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Traffic tunnels must be safe and remain open 24 hours a day. The failure of a rescue system or the automatic closing of the tunnel due to surge voltage not only may cause economic damage, it may also endanger human lives. An effective surge voltage protection design minimizes such risks significantly.
In order to plan a tunnel correctly, it is necessary to understand the risk of lightning strikes and associated damage in the tunnel. The IEC 62305-2 standard provides a method for risk assessment for physical structures with respect to lightning protection. Among other things, it assesses the local lightning density, the dimensions of the structure, and the properties of the power supply and communication cables. However, the risk assessment according to this method extends only to service and facility management buildings of above-ground tunnel structures. Underground physical structures and the tunnels themselves are not taken into account.
Lightning protection zone concept in the tunnel
However, in order to create an effective lightning protection concept, it is recommended to use the lightning protection zone concept from the IEC 62305-1 standard. This standard divides the structure into multiple lightning protection zones (LPZs) that are electrically and electromagnetically isolated from each other (see table). This is achieved on the one hand by surge protection devices (SPDs) at the zone transitions, and on the other hand by using electromagnetic shielding or appropriate physical spacing.
According to this approach, the tunnel is also divided into multiple zones. Account is taken of the fact that the tunnel is below the ground, the conductivity of which can vary widely. In addition, the tunnel has an extensive meshed ground system. Each segment of the tunnel must be grounded as well as connected to the equipotential bonding system. In the event of a lightning strike in the vicinity of the tunnel, the grounding may result in a voltage increase in the tunnel, the magnitude of which is unpredictable. Thus, in the best case, the entire tunnel may be defined as LPZ 1.
The IEC 60364-4-44 standard describes so-called surge voltage categories. They range from Category IV to I and specify the minimum surge voltage capacity of devices, depending on their rated voltage and their point of installation. Category IV is located at the entrance of the installation (LPZ 0B) and relates, for example, to electricity meters and transformers. Category I relates to sensitive devices such as controllers or communication technology.
Surge voltage category III, which applies to LPZ 1, corresponds to a dielectric strength of 4 kV in a 230/400 V system. However, devices such as controllers or sensors provide lower dielectric strengths. Therefore, they can only be installed in LPZ 2 (230/400 V: 2.5 kV) or LPZ 3 (230/400 V: 1.5 kV). In order to equip these zones, control cabinets having Protection Class 1 are often used. They are highly suitable for this task due to their electromagnetic shielding effect in combination with SPDs.
Protecting the power supply
Every application in the tunnel, whether safety-related or not, must be supplied with electric power. The cables come from the outside into the tunnel, meaning that they may transport lightning currents and conduct them into the tunnel. However, an error-free power supply is of vital importance for the tunnel to operate reliably. For this purpose, all power supply cables that are routed into the tunnel are connected to Type 1 SPDs. This takes place ideally in a single control cabinet that is placed upstream from all other devices in the tunnel. Thus, protected and unprotected cables do not run parallel to each other, and inductive coupling does not occur.
The Flashtrab Compact FLT-CP lightning current arrester from Phoenix Contact, a combination of a Type 1 and Type 2 SPD, is used for this purpose. This arrester combines the high current-arresting capability of a spark gap with excellent surge voltage limitation of a varistor.
Protecting data lines and instrumentation & control lines
Data lines and instrumentation and control lines are particularly susceptible to the effects of power supply cables running in parallel. A lightning current flowing through a power supply line creates an electromagnetic field around the line due to the fast rise of the current. If a data cable or an instrumentation and control cable is located in the magnetic field, a voltage is induced in this cable, which is then present at the connected device. The connected data devices and instrumentation and control devices are also sensitive to voltage surges. Distorted measurement readings or corrupt control commands may result, and these devices may get damaged or may fail. If the devices are safety-critical systems such as visibility measurement, heat detection, or the bus system, this may result in an automatic shutdown of the tunnel.
In tunnels in which power supply, data, or instrumentation and control lines run in parallel over long distances, the latter are particularly affected by inductive coupling. Selecting the correct SPD is just as important as correct installation. The placement of the SPD directly upstream from the device to be protected and the physical separation of unprotected and protected cables have a major impact on the protection. The unprotected cables are routed along the outer walls in the control cabinet, while the protected cables are routed along the cable channel in the center. The physical separation prevents inductive coupling of the unprotected cables to the protected cables.
In SPDs for instrumentation and control and data applications, multiple protection levels are normally combined: lightning, surge voltage, and device protection. Therefore, a single protection module upstream from each device is sufficient. The types are classified like those for power supply, but their designations are different. Lightning protection is designated as D1 instead of Type 1, surge voltage protection as C2 instead of Type 2, and device protection as C1 instead of Type 3.
On each line, different kinds of signal types can be transmitted. Therefore the signal defines the SPD which needs to be used to protect the line (for instance, a SPD for an analog signal is built up differently than a SPD for the CAN protocol). In order to turn unprotected lines into protected ones, the appropriate SPD needs to be chosen. The intelligent Plugtrab PT-IQ protection system from Phoenix Contact is suitable for this application. It provides reliable protection from surge voltages, detects and displays the state of the arresters, and forwards this information via remote signaling, for example, to a programmable logic controller (PLC). The advantage of this design is that critical arresters that still operate but are approaching the end of their service life are also detected. They can then be replaced during the next maintenance session, before they become defective.
The potential for lightning and surge voltages to affect the power supply and control systems of a tunnel is great. If SPDs are placed correctly and the lightning protection zone concept is applied, effects can be reduced to a minimum. On the one hand, it is important to define the zone transitions clearly and to equip each cable that passes through the transition with an SPD. On the other hand, SPDs function properly only if grounding and equipotential bonding system is correctly designed in the structure.