Lightning and Surge Protection for Motorway Overhead Gantry

Lightning and Surge Protection for Motorway Overhead Gantry

Created by: Glen Zhu | Updated Date: January 29th, 2024

Smart Motorway and Overhead Gantries

Smart motorways revolutionize traditional highway systems with dynamic modern equipment, such as overhead gantries, that redefine the landscape of traffic management. Unlike conventional motorways, intelligent networks feature variable speed limits and lane management, adapting in real-time to traffic flow and conditions.

Modern overhead gantries in motorways serve as integral structures housing sophisticated apparatus for traffic monitoring systems and cameras, along with open-road tolling systems. In the event of congestion or accidents, gantries promptly detect and relay real-time information, facilitating swift responses such as adjusting variable speed limits, implementing adaptive lane management, and optimizing traffic flow.

Lightning and surge protection

Overhead gantries on motorways, often found in remote areas, face high susceptibility to potential lightning strikes, thus elevating the hazards to the safety of both vehicles and individuals.

Gantry structures used on motorways and all-purpose trunk roads, whether wholly spanning or partially cantilevered over the carriageway, central reserve, hard shoulder, and/or message signs, pose unique challenges for lightning protection due to their expansive and interconnected design.

Figure 1 – Motorway overhead gantry structures

Direct lightning strikes on overhead gantries pose multifaceted risks that include structural damage, electronic system vulnerabilities, fire hazards, and operational disruptions. The accompanying heat may ignite flammable materials, thereby increasing the risk of fire.

Disruptions to normal operations may impact traffic management as well. To avoid potential damage, a comprehensive lightning protection system must be installed to prevent damage, maintain functionality, and ensure safety.

Another great threat faced by overhead gantries that is often overlooked is electrical surges resulting from lightning strikes or power system faults. These surges can cause significant damage to the entire smart motorway structure.

The elevated voltage levels associated with surges can cause electrical damage to critical components within the gantry, including sensors, cameras, and communication equipment. This may lead to gantry malfunction.

Furthermore, the surges may result in data loss, compromising the gantry’s ability to accurately monitor and report on traffic conditions. This may have a temporary or permanent impact on the gantry’s role in traffic management and safety monitoring.

In extreme cases, the heat generated by surges may potentially cause a fire event that could endanger the gantry structure and its surrounding area. Implementing comprehensive surge protection measures, including surge protective devices and grounding systems, is crucial for mitigating the negative effects of power surges.

Comprehensive Lightning and Surge Protection System

Air terminals and grounding

In practical scenarios, air terminals, commonly known as lightning rods, are strategically placed to mitigate the impact of direct lightning strikes. The rods serve as conduits for redirecting lightning currents safely to the ground through high voltage-resistant down conductors that establish a connection with the earth.

To enhance the resilience of both steel and concrete gantries, the solution involves strategically situating radial earth electrodes or earth rods between the carriageways. Buried earth electrodes are often made of stainless steel due to its superior corrosion resistance, mechanical strength, and chemical stability.

The meticulously connected system ensures a secure and reliable interlink, as well as protects the cables underneath the control cabinet from potential flashovers caused by lightning strikes.

If the control cabinet incorporates a steel-reinforced foundation, it is integrated with the radial earth electrode or earth rod of the control cabinet. The length of these earth electrodes is precisely calibrated based on the lightning protection level stipulated in the lightning protection standard IEC 62305.

All individual components of the gantry shall be earth-bonded, and the base of the structure must be connected to the earth in accordance with the earthing system specified in the ‘Code of practice for earthing’ BS 7430 [Ref 2. N], meeting the performance requirements outlined in ‘Requirements for Electrical Installations – IET Wiring Regulations’ BS 7671 [Ref 30. N]. It is essential to provide electrical connections between the reinforcements in the foundation by holding down bolts and metal gantries to achieve adequate grounding.

Within the framework of an overhead gantry structure, all installed components are subject to equipotential bonding. By Maintaining a uniform voltage level across all elements, the system helps prevent the spread of damaging electrical surges. Thereby, the risk of potential differences or uneven electrical distribution is greatly minimized.

Surge protection

To ensure the resilience of motorway overhead gantries, it is necessary to protect all cables that are either unshielded or earthed on both ends with surge protective devices (SPDs). Unshielded or improperly earthed cables are at a heightened risk of experiencing damage due to transient voltage surges.

The selection of surge protectors to be used depends on the distance between the gantry and its control cabinet. Type 1+2 Surge Protective Devices (SPDs) are used when the distance is less than 5 meters, whereas Type 2 SPDs are used for distances greater than 5 meters. FLP12,5-275/3S+1 type 1+2 surge protective devices for TT systems are installed to safeguard the low voltage supply of the roadside controller.

In the case of Variable Message Signs (VMS), proper surge protective devices, for example, type 2 and SPDs for data and signal line, for example, SLP40-275 and FRD2-24 are required to be installed at the entrance connecting to the power distribution cabinets. This dual protection strategy aims to secure the communication lines responsible for dynamically displaying variable messages on signs. It addresses the need to shield against both direct lightning strikes and internal surges that may originate within the electrical system.

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