Choosing fire alarm cable: fire resistance and halogen-free cable

Why is alarm cable different from ordinary cable?

A fire alarm system is a system that has to work at the worst moment of a building. When a fire starts, the cables can be exposed to heat, smoke and flames. Under these conditions an ordinary cable quickly melts, short-circuits or breaks and the sounders fall silent. In addition, the dense and toxic smoke it gives off while burning creates an extra danger for people on the evacuation route.

That is why fire alarm cables are assessed on two properties. The first is what it gives off while burning; halogen-free cables give off less smoke and less toxic, corrosive gas while burning. The second is how long it keeps working during a fire; fire-resistant cables keep conducting electricity for a certain time.

These two properties are different from each other. A halogen-free cable may not be fire-resistant; the outer sheath of a fire-resistant cable may also not always be halogen-free. When choosing cable, the two properties are checked separately.

Halogen-free cable

Halogens are elements such as chlorine and fluorine and are found in some cable sheaths such as PVC. When these sheaths burn, they give off dense black smoke and corrosive gases. These gases harm people, block vision and can later damage electronic devices that survived the fire.

Halogen-free cables give off less smoke when they burn and keep visibility on escape routes for longer. Using halogen-free cable is especially important in crowded buildings or ones that are hard to evacuate, such as schools, hospitals, hotels, shopping centers and tunnels. Most fire alarm cables are made with a halogen-free sheath; this information is on the print on the cable and in its technical datasheet.

Fire-resistant cable and PH classes

Fire-resistant cables keep circuit integrity for a certain time even though they are exposed to fire. This time is determined by standard tests. One of the test standards commonly used for small cross-section alarm and control cables is EN 50200, and cables are defined with classes such as PH30, PH60, PH120. The number here shows how many minutes the cable keeps working under test conditions.

Which class is needed is determined by the type of building, the evacuation time and the duty given to that line in the project. Sirens and announcement speakers that must work during evacuation, lines that pass through a zone affected by fire and connections between panels are examples of lines run with fire-resistant cable.

Even if the cable itself is fire resistant, if the clips and trays that hold it melt, the cable falls and breaks. On fire-resistant lines, the cable fixings and supports must also be fire resistant. A fire cable fastened to the ceiling with a plastic clip cannot deliver its test performance on site.

Expert note: Photograph the print on the cable reel and the technical datasheet at handover and add them to the project file. During an inspection, or years later during a renovation, this may be the only document that shows which cable was used on which line.

Shielding and cross-section

Fire alarm lines run near power cables, motors and lighting ballasts in the building. These sources produce interference and can disrupt communication between the panel and the devices, especially in addressable systems. A shielded cable reduces interference thanks to the foil or braid around the cable. The shield must be properly grounded at the panel end.

The cable cross-section is chosen according to the length of the line and the current drawn from it. If a thin cable is used on a long sounder line, not enough voltage reaches the sounder at the end of the line during an alarm, and the sounder sounds weakly or not at all. The cross-section is calculated from the currents of the devices and the line length. The cable specifications recommended by the panel manufacturer are also taken into account in this calculation.

What to watch for along the cable route

Running the cable correctly is as important as choosing the right one. Common mistakes on site and points to watch are as follows:

• Alarm cables must not be put in the same conduit as power cables
• If a splice is needed, use a proper junction box
• On fire-resistant lines, the supports must also be fire resistant
• On loop-type addressable lines, the outgoing and return runs must take different routes
• Keep cables away from wet and hot surfaces
• Wall and floor penetrations must be sealed with fire stopping

Can the existing cable be reused in a renovation?

When renewing an old system, whether the existing cables can be reused is a common question. If the cable type, cross-section and condition are not known, the lines are measured first. Insulation resistance and continuity tests show whether the cable is sound. When moving from a conventional system to an addressable one, the old cables can sometimes be reused, but this decision is made based on the new panel manufacturer’s cable recommendation and the measurement results.

If the old cable is halogenated, or if lines that must work during a fire were run with ordinary cable, the renovation should be used as an opportunity to correct these lines. The cable is often the cheapest part of the system but the hardest to replace.

In addressable systems, running the outgoing and return lines of the loop along separate routes means that if the cable is damaged at one point, the loop continues to be fed from the other end. If both lines run through the same conduit, a single damage cuts both and the loop’s redundancy is useless.

How to check the cable in the quote and at handover

In quotes, the cable often appears as a single line, in meters. The cable type, whether it is shielded, whether it is halogen-free, its fire resistance class and its cross-section should be written separately. Sometimes the difference between two quotes lies only in this line.

At handover, the measurement results of the lines and the documentation of the cable used are requested. In a system commissioned without measurements, an insulation problem that appears years later shows up as intermittent faults and false alarms.

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