Investigations have pinpointed the cause of the July 7 massive rail disruption to a third rail insulator that was contaminated, the Land Transport Authority and SMRT said on Wednesday.

This caused a power trip that led to the breakdown of the North-South and East-West lines.

In a media briefing, LTA deputy chief Chua Chong Kheng said there was a tunnel leak at Tanjong Pagar station and water had dripped near one of these rail insulators.

The insulators insulate the electrified third rail from the ground and ensure that electricity from the third rail only flows to the trains via the trains’ current collector shoes.

Tests found that there was a high chlorine content in the water and this led to surface contamination of the insulator.

Chua explained that there were other third rail insulators which were also found with weak electrical resistance.

This resulted in electricity flowing through the insulator to the ground, resulting in a higher than normal voltage difference between the running rail and the ground, he said. It was this higher-than-usual voltage difference that tripped the power system.

As a follow-up measure, LTA is changing out all third rail insulators with low resistance in the next four to six weeks.

SMRT chief executive Desmond Kuek said SMRT’s maintenance regime has been complied with. His staff comb through the rail network to search for leaks every four to seven days. — The Straits Times /Asia News Network