Lightning and surge protection for highway gantry signs
November 2015
Electrical Power & Protection
Highway gantry signs are used to hold traffic-specific detection systems such as CCTV cameras, induction loops and radar detectors. On toll roads, additional detection devices are installed which ensure fully automated toll data identification.
To prevent a direct lightning strike to the display modules, it is advisable to install an isolated lightning protection system which is directly fixed on the structure by means of air-termination rods and isolated spacers, or a patented high voltage resistant and insulated HVI conductor. Thus, flashover of lightning currents to the components of the highway gantry sign is eliminated.
The highway gantry sign profile between the carriageways is earthed via radial earth electrodes or earth rods, such as those made of stainless steel. The power and control cables between the control cabinet and the gantry sign are protected against lightning strikes by a buried conductor, also potentially stainless steel. This is the second earth-termination system of the highway gantry sign. In South Africa, stainless steel is a viable alternative to copper, given the high theft rate of copper in the country.
In the control cabinet, direct equipotential bonding is established and the required surge protective devices are installed. In principle, all unshielded and unearthed cables must be protected by surge protective devices. The low-voltage supply of the roadside controller is protected by a lightning current carrying combined arrester, for example DEHNventil M TT 255 (FM). The information technology cables of the roadside controller are typically connected to the traffic control centre via optical fibre cables. In this context, it must be ensured that their metal sheaths are connected to the equipotential bonding system.
For more information contact Alexis Barwise, Dehn Africa, +27 (0)11 704 1487, [email protected], www.dehn-africa.com
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