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General information

Conduit systems directive & calculation of cross cable sections


Conduit systems directive

The Model Conduit Systems Directive (MLAR) governs: Conduit systems in required stairwells and in rooms between required stairwells as well as the routing of cables through room-closing elements (walls and ceiling).

The MLAR has been introduced as a technical rule and must be applied. While possible, deviations must have no negative impact on the safety objective. Certification may be required and must be provided in the form of approval in an individual case (ZiE).
Excerpt from the MLAR:
5.3.2 The conduit systems must retain functional endurance for at least 30 minutes when used in […]
e) natural smoke exhaust units (smoke removal through thermal lift); exempt are systems that will open automatically in case of a power failure and conduit systems in rooms that are monitored by automatic fire detectors and in which the activation of a fire detector by smoke causes the system to open automatically.

FEflame impact at 750 °C
180no shirt circuit or interruption for 180 min.
Efunctional endurance of a wiring system
30/60/90

current flow and no short circuit
after at least 30/60/90 at > 840 °C


Electrical wiring systems (cables + supporting structure) with functional endurance must be installed in accordance with DIN 4102-12. DIN VDE 0100-540 mandates that the green / yellow protective earth wire be used only as a protective earth conductor and for no other purposes.

Calculation of cross cable sections

Formula for calculating the necessary cross cable sections

A cross section in mm²
I total current consumption all drives
L single cable length
2 multiplier for consideration of returning cable
2,5 V permissible voltage drop
58 Sm/mm² electrical conductivity of copper

Note: Varying with the drives used, the calculation of the cross sections and the application of the cable length diagram must include the currents associated with the starting torque and the cut-out torque.

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