Long LV Circuits in Road Tunnels

This paper discusses the design criteria of low voltage circuits for road tunnels where they can reach considerable lengths. The "comb distribution" applicable in road tunnels is a special distribution compliant with the two criteria of their zoning and subdivision into modules that provide intrinsic resilience to fire exposure in the event of an accident. This distribution provides primary circuits to power the tunnel modules and the fans and to be installed in a protected manner at the bottom with reduced exposure to high temperatures. Each primary circuit in its module feeds a uniform configuration of terminal circuits. This paper highlights how for long circuits the minimum short circuit currents must not only be promptly interrupted by the coordinated protection devices, but cannot assume a value lower than a minimum value adequate to guarantee the admissible voltage drop. The maximum length of a circuit that guarantees a given voltage drop is generally less than and in any case different from the maximum protected length. For this purpose, a parameter is introduced that can be evaluated in a simple way, which is the characteristic current of the minimum prospected short circuit for a conductor of given length, section and cable ampacity. The ratio between the minimum characteristic current and the effective load current of the circuit is an index parameter which, if it remains below the minimum value required by the voltage drop, highlights the inadequacy of the circuit at a preliminary design level.