The track circuit is a very important part in the TIAA MRT signaling system to continuously monitor the status of track clearance and occupation. Its principle of operation is to intercept an appropriate length of rail tracks and insulate both ends thereby constituting an independent circuit section. When there is no train operating on the section, the current will arrive at relay via the rail and simultaneously suck the circuit contact point upwards to turn on the green light circuit.
When a train is operating in the section, the current in the rail will short circuit by the train axel; the current will not reach relay and the contact point will drop down and turn on the red light circuit. When the train leaves this section, the relay will re-excite and turn the green light on once again allowing other trains to enter the section.
In the testing phase of TIAA MRT Project, there were persistent track circuit occupational errors and after onsite investigation, was suspected to be water vapor entering the cable terminal in the TU box that led to incomplete contact caused by oxidation of the cable. After discussion by experts and scholars, improvement measures were decided to use self-recovery waterproof adhesive to fill the interspace.
The self-recovery waterproof adhesive (commonly known as jelly adhesive) is extremely soft and is suitable for filling, waterproofing and buffering purposes. Its features are as follows: able to be mixed and applied at room temperature, can penetrate extremely small space between elements to form protection, transparent to the interior of the colloid and test probes can be inserted directly into the colloid for testing. By applying the jelly adhesive, waterproof features of the TU box will be enhanced and damage of colloid-coated components caused by vibration will be reduced. Additionally, the adhesive will not cause electromagnetic or frequency interference and blockage so will not affect the functionality of the original equipment.
To ensure a smooth testing process of TIAA MRT Project, the contractor has finished 545 sets of adhesive-filling for TU boxes on the entire track circuit during 2015 October to 2016 January. According to the equipment malfunction record, the number of rail system failure (RSF) incidents has substantially declined. There were 130 incidents per month before the adhesive-filling procedure and now, from 2015 October to 2016 January, there were only an average below 20 incidents per month and the outcomes are continued to be observed.