average draw of (0.20 x 11) + (0.20 x 2) + (0.60 x 1.5) = 3.5 A.

For 4 hours: 3.5 A x 4 h = 14 Ah, divided by 0.8 for depth of discharge is 17.5 Ah, plus 25% for ageing is roughly 22 Ah — call it a 26 Ah string. For 12 hours the same chain gives 42 Ah, then 52.5 Ah, then about 66 Ah — a 65 to 75 Ah string. That is a real jump in cabinet size and floor loading, so the 4-versus-12-hour decision belongs at design stage.

What gets missed at handover?

Three things, consistently. Nobody load-tests the batteries — an installed battery is not a proven battery, and a timed discharge test belongs in the acceptance procedure alongside the coverage grid survey. Nobody monitors them, so the first anyone learns of a dead string is during the incident it was bought for. And nobody owns replacement: VRLA batteries in a warm Singapore riser will not reach their rated design life, so a system commissioned five years ago with no battery replacement in scope is almost certainly not delivering its hours.

Related questions

Can the repeater just run off the building's emergency generator instead? It can supplement it, not replace it. Generators take time to start and transfer, and the radio has to survive that gap — precisely when an evacuation is being called. Specify both: batteries for ride-through, generator for duration.

Does a passive DAS need any power at all? No. Antennas, coax, couplers and taps are passive. Only the head-end repeater or BDA needs a supply, which is why backup is a single-cabinet problem.

How long do the handsets themselves last? A professional portable on a 5/5/90 duty cycle typically gives 12 to 24 hours, and considerably less under continuous incident traffic. Backing up the infrastructure is pointless if the handsets are flat.

How Suneast handles this

We size the secondary power supply as part of the RF design, not as an afterthought at procurement: head-end current draw taken from the actual datasheet at an incident-realistic duty cycle, deratings applied, and the resulting amp-hour figure and backup duration stated in the design documentation so it can be audited. A timed discharge test goes into commissioning and battery condition checks into the annual maintenance scope — so the hours you specified are the hours you still have in year five.