The Land Transport Authority (LTA) has released its report on the 14 October 2020 train service disruption which occurred on the North-South and East-West Lines (NSEWL) from Woodlands to Jurong East stations and Queenstown to Gul Circle stations at 6.58pm, and on the Circle Line (CCL) from HarbourFront to Serangoon stations at 7.34pm.

2        Investigations revealed that the incident was caused by the occurrence of concurrent faults. It started with a 22kV (kilovolt) power cable fault in the electrical zone between Tuas Link and Tuas West Road stations on the Tuas West Extension (TWE). This would not cause a disruption if the circuit breaker had kicked in and isolated the affected electrical zone, which would allow train services to continue uninterrupted. However, the circuit breaker at Tuas West Road station, where the section of faulty cables was, also malfunctioned.

3        It was discovered that the trip coil, which is a component in the circuit breaker, was faulty. As a result, the circuit breaker meant to isolate the 22kV cable fault within its electrical zone between Tuas Link and Tuas West Road stations did not automatically activate as designed. A secondary protection mechanism then kicked in and isolated a larger electrical sector as a precaution. This caused the power supply from Tuas Depot Intake (TDI) substation to be cut off, disrupting the power supply to the affected sector on the NSEWL.

4        The cable fault should have been first isolated within its electric zone before power is restored to the affected electrical sector, but this was not done. An attempt was then made to draw power from the Buona Vista Intake (BVI) substation, which also supplies power to the CCL. As the faulty cable at Tuas West Road station had yet to be isolated, this led to a voltage dip at BVI, which activated the safety mechanism that tripped to protect the CCL’s power system. This in turn affected train services along parts of the CCL.

5        After 40 minutes into the disruption, SMRT assessed that the power supply for the NSEWL and CCL could not be restored quickly. It proceeded to detrain commuters from 12 stalled trains along the NSEWL and three stalled trains along the CCL. All necessary safety precautions were taken. SMRT staff walked along the tracks to reach the stalled trains to check on the well-being of the commuters on board, and assisted and guided commuters to the nearest station. Detrainment on the three affected trains along the CCL was safely completed within 20 minutes.

6        At 8.00pm, SMRT managed to restore power to the affected sector on CCL by drawing power from Bishan Depot Intake. Train services on the CCL were progressively restored at about 8.43pm.

7        Once SMRT isolated the electrical zone with the section of faulty cables at Tuas West Road station, power was restored to the NSEWL. Detrainment operations for 11 out of 12 stalled trains on the NSEWL concluded at 8.42pm. Due to inclement weather and lightning risk, detrainment for the last NSEWL train near Bukit Batok station had to be suspended and was only completed at 9.43pm. With the detrainment operations completed, and following safety checks, train services were progressively restored on the NSEWL at 10.34pm.

Remedial Measures

8        Site investigations conducted during engineering hours on 15 October 2020 found that a section of 22kV cables located about 500m from TLK station was burnt through. The faulty trip coil and faulty cables have since been replaced.

9        As an additional precautionary measure, the maintenance team combed through all TWE stations to ensure all the circuit breakers’ trip coils are functional. Two other trip coils were found to exhibit inconsistent performance and were immediately replaced on 15 October 2020. The faulty trip coils have been sent for further forensic examination.

Pre-emptive Changeout 

10       The TWE 22kV network configuration and protection systems are similar to Singapore Power’s network. Previous batches of 22kV cables of similar design and by the same manufacturer have been used in other parts of our MRT network. The TWE circuit breakers are of an established design and are widely used in the power industry.

11       In the case of TWE, since the line commenced commuter service in 2017, there had been four instances of faults at different locations in the upper layer of the power cables. These instances did not lead to any service disruption because the circuit breaker kicked in as intended and isolated the faults. LTA treated this as a matter of significant concern and raised the issue with the main contractor Alstom in January 2020. Alstom agreed to replace the upper of two layers of TWE power cables, which was where the faults had all occurred. Replacement works had been scheduled to commence in October 2020.

12       In light of the 14 October 2020 incident, Alstom will replace all the 22kV cables, amounting to 150km of cable. To provide additional assurance that power cable faults will not lead to another service disruption, Alstom will also replace all 113 trip coils along the TWE. LTA will implement weekend Early Closures from November 2020 to complete replacement of the trip coils by end 2020. As the replacement for the lower layer cables will take time to arrive, LTA will schedule Early Closures/Late Openings on weekends or Full Sunday Closures for limited periods in 2021 to facilitate the physical cable replacement works.

13       While the cables are being replaced, SMRT will increase the frequency of checks on the trip coils and 22kV breakers.  This includes visual checks of the trip coil health status every 10 days, and mechanical testing every six months instead of 12 months.  SMRT will also enhance their standard operating procedures for power recovery so that similar errors of judgement will not recur.

Working Together

14       Once again, we apologise for the inconvenience caused to commuters that evening. LTA, SMRT and Alstom will work together to address the underlying issues to minimise the risk of a reoccurrence.

Annex: Report on 14 Oct 2020 Service Disruption