NEL, SPLRT disruption: Electrical surge shut down backup power switchboard, says LTA

SINGAPORE - Early investigations into

the Aug 12 disruption on the North East MRT Line (NEL)

and the Sengkang-Punggol LRT (SPLRT) system have shed more light on

the cause of the breakdown

and why it took hours for operator SBS Transit to restore services on the affected lines.

The Land Transport Authority (LTA) said in a Facebook post on Aug 13 that a surge in electricity had damaged one of two switchboards at a substation in the NEL depot in Sengkang, which fed power to the affected rail systems.

At the same time, the electrical surge also triggered a safety feature meant to protect the second switchboard from damage, causing it to shut down as well, LTA noted.

A switchboard distributes power from electrical sources to several smaller circuits.

The authority said the two switchboards would normally have acted as backups for each other, so that power could continue to be supplied even if one were to go down.

But on Aug 12, the power supply from Sengkang Depot was cut off completely as a result of both switchboards tripping, and SBS Transit had to manually connect the second working switchboard to a backup power source in Dhoby Ghaut to restore train services progressively.

LTA said that while it continues to probe the incident, which was the second lengthy service disruption to hit Singapore’s rail network in two weeks, it is also reviewing with SBS Transit how to make the power systems more resilient.

Owing to the power fault on Aug 12, a number of passengers had to walk along the NEL tracks to get to the nearest stations, as trains travelling between an 11-station stretch of the NEL between Farrer Park and Punggol Coast stalled.

Several NEL stations were also plunged into darkness, and free bus services were activated.

The disruption on the NEL lasted about three hours, from just after 11am to 2.10pm, when train services were fully restored.

The SPLRT was worse hit, with train service resuming fully 10 hours later at about 9.20pm.

SBS Transit group chief executive Jeffrey Sim earlier said the failure of a power switchboard at Sengkang Depot’s substation had caused an unintended discharge of electricity in a device used to regulate the power supply. This resulted in the power trip that crippled the NEL and SPLRT.

This device, called a voltage transformer, converts high voltage to a lower, safer level.

Mr Sim said the faulty voltage transformer could not be repaired immediately after the power trip, which is why SBS Transit decided to draw power from its other substation at Dhoby Ghaut.

LTA said on Aug 13 that LTA and SBS Transit engineers worked through the night to restore power supply to the NEL depot substation and ensure the safety of the system.

In addition, about 350 personnel from LTA and SBS Transit, on top of regular station staff, were deployed to the affected stations to guide commuters during the breakdown.

Mr Simon Lee, director of engineering consultancy Bescon Consulting Engineers, said the probability of a voltage transformer failure is “extremely low”.

This occurs when its insulation system cannot handle the electrical stresses generated during train operations, or when it experiences electrical disturbances such as lightning surges or temporary voltage spikes, added Mr Lee, who also co-chairs the qualified electrical contractor registry of the Institution of Engineers, Singapore, and Association of Consulting Engineers Singapore.

Associate Professor Jimmy Peng from the department of electrical and computer engineering at the National University of Singapore, who specialises in electrical power systems, said power switchboard failures are “very rare” due to regular preventive maintenance and backup safety measures.

He noted that in this incident, protection relays – devices that detect abnormal electrical conditions – likely tracked down the fault and disconnected the affected equipment from the damaged switchboard.

This contained the problem and prevented damage to the second switchboard and the wider power grid, added Prof Peng.

Acknowledging that it would take some time for the rail operator to check the condition of the undamaged portion of the electrical system, Mr Lee said three hours was a reasonable amount of time for SBS Transit to get NEL services running again.

Prof Peng described the restoration of power to a network as “a complex and delicate process”, which is not as simple as resetting a household circuit breaker.

He noted that recovery speed depends on several factors, such as the severity of the fault, safe isolation of damaged equipment, rerouting of power, and system-wide safety checks.

Gradual restoration is also key to avoid overloading the backup power supply or causing other failures.

Given that major train power failures across the world could take many hours or days to be resolved completely, Prof Peng said the three-hour period taken to restore NEL services and 10 hours for full reinstatement of SPLRT services were “within international norms”.