NEL train disruption on Oct 26 caused by melted wires which snapped, say SBS Transit & LTA

A sign at Serangoon MRT station on Oct 26, when the North-East Line was down for two hours.
A sign at Serangoon MRT station on Oct 26, when the North-East Line was down for two hours. PHOTO: ST FILE

SINGAPORE - The causes of two major train disruptions on the North-East Line last month has been determined, SBS Transit and the Land Transport Authority (LTA) said in a joint statement on Friday (Nov 27).

NEL train services were disrupted at five stations for one hour and 40 minutes during the morning peak on Oct 13. There was another two-hour disruption on the morning of Oct 26.

The disruption on Oct 26 happened after a new train undergoing testing damaged the overhead catenary system (OCS), which supplies power to the trains. It was returning to the depot.

The catenary system is the power supply system installed on the ceiling of the train tunnel.

Investigations revealed that the train was being manually driven from the mainline back to the depot when the driver saw sparks from above the train and stopped the train immediately.

The train stopped at a stretch where the mainline and depot catenary system wires overlap, resulting in arcing between the two sections of wires.

The sustained arcing caused the catenary system wires to melt, and eventually snap.

Arcing occurs at the overlap point because of the voltage difference between the mainline and depot catenary system. This is not a problem during normal operations as the NEL trains are driverless and will not stop at this overlap point, the statement said.

Power to the mainline tracks between Hougang to Buangkok was switched off to facilitate urgent repair works.

This affected the launching of trains to the mainline, and hence the disruption to NEL services. Power to the rest of the mainline and depot was not affected, the statement said.

As an interim precautionary measure, SBST has marked all overlap zones on the reception tracks to indicate these as non-stopping zones.

LTA and SBST are working together to enhance the design of the catenary system to minimise the likelihood of a recurrence.

This includes reducing the electrical voltage difference between the catenary system wires at the overlap point, in case trains still stop at this stretch.

The NEL train service disruption between Farrer Park and Hougang stations on Oct 13 was due to a power fault, which caused the main circuit breaker to trip.

The power fault was triggered by a "malfunction of a component in the electrical switchgear at a substation" that supplied electricity to power the trains running between these stations.

This caused the main circuit breaker to trip. Then, power was diverted to a backup breaker which also tripped.

After investigations, LTA and SBST said that the original manufacturer protection setting of the backup breaker was overly conservative.

As an interim measure, SBST has increased the protection setting of the circuit breaker to cater for the projected maximum load of the network.

For the long term, LTA and SBST are working together to enhance the power system to minimise the likelihood of a recurrence.