The two rail disruptions on the North-East Line (NEL) last month were electrical in nature.
In a joint statement released yesterday, the Land Transport Authority (LTA) and train operator SBS Transit (SBST) said they had completed investigations into what brought trains to a halt on Oct 13 and 26.
On Oct 13, train services were disrupted between Farrer Park and Hougang stations because of a malfunction of an electrical switchgear at a substation.
This caused the main circuit breaker to trip. As a result, power was diverted to a backup breaker, which also tripped.
The LTA and SBST said the manufacturer's protection setting of the backup breaker was "overly conservative". As an interim measure, they have raised this setting to cater for the projected load of the network. This means that the circuit breaker - a safety feature - will not trip as readily as before.
Both parties said the new setting is safe but, in the longer term, they will work at "enhancing the power system" of the 12-year-old line, but did not say what this entails.
The breakdown, which happened during the morning rush hour, inconvenienced thousands of commuters, like the breakdown on the morning of Oct 26.
That day, a new train undergoing testing damaged the overhead catenary system (OCS) which supplies electrical power to trains. This happened when it was returning to the depot via one of the reception tracks.
Power to the mainline tracks between Hougang and Buangkok was switched off to facilitate urgent repairs. Hence, trains could not run.
Investigations found that the new train, which was manually driven, was stopped immediately when the driver saw sparks from above.
The train stopped at a stretch where the mainline and depot OCS wires overlapped, resulting in electrical arcing between the mainline and the depot OCS wires. Sustained arcing caused the OCS wires to melt and snap.
Arcing occurred at this overlap point because of a voltage difference between the mainline and depot OCS. This is not an issue during normal operations as trains on the line are driverless and will not stop at this overlap point.
Asked why there was a difference in voltage in the first place, an LTA spokesman said this was because power came from two different sources - one from the depot and the other from Buangkok station. LTA and SBST said they will reduce the voltage difference.
In the meantime, the operator has marked all overlap zones on the tracks to indicate these are no-stopping zones.
Yesterday, rail operator SMRT Corp announced it is upgrading the electrical system of the North-South and East-West lines. "We have started the mass implementation of power substation upgrading and renewal works," said SMRT spokesman Patrick Nathan.
Work started in the middle of this year and is slated to be completed by 2019. There are 72 substations serving the two lines.
The project is one of several to bring back reliability to Singapore's two oldest MRT lines. Others include replacing the rail sleepers, power-supplying third rail, signalling system and train components.
The project comes as the operator grapples with a rise in power-related breakdowns. On Wednesday, a circuit breaker fire caused service on the North-South Line to be disrupted for more than two hours during morning peak hours.