SINGAPORE - The Land Transport Authority (LTA) and SMRT have pinpointed a contaminated electrical component as the cause of a massive disruption that crippled the entire North-South and East-West MRT lines on July 7.
Salt deposits had accumulated on an insulator for the power-supplying third rail along a stretch of the East-West Line between Tanjong Pagar and Raffles Place stations.
This was one of a "confluence of factors" that caused a power trip leading to the breakdown of the North-South and East-West lines, the LTA and SMRT said on Wednesday.
In a media briefing some three weeks after the incident, LTA deputy chief executive for infrastructure and development Chua Chong Kheng said a tunnel leak at that stretch resulted in water with high chloride content dripping near one of these rail insulators.
This led to surface contamination of the insulator, hence reducing its effectiveness. Mr Chua said there were other third rail insulators which were also found with weak electrical resistance
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.
The contaminated insulator resulted 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 followup measure, SMRT is changing out third rail insulators with low resistance in the next four to six weeks.
The eventual plan is to replace all 30,000 insulators on the network with new one.
SMRT aims to change out 16,000 of them by the first quarter of 2016, and the remaining 14,000 by the first quarter of 2017.
SMRT chief executive Desmond Kuek, who was also at the briefing, said SMRT has complied with its maintenance regime. His staff comb through the rail network to search for tunnel leaks every four to seven days, he said.
Even so, SMRT will be stepping up its maintenance procedure and explore ways to pre-empt problems, he said.
SMRT added that it will be de-sensitising the touch voltage protection relay, a safety feature on both lines called the 64P, by raising its setting from 136V to 200V. This will make it less susceptible to power trips.
It added that 200V is in line with international standards, and is the setting on newer MRT lines such as the Circle Line and Downtown Lines.
"Commuter safety is not compromised," SMRT and LTA said.
The findings released on Wednesday come after a joint study by the LTA and SMRT, together with rail specialists brought in from Sweden's Parsons Brinckerhoff and Japan's Meidensha Corporation, after the July 7 incident.
That day, train services on the North-South and East-West lines were disrupted on an unprecedented scale for more than two hours from around 7pm, causing commuters to be stranded during the evening peak hour.
An estimated 250,000 commuters were affected that night as two of Singapore's oldest and most heavily-used lines were shut down.