Laying Fire-Resistant Cables: Rules for Installation, Fastening, and Protection
Laying a fire-resistant cable is not just the installation of an electrical line, but the creation of critically important infrastructure that must operate reliably in fire conditions. Whether fire protection systems can perform their functions and ensure the safe evacuation of people depends on how correctly these works are carried out. Even the highest quality certified cable with a fire resistance rating of 180 minutes can fail after 30 minutes of a fire if errors were made during its installation.
In this guide, we will detail the key rules and features of laying fire-resistant cables: requirements for cable support systems, the critically important “snake” laying principle, permissible bending radii, types of fastening elements, and additional protection measures. The material is based on current regulatory requirements and practical experience.
1. Main Types of Cables for Fire-Resistant Wiring
Before discussing installation, it is important to understand which fire-resistant cables are used. Their main feature is the ability to remain operational under the influence of open flame. This is achieved through the use of special materials in their construction.
- Fire-resistant cables with mica-containing tapes: The most common type. Fire resistance is ensured by wrapping each core with mica-based tapes. During a fire, the tape sinters into a durable ceramic insulation, preventing short circuits. Such cables are marked, for example, as KUIN-SP with indices FR, FRLS, FRHF.
- Mineral insulated cables: Cores are enclosed in a sheath of compressed mineral insulation. They have exceptionally high fire resistance but are complex to install.
- Shielded fire-resistant cable: Additionally equipped with a screen, which protects transmitted signals from electromagnetic interference. This is especially important for fire alarm and communication circuits to avoid false alarms or signal loss.
The key element uniting all these types is the fire-resistant cable insulation, which allows it to function in fire. Such cables are tested at temperatures around 750°C and must remain operational for at least 180 minutes.
2. The Main Danger During a Fire: Thermal Expansion
The primary reason for the failure of a correctly selected but improperly installed fire-resistant cable is the failure to account for the thermal expansion of metal cable support systems.
During a fire, the ambient temperature quickly rises to 750°C and above. The steel from which trays and trunking are made expands. For example, a 10-meter steel trunking at such a temperature can lengthen by almost 100 mm! If the fire-resistant cable is rigidly fixed to this trunking without slack, then when the trunking lengthens, the cable will inevitably be stretched. This tension leads to:
- Rupture of the conductive cores.
- Violation of the integrity of the fragile mica-containing insulation.
- Breakage of the cable at the fastening points.
As a result, the line ceases to function, even if the cable itself is rated for 180 minutes of operation in fire. Compensating for thermal expansion is the main imperative during installation.
3. Basic Rules for Laying Fire-Resistant Cables
Adherence to the following rules is mandatory to ensure the operability of the cable line in fire conditions.
3.1. Requirements for Cable Support Systems and Their Fastening
- Fire resistance of fasteners: All fastening elements must be steel and have a fire resistance rating not lower than that of the cable line. The use of plastic clips, dowel clamps, and ties is strictly prohibited — they will melt in the first minutes of a fire.
- Reliable fixation of trays and trunking:
- Trays and trunking must be rigidly attached to building structures. The fastening spacing should be no more than 1-1.2 meters.
- Each trunking is attached to the bracket at a minimum of two points. To increase reliability, it is recommended to additionally secure the bracket shelf to the ceiling with a steel stud.
- Trunking must be installed strictly horizontally to eliminate additional mechanical impact on the cables during possible displacement.
3.2. Laying the Cable with Compensation for Thermal Expansion (“Snake” Laying)
This is a key rule for laying fire-resistant cables. A cable of any cross-section must be laid in trays or trunking not taut, but with a free sag — in a “snake” pattern.
- Amount of slack: Between every two fastening points, a cable length slack of at least 3-4% of the span length must be provided. If the span between fastenings is 1 meter, the slack should be 30-40 mm.
- Result: This slack will allow the cable to “take up” the slack when the trunking lengthens due to heat, avoiding critical tensions and damage.
3.3. Ensuring Free Fastening
- Inadmissibility of rigid fixation: It is forbidden to rigidly press the cable against the tray or trunking with bands, ties, or clamps. The fastening must be free, allowing longitudinal movement of the cable when heated.
- Group laying of thin cables: For fire-resistant fire alarm cables and other small cross-section cables laid in a group, any compression of the bundle by fasteners must be excluded. The cables should lie freely.
- Fastening large cross-section cables: For large cross-section cables, fastening with partial compression of the sheath is allowed. This is done to avoid damaging the cable due to dynamic loads during a short circuit, but it should not impede its longitudinal movement.
- Vertical laying: For vertical laying, the fastening spacing is reduced, and small cross-section cables may be fastened with partial compression. However, the principle of free movement must be maintained.
3.4. Types of Fastening Elements
- Only metal clamps and clips made from materials capable of performing their functions for a long time at high temperatures should be used for fastening fire-resistant cables.
- It is important that these fastening elements have no sharp edges or burrs that could damage the cable sheath during heating and expansion.
3.5. Increased Bending Radius
- Problem: At cable bend points, the mica-containing tapes that ensure fire resistance can delaminate or their overlap can decrease. This reduces the protection of the cores from fire.
- Solution: The minimum permissible bending radius for fire-resistant cables should be increased by 1.5 times or more compared to ordinary cables. The specific value must be clarified in the cable’s technical documentation, but the general rule is: the larger, the better for preserving insulation integrity.
4. Additional Protection Measures
- Fire-retardant trunking and coatings: To slow down the heating of the cable and increase its operating time, special trunking made of fire-retardant plasterboard or other materials, often with additional thermal insulation from mineral wool mats, can be used. This is especially important for transit lines passing through rooms with high fire loads.
- Protection of transit lines: If a cable powering equipment in one room passes in transit through other rooms, it must be additionally protected from possible fire exposure in those rooms.
- Cable penetrations: Places where fire-resistant cables pass through walls and ceilings must be sealed with special fire-resistant materials that restore the fire resistance rating of the barrier and prevent the spread of fire and smoke.
5. Conclusion
Laying fire-resistant cables is a complex engineering task requiring strict adherence to technology. Using certified fire-resistant cables, including shielded versions for signal circuits, is only the first step. The correct selection and installation of cable support systems, ensuring compensation for thermal expansion, using appropriate fasteners, and increasing bending radii are of decisive importance. Only such an integrated approach guarantees that fire alarm, warning, and automatic fire extinguishing systems will perform their functions and ensure people’s safety in a fire.
For critical facilities of any complexity, JSC “Kazenergokabel” offers a full range of certified cable products, including fire-resistant cables of the KUIN-SP series in various versions, fully complying with the requirements of technical regulations and GOST. Our specialists are ready to provide consulting support in choosing a cable for your project.
