Cable trays on longer supports
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 starting point
When installing cable systems with functional integrity in accordance with DIN 4102-12, it may be necessary to deviate from the specifications of the general building inspection test certificates (abP) for structural reasons.
A common application is the use of longer brackets when pipes, thermal insulation, wall projections or recesses, or other technical installations cross the direct route of the cable tray.
The test certificates define cable trays and brackets with the same nominal width or length, for example, a 400 mm wide cable tray on a 400 mm long bracket or a 200 mm wide cable tray on a 200 mm long bracket. If, on the other hand, a 200 mm wide cable tray is mounted on a 400 mm long bracket, the effective lever arm changes compared to the tested arrangement. With the same load, this leads to a higher bending moment, which could impair stability and thus functional integrity in the event of a fire.
Calculation and adjustment of parameters
In order to ensure safety and functional integrity classifications, it must be mathematically proven in such cases that the effective bending moment in the cantilever arm ( ) is not greater than that proven by the fire test.
In the specific example, the calculation shows that the fastening distance must be reduced to a ≤ 1.13 m or the cable's own weight must be limited to q ≤ 14.5 kg/m in order to comply with the permissible values. Taking the above considerations into account, it can be assumed that the behavior of the cable systems will not be significantly affected with regard to the classifications issued in the general building inspection test certificate P-MPA-E-16-011.
Structural background and practical recommendation
Such situations often arise when vertical installations such as pipes, cable trays, thermal insulation, or wall projections cross the cable route.
Since neither DIN 4102-12 nor 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 resort to individual solutions.
The installation of bypasses with fittings or cable support elements cut to size on site is often complex, cost-intensive, and statistically unfavorable, as they can lead to unacceptable deformation in the event of a fire. In addition, functional integrity cables are subjected to greater mechanical stress due to changes in direction, especially if the minimum bending radii are not observed.
In many cases, it therefore makes more sense to create the required distance by using longer cantilevers and to lay the route in a straight line without changes in direction – but always in compliance with the requirements of the general building inspection test certificate.
