Why cables are a special case
Cable runs are one of the most common causes of fire spread inside buildings and industrial facilities. The insulation and sheaths of most cables are combustible, and cable trays and shafts form long, continuous “wicks” that pass through walls and floors. A fire that starts in one room can travel along a cable bundle to another storey or a neighbouring compartment, and the smoke from burning insulation is dense and toxic.
Cable fire protection serves three different purposes, and it is important not to confuse them:
- limiting flame spread along cable bundles over the length of the route;
- maintaining the fire resistance of separating elements where cables pass through walls and floors (cable penetration seals);
- circuit integrity of cable runs serving life safety systems for a specified time — to power fire pumps, alarm systems and smoke extraction.
Protection methods
Fire protection coatings for cables
Intumescent and ablative compounds are applied to the surface of cables and bundles. They stop flame from spreading along the sheaths and reduce smoke emission. The coating must be flexible, so that it does not crack under thermal movement or during work on the route, and it must not impair heat dissipation from the cable — the manufacturer usually states whether a reduction in current rating (derating) needs to be allowed for.
Fire-resisting enclosures and casings
Calcium silicate board enclosures or purpose-made casings form a fire-resisting envelope around the route. This is the main method where circuit integrity must be maintained on critical circuits or the route must be protected from an external fire. At oil and gas facilities, cable trays are sometimes protected with epoxy intumescent coatings that have also been tested on cable ducts.
Cable penetration seals
Where a cable bundle passes through a wall or floor, the penetration must have the same fire resistance as the element itself. This is achieved with coated boards, foams, sealants, blocks and pillows. See the “Fire stopping of penetrations” page for details.
Fire-resisting cables
Cables of special design maintain circuit integrity in a fire on their own. This does not replace fire protection of the route as a whole, but in some cases it takes care of the circuit integrity requirement.
How to choose a solution
- The task in hand. Limiting flame spread, sealing a penetration and circuit integrity involve different requirements and different materials.
- Design requirements. The fire resistance rating of the penetration seal (usually not lower than that of the wall or floor), the circuit integrity time, and smoke production requirements.
- Cable type and installation. Cable designation, diameter, bundle density, trays or trunking, vertical or horizontal route.
- Service conditions. Humidity, temperature, aggressive environments, outdoor location.
- Maintainability. If more cables are to be added later, materials that allow the protection to be opened up and reinstated are more practical — pillows, blocks, foams.
- Thermal conditions. Continuous coatings and enclosures reduce cooling — the electrical engineer allows for this when sizing the cables.
Common mistakes
- Using a generic “fire-rated foam” without an approved solution for cable penetration seals.
- Incomplete treatment of the bundle: coating applied on top only, without working it in between the cables.
- Coating over dust and oil — poor adhesion and peeling.
- Penetration seals opened up to add cables and never resealed — the most common defect in occupied buildings.
- Substituting materials from one system with those of another within the same penetration seal.
Materials from leading manufacturers
- Hilti CFS-CT — Hilti — fire protection coating for boards and for penetration seals with cables, trays and pipes.
- Hilti CFS-F FX — Hilti — two-component fire protection foam for cable and mixed penetration seals, allowing cables to be added later.
- Hilti CFS-BL — Hilti — ready-made fire protection blocks for cable penetration seals.
- Hilti CFS-CU — Hilti — fire protection pillows for temporary and permanent penetration seals.
- PROMASTOP-CC — Promat — water-based “hybrid” coating for cable and mixed penetration seals.
- PROMASEAL-AG — Promat — intumescent acrylic sealant for cables, bundles and conduits.
- 3M Fire Barrier Sealant CP 25WB+ — 3M — intumescent sealant for penetration seals.
- 3M Fire Barrier Moldable Putty Pads MPP+ — 3M — intumescent putty for cables and electrical back boxes.
- PROMATECT-L500 — Promat — boards for fire-resisting cable enclosures.
- Thermo-Lag 3000-P — Carboline — epoxy coating, also used for cable ducts at petrochemical facilities.
- Hilti CP 678 — Hilti — water-based intumescent coating for cables and cable bundles on trays indoors.
- Hilti CP 679A Plus — Hilti — ablative fire protection coating for cables and trays, indoors and outdoors.
- Hilti CFS-T — Hilti — modular fire-, water- and gas-tight transit system for cables and pipes.
- Hilti CFS-D 25 — Hilti — pre-formed firestop disc for small cable penetrations up to 25 mm.
- Hilti CFS-CC — Hilti — firestop cable collar for fully filled penetrations up to 108 mm, EI 120.
- HENSOMASTIK 5 KS — Rudolf Hensel — ablative coating preventing flame spread along cables, indoors and outdoors.
- HENSOMASTIK Acrylic Penetration Seal 2 x 50 mm for Cables — Rudolf Hensel — coated-board soft seal for cable penetrations, up to EI 120.
- PROMATECT-LS — Promat — lightweight calcium silicate board for fire-rated service enclosures and cable protection.
- 3M Fire Barrier Pass-Through Device — 3M — re-enterable firestop sleeve device for cables and pipes.
- Protecta Service Coat FR-1 — Protecta (Polyseam) — water-based intumescent coating for cables, trays and metal pipes, EI 30–EI 120.
Frequently asked questions
Does a cable need coating if it runs in a steel tray?
The tray does not prevent fire from spreading along the sheaths. Whether a coating is required is decided by the design, based on the cable type and installation conditions.
Can cables be added after fire protection has been applied?
Yes, but the protection must be reinstated with the same material. For routes that change frequently, choose solutions that are easy to open up and reinstate.
Does the coating affect how hot the cable runs?
It can. Manufacturers state whether the permissible current load must be reduced, and the electrical designer takes this into account.