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Update: Weak resistance of third rail insulator 'root cause of July 7 train breakdown'

Signs outside City Hall MRT station during the breakdown.

THE weak electrical resistance of the third rail insulator has been determined to be the root cause of the massive train breakdown on July 7, 2015.

The evening disruption of the North-South and East-West Lines (NSEWL), which affected 250,000 commuters, was caused by the intermittent tripping of the rail power system at multiple locations.

The Land Transport Authority (LTA) and train operator SMRT Corp have ascertained that this was due to the lower electrical resistance pathway at a third rail insulator.

When functioning properly, the third rail insulators insulate the electrified third rail from the ground and ensure that the electricity from the third rail flows to the trains only via the trains' current collector shoes.

However, the weak resistance of an insulator can allow electricity to flow through the insulator to the ground, resulting in a higher-than-normal voltage difference between the running rail (which has a negative voltage relative to the ground) and the ground.

This higher-than-normal voltage difference was aggravated by the movement of the trains and resulted in the activation of the 64P safety mechanism at multiple locations in the network.

The activation of the 64P safety mechanism is what tripped the power system.

The third rail supplies electrical power to the trains, while the running rail is the surface on which the train wheels run and through which the electricity "returns" to the source to complete the circuit.

The 64P, on the other hand, is the Touch Voltage Protection Relay. It is a safety feature on the NSEWL network and is used in rail systems all over the world. The 64P is activated when the voltage difference between the running rail and the ground becomes too high.

LTA, SMRT and five overseas experts carried out comprehensive system-wide checks across more than 200km of train track and components such as the third rail, power cables and the signalling system to identify the root cause of the disruption.

They also checked all 141 trains and analysed train logs from the day of the incident.

The checks and trackside investigations found that a confluence of factors had triggered the incident.

In the stretch of the tunnel between Tanjong Pagar and Raffles Place, water was found to be dripping onto the third rail cover near an insulator. Test results on the sample residue from the third rail cover, and on water collected from the tunnel leak, found mineral deposits with a high chloride content.

The presence of chloride on the insulator, coupled with a wet environment, would have significantly reduced the effectiveness of the insulator.