For many years it has been common practice to install VLD-F anywhere in DC and AC powered railway lines. This installation has often not been associated with infrastructure corrosion problems. With increasing traffic on the lines, changing power infrastructure and train propulsion systems, there appear more and more frequently infrastructure failures and damages (corrosion) that can be clearly attributed to improperly installed and incorrectly checked VLD-Fs designed on the basis of gas discharge tubes (GDT). The crux of the problem lies in the combination of the behavior of GDT at different voltage levels.
How do standard VLD-F systems work?
The current practice of DC and AC systems uses VLD-F designed on the basis of a gas discharge tube connected to an automatic short-circuiting device, which in the event of a fatal traction fault always operates in a non-repeatable mode, i.e. it goes into a permanently conductive state. This complies with the requirements of EN 50526-2 and other standards. However, if such an event occurs, a permanent short-circuit opens up between the isolated rail and the ground, through which stray currents escape permanently.
This permanent current leakage has a very adverse effect on the service life of the metal infrastructure (poles, bridges, waiting shelters, cables, reinforced concrete structures, etc.) alongside the railway track due to electrochemical corrosion caused by the current flow. Current solutions offer only one way to minimize this damage, consisting in frequent physical inspection of the installed VLD-Fs, which is a challenging task because a reliable verification of the function necessitates to disconnect the VLD from the rail, perform impedance measurements with a sufficiently powerful current source, followed with reconnection of the VLD-F to the rail. This takes quite a lot of time (sometimes around 10 minutes) and costs considerable operating money, so that the intervals between checks are often extended and the risk of damage to the infrastructure increases disproportionately.
How do SALTEK's innovative VLD-Fs work?
The new, patented VLD-F SALTEK solution means revolutionary changes in the use and of VLD-F and minimization of the complications associated with it. The short-circuiting device inside the new SCG is of special design that reacts very quickly to the current flowing through the discharge tube and, at the moment of its overload or destruction threat, shunts it out and ensures a permanent short-circuit with minimal energy loss so that there is no physical damage to the product housing that could be dangerous to the environment.
This short-circuiting device is wirelessly connected to detection electronics that registers such a change and is capable of permanently indicating it. The electronics consists of a special RFID tag that can be read remotely by a commercially available RFID reader in a fraction of a second without any manipulation (shunting, disconnecting, measuring) to the VLD. Reading is possible even while moving, making the inspection of the installed VLD-F simple and operationally inexpensive. This allows more frequent VLD inspections and minimizes the time when dangerous corrosive current leaks through the activated VLD.
Another significant advantage of this solution is the significantly simpler and more transparent logistics, because in addition to the indication of immediate necessity to replace the VLD-F, other identification parameters of the VLD to be inspected (e.g. seriál number, installation coordinates, etc.) are stored in the controller memory, which can then be used as a basis for operational statistics, analyses, etc.
The use of this modern VLD-F therefore results in a significant reduction of operating costs for the management of the railway protective bonding system, a minimization of stray currents and thus an increase in the lifetime of railway, which applies also to third-party infrastructure last but not least the possibility of advanced analyses entering predictive maintenance systems for the purpose of inspection optimization and further reduction of operating costs. The integrated protection against lightning and industrial short-term pulses is of course integrated into the product and operates in a repeatable mode (i.e. the internal shortcircuiting device does not react after a lightning strike).
