Cable weight exceeds that permitted by proof of use

The basis for this is DIN 4102-12 and the Model Cable System Guideline (MLAR). The general building inspection test certificates (abP), which prove the usability of tested systems in construction, are decisive in this regard. In practice, however, structural conditions often lead to deviations from these specifications. Since neither DIN 4102-12 nor the MLAR, the Model Building Code (MBO), or the Model Administrative Regulation for Technical Building Regulations (MVV TB) provide specific guidelines in this regard, planners and installers often have to resort to individual design solutions.

Technical initial situation

When installing a cable system with integrated functional integrity in accordance with DIN 4102-12, a 400 mm wide cable ladder is to be mounted on a 400 mm long bracket.

The problem: The planned cable load is 45 kg/m, while the general building inspection test certificate (abP) only allows a load of 20 kg/m with a fastening distance of 1.5 m.

Exceeding these values would lead to a higher bending moment in the cantilever, which could negatively affect the deformation behavior of the system in the event of a fire and thus jeopardize functional integrity.

Recommended solution: Adjustment of the fastening distance

In order to ensure static safety and the classification of functional integrity, the bending moment must be reduced mathematically to the limit value proven by the fire test.

This can be achieved by shortening the fastening distance: With a cable load of 45 kg/m, the fastening distance must be reduced to a ≤ 1.05 m.

This brings the effective bending moment back into line with the tested reference value from the abP. In this way, the functional integrity of the cable system is maintained even under higher loads without the system having to be retested.

Structural background and practical recommendations

In practice, it often happens that cable trays need to be retrofitted with heavier loads or designed for future retrofitting. In doing so, it must always be taken into account that both the mechanical stability and the fire protection classification must be maintained.

A practical example illustrates this: In a construction project, cables weighing 23 kg/m were laid in a cable tray. Theoretically, reducing the fastening distance to 1.4 m would have been sufficient. However, the installer opted for a distance of 1.25 m in order to create a reserve for later retrofitting. This allowed the system to carry a permissible load of up to 30 kg/m without losing its classification. If, instead, cables had been added later at the original fastening distance of 1.4 m, an additional fastening would have had to be installed between each existing fastening. The new distance would then have been only 0.7 m.

These examples show that

• forward planning of cable loads and fastening distances is crucial for the cost-effectiveness and safety of a system.
• By specifically adjusting the distance, functional integrity in accordance with DIN 4102-12 can be ensured even with higher cable weights.

At the same time, unnecessary rework and cost-intensive new installations are avoided.