The Land Transport Authority (LTA) said it will avoid testing new trains during off-service hours on Sunday nights, following a disruption on the North East Line (NEL) on Monday morning.

This is to minimise the impact on passenger service during the morning peak hour the next day, should a fault occur. On Monday, 41,000 NEL commuters, including students on their way to sit A- or O-level exams, were hit by a disruption of about two hours. A test train had pulled a wire on the line's overhead power system during off-service hours, causing a power fault.

This led Transport Minister Khaw Boon Wan to ask yesterday, "why was the testing done during a major exam period, especially on its first day?" On his blog, he wrote that LTA had agreed, in hindsight, that "it could have limited the testing to only Saturday night/Sunday morning, rather than Sunday night, eating into Monday morning".

LTA said that, moving forward, it will "adopt a risk-based approach" in developing the testing schedule for new trains.

Mr Khaw said LTA explained to him that testing was not a daily affair and was done progressively - first during engineering, or off-service, hours over weekends, then during off-peak hours and, finally, when ready, into peak-hour traffic.

The new train in question was from a batch of five acquired to increase the NEL's capacity; it had clocked 200km on test tracks, before being tested on the line's tracks over the past two weeks.

In his blog, Mr Khaw praised the strong relationship between the Singapore Examinations and Assessment Board and LTA. Contingency plans worked out beforehand were activated after SBS Transit and LTA alerted the board of the delay.

"Since breakdowns cannot be completely eliminated, we must be prepared for Murphy's Law and expect the worst," said Mr Khaw.

"Even when things are tried and tested, we must anticipate and buffer for further glitches and failures, so we do not let the stress of something unplanned happen during stressful events, like our children's national examination."