The contact resistance characteristics of fishplates and their compatibility requirements with the track circuits of electrified railways
Why do electrified railways have much stricter requirements for the contact resistance of fish plates than non-electrified railways?
Fish plates on non-electrified railways only perform a mechanical connection function, with no strict limits on contact resistance. In electrified railways, rails are both load-bearing structures and signal transmission channels for track circuits, as well as conductive channels for traction return current. As a connecting component of rail joints, excessively high contact resistance of fish plates causes attenuation of signal current in track circuits, leading to abnormal signal display and even "red light band" faults; meanwhile, resistance loss of traction return current generates heat, exacerbating contact corrosion between fish plates and rails and forming a vicious cycle. Therefore, controlling contact resistance is a core design indicator for fish plates in electrified railways.
What structural and material factors mainly affect the contact resistance of fish plates?
In terms of materials, ordinary carbon steel fish plates have high resistivity and large contact resistance; alloy steel or copper alloy composite fish plates have low resistivity, significantly reducing contact resistance. Structurally, surface roughness of the contact surface is critical-moderate roughness (Ra 3.2-6.3μm) increases the effective contact area and reduces resistance; insufficient fitness of the contact surface (e.g., machining errors) forms gaps, causing a sharp increase in contact resistance. Additionally, insufficient bolt preload on fish plates reduces contact surface pressure and effective contact area, also increasing contact resistance.
What is the direct correlation between the "shunting failure" fault in track circuits and excessively high contact resistance of fish plates?
Shunting failure means that when a train wheel occupies the track, the track circuit cannot be effectively short-circuited, and the signal still shows "clear." When fish plate contact resistance is excessively high, the signal current of the track circuit is greatly attenuated at the joint, making the voltage at the receiving end of the track circuit higher than the shunting threshold. At this point, the short-circuit effect of train wheels is offset by the high contact resistance, and the track circuit cannot identify the train occupation state, causing shunting failure. This fault is particularly frequent in joint-dense areas such as turnouts and long tunnels, posing a major safety hazard in electrified railways.
What special contact resistance optimization measures must fish plates adopt to adapt to high-frequency frequency-shift track circuits?
High-frequency frequency-shift track circuits have high signal frequencies (e.g., 10-2000Hz), with significant skin effect-current is mainly transmitted along the rail surface. Optimization measures include: 1) Surface silver plating or tin plating-silver and tin have low resistivity and stable chemical properties, forming a conductive protective layer on the fish plate contact surface to reduce high-frequency contact resistance; 2) Designing a multi-contact structure-adding raised conductive contacts on the contact surface between the fish plate and rail to increase effective contact points for high-frequency current; 3) Using supporting high-strength insulated bolts to avoid bolt current shunting and ensure signal current is transmitted along the fish plate contact surface.
How to measure the contact resistance of fish plates on-site and determine if they meet the adaptability requirements of track circuits?
A track circuit contact resistance tester is used, which can simulate the operating current (power frequency or frequency-shift) of track circuits to directly measure the contact resistance of fish plate joints. During testing, clamp the two test clips of the instrument on the rails on both sides of the fish plate, 1m from the joint center each, to avoid interference from the rail's own resistance. For power frequency track circuits, the fish plate contact resistance should be ≤1mΩ; for frequency-shift track circuits, ≤0.5mΩ. If the measured value exceeds the limit, treat it by grinding the contact surface, retightening bolts, or replacing with conductive fish plates to ensure normal operation of the track circuit.