Fire Alarm Cable and Backup Power: Which Cable You Need and Why a Backup PSU Matters

Cable and power are the circulatory system of a fire alarm. You can buy the best panel and detectors, but if the sounders are wired with ordinary cable they will fall silent a few minutes into the fire.

Here is which cables you need, how to run them and why backup power is not a formality.

What cable is used for a fire alarm?

Detection circuits and addressable loops use flame-retardant, low-smoke, low-toxicity cable (marked "ng(A)-LS" in the CIS). Circuits that must work during a fire — sounders, power supply and smoke extraction control — need fire-resistant cable ("ng(A)-FRLS"), for example KPSng(A)-FRLS.

The RS-485 interface uses shielded twisted pair. Cutting costs on cable is a frequent cause of inspection findings and of systems failing during a fire.

Can fire alarm cables share a route with power cables?

Fire alarm circuits must not run in the same conduit or tray as power and lighting cables — interference causes false alarms and communication failures. The standards require separate routes or divider barriers, and minimum distances for parallel runs.

This is one of the typical mistakes of installers trying to save money.

What is a backup power supply and why is it needed?

A backup power supply (RIP in the Bolid range) powers the panel and sounders from the mains and switches them to batteries when the mains fails. Modern units (for example RIP-12 isp.60) monitor battery charge and condition and report it to the panel.

Batteries need replacing every 3–5 years — one of the most common faults found during inspections.

The cables we use in practice

On our projects we use, for example: KPSng(A)-FRLS for 12 V power and sounder circuits, KSVVEng(A)-LS for signal lines, KPKVng(A)-FRLS for voice/sounder lines, KSPEVG shielded twisted pair for RS-485, and 16 mm PVC conduit to protect the cable.

Backup power is provided by RIP-12 units with battery monitoring over the interface.

How battery capacity is calculated

The designer adds up the current drawn by all devices in standby and in alarm (with sounders and signs on), multiplies by the operating time required by the standards and adds a margin for battery ageing. This determines the number and capacity of batteries in the power supply.

If sounders or detectors are added later, the calculation must be repeated — otherwise the system won't run for the required time.

How cable circuits are checked

During maintenance technicians check insulation resistance and continuity, look for mechanical damage, and check fixings, labels, twisted joints and unprotected sections. An addressable system reports open and short circuits by itself.

Sounder and power circuits need special attention: they must be fire-resistant along their entire length, including junction boxes.