The Circle Line disruption on Sep. 17 was caused by a power trip, whereas the one on Sep. 18 was caused by a fire in one of the systems.
On Sep. 26, SMRT issued an explanation on Facebook explaining why the Circle Line MRT trains stalled on Sep. 17 and 18, 2024.
Sep. 17 incident
A circuit breaker in the Traction Safety Shutdown System (TSSS) – the system which cuts off traction power supply during emergency events – on the Circle Line tripped.
The circuit breaker was subsequently reset by the maintenance team, but they were concerned that the fault might re-occur and disrupt the withdrawal of trains to the depot later in the day.
The team decided to replace the main circuit breaker entirely, and powered down the TSSS cubicle as a safety precaution.
However, this action caused another circuit breaker – which is part of a fail-safe design – to trip.
This resulted in a loss of traction power across the entire Circle Line.
The circuit breaker is part of a fail-safe design intended to trigger a line-wide shutdown when necessary for operation and commuter safety.
SMRT added that standard protocol required the maintenance team to seek clearance from the head of Circle Line operations before conducting corrective maintenance during service hours, to ensure that maintenance in a live system is carefully controlled.
SMRT added: "Unfortunately, in their eagerness to rectify the fault, the team overlooked this critical step."
Standard operating procedure indicated that such actions should have been carried out during engineering hours, when train services are not running.
The resumption of normal train services also took longer than anticipated to make sure no further technical issues were unresolved, and for all trains to be accounted for before normal services resumed.
The rail operator subsequently activated diversion plans and advised commuters to use other MRT lines or buses.
In-train and station announcements were made between 5:46pm and 7:30pm, except for a 12-minute break between 6:35pm and 6:47pm, when a technical glitch required a public announcement system restart, SMRT said.
Throughout the night of Sep. 17, SMRT engineers worked with Land Transport Authority (LTA) and OEM engineers to diagnose and resolve the issue, ensuring normal train services resumed on schedule the following morning," SMRT said.
All lines resumed operations the following morning.
Sep. 18 incident
SMRT also explained that the Sep. 18 incident stemmed from a fire.
At around 8pm, the Circle Line Integrated Supervisory Control System (ISCS) team detected a fire in one of the TSSS cubicles at Kim Chuan Depot.
This caused a power trip across the entire Circle Line, and the global emergency shutdown (GES) system was automatically activated, triggering a line-wide power shutdown.
The fire was quickly extinguished by ISCS staff and the area was assessed by the Singapore Civil Defence Force.
Once safe, traction power was restored within 10 minutes, and full train services resumed at 8:15pm.
LTA, SMRT and OEM engineers worked throughout the night to conduct checks and maintenance.
"Our priority was to ensure safe and smooth train operations, especially during the ongoing Formula 1 period," SMRT said.
The damaged TSSS cubicle at Kim Chuan Depot has since been isolated from the Circle Line power network, and operational control for the TSSS and GES have been transferred to the backup Operations Control Centre at another site.
The root cause of the fire is still under investigation, the rail operator added.
Improvements to be made
In addition, an SMRT engineering team is working with LTA to review the overall design of the TSSS and GES systems.
Enhancements to the Circle Line's signalling and PA systems, and current fail-safe features will be improved.
The TSSS cubicle will also be replaced with enhanced features.
Top photo via Mothership reader