Protection aim 3: Basic principles

Technical equipment with function maintenance is required for the following buildings and areas: Hospitals, hotels, restaurants, tower blocks, meeting places, shops, closed indoor car parks, metro systems, the chemical industry, power stations and tunnels. These buildings are frequently visited by large numbers of people, creating an increased safety risk for gatherings of people. However, with certain systems, property and the environment must also be protected. The requirement for electrical installations with function maintenance is a component part of the construction regulations. Function maintenance only relates to those areas which provide the power supply to safety-relevant electrical systems, such as emergency lighting, alarm systems, fire alarm systems, automatic extinguishing systems, smoke extraction systems, etc. Here, the regulations require that the power supply must be guaranteed for a specific period of time, even if there is a fire.

30 minutes: Function maintenance for rescue and a safe evacuation

The first 30 minutes after the start of a fire play an important role when clearing the affected building. During this time, function maintenance must be guaranteed for the following units:

• Safety lighting systems
• Lifts with fire control
• Fire alarm systems
• Alarm systems and systems for issuing instructions
• Smoke extraction systems

60/90 minutes: Function maintenance for effective firefighting and difficult evacuation

To support firefighting operations, it is imperative that certain technical equipment is supplied with sufficient power for 60 or 90 minutes after a fire breaks out in a building.
This equipment includes:

• Automatic extinguishing systems
• Water pressure increase systems for fire water supply
• Mechanical smoke extraction systems and smoke protection pressure systems
• Fire brigade lifts
• Bed lifts in hospitals and similar equipment

A cable system with integrated function maintenance is, according to DIN 4102 Part 12, the combination of the routing system (cable ladder, cable tray, fire protection duct, etc.) and cables. The proof of the function maintenance of electrical installation material must be obtained by a fire test, carried out by an independent materials' testing agency.

The test body, i.e. the cable system, must have a testing length of at least 3,000 mm and is installed in a special oven. The cables are routed on the support systems. The test criterion is: No failure of the cables through short-circuits or conductor breaks through the required testing time.

If there is a fire, the cables and conductors are subjected to extreme loads from flames and heat. Nevertheless, cables used for a safety installation must be able to withstand temperatures of up to 1,000 °C and higher for a specific period of time, without there being a short-circuit of the copper conductors. As the copper conductor may begin to anneal at these extreme temperatures, thus impairing its own mechanical stability, the support system serving as a "support corset" has a special significance. Due to the temperature development in the cable, in the case of cables with integrated function maintenance, the insulation has a special role to play. There are two construction types: The cables possess a special winding around the copper conductor made of glass silk or mica tape. If there is a fire, then the cable insulation burns completely, creating a layer of ash. This is kept together by the windings and ensures that the copper conductors are kept apart and that no short-circuit of the support system can take place. More modern cable types use special ceramising plastic insulation instead of the coils. The main component of the insulation is aluminium hydroxide, which forms a soft ceramic sleeve when it burns. This creates the desired insulation of the wires carrying current, both between each other and naturally also to the support system.

Cable systems according to DIN 4102 Part 12 also include cable ducts. The different construction types of the ducts must ensure that the cables routed in the interior continue to function in a fire on the outside. This is ensured using different duct materials. Therefore, no special function maintenance cables need be routed in fire protection ducts. It is possible to use normal, PVC-insulated cables, tested according to the standard. As cables with integrated function maintenance are usually created with a nominal voltage of 0.6/1 kV, there are no options in the field of cable systems for routing medium-voltage cables with function maintenance, for example. However, these cable types can be routed in the fire protection duct, achieving the protection aim of the safe supply of a safety-relevant system.

There are various routing options for routing cables with integrated function maintenance. Besides the type and number of cables to be routed, economic aspects are naturally also of importance. There are many variations, from the tried and trusted standard support structures with which planning is possible, irrespective of the cable type, right through to economical, cable-specific solutions.

Benefits of standard support structures

• Free cable selection, as the combinations of cables and standard support structures possess proof of applicability.
• No binding to specific cable types.
• The structures are ideal for smaller projects.
• Testing means that the countless installation variants are approved for many years.

When selecting the products approved for function maintenance, observe the specifications of the planner and the details of the test certificates. All the parameters on mounting and the usable components must be taken from the test certificates. The systems were tested at German testing institutes in cooperation with renowned manufacturers of safety cables (Dätwyler Cables, Kabelwerk Eupen, Leoni Studer, Nexans and Prysmian). In addition, certain routing types were tested and approved appropriately at local testing offices along with cable manufacturers from other countries according to DIN 4102 Part 12.

The following cable-specific support structures and routing types are available:

• RKSM cable trays
• GRM and G-GRM mesh cable trays
• LKM cable routing ducts
• SL cable ladders
• Tunnel systems made of VA

Many combinations with cable spacing clips and clamp clips with larger fastening spacing distances were tested and proven through various cable manufacturers. Even the routing of function maintenance cables in pipes was covered. To obtain a clear overview, OBO compiles a so-called cable list with the tested and approval routing system and cable combinations available at regular intervals.

Cables on rising sections must be effectively supported in the transition area between vertical and horizontal routing, to prevent bending or sliding. Continuous cable systems only receive the appropriate function maintenance classification when there is effective support at a spacing of max. 3.5 m.

For system fastening, the approvals of the cable systems with integrated function maintenance according to DIN 4102 Part 12 require metal anchors with a general construction approval or a European Technical Approval/Evaluation. In contrast to normal "cold" fastening, these anchors must be set at least twice as deeply for a fire protection application. Alternatively, anchors are used which have proven their load capacity and fire resistance length in a fire test. With these solutions, the necessary setting depths according to the load are listed in the approval documents or in the appropriate fire protection reports. It must also be noted for which substrates and resistance classes the anchors are approved.

The most important solutions for anchoring small to very large loads in most substrates are:

• Metal spreading anchors for use in concrete:
  Heavy-duty ties, nail ties, interior thread anchors, cavity ceiling ties
• Injection ties for use in concrete, hollow brick and porous concrete:
  Anchor rods inserted in plastic or metal sieve sleeves with special mortar
• Bolt ties for use in concrete and various types of masonry:
  Self-tapping concrete screws with various head shapes
• Wood screws with a large embedment depth

Wood is a combustible material and, as with steel constructions, wooden components are firstly only suitable under certain conditions for the fastening of fire-tested electrical installations. Coatings and panelling are also used in the constructions, in order to achieve a fire resistance class at all. However, if there is a fire, wood has a very good property: When burning, an insulated layer is created which delays further combustion. The wooden component must be dimensioned in such a way that failure cannot occur at an early stage. Taking the combustion rates into account, various cable support systems for electrical safety systems with the function maintenance classes E30 and E60 can be fastened to wooden components. Wood screws with a suitable steel cross-section and sufficient embedment depth are used for fastening. The long screws drill deep into the cross-section of the wooden beam, ensuring a secure hold of the mounted support systems, despite burning. Various mounting variants are documented in a fire protection survey.

Simply explained, comprehensively protected. Our tutorials explain exactly what is important with the second protection aim.