SINGAPORE - Measures to avoid a recurrence of the massive MRT breakdown on July 7 will be prioritised, and longer-term track upgrade and renewal programmes will be rescheduled, transport operator SMRT announced on Friday.

This is so that "the priority for track access during the limited maintenance hours at night can be given to immediate remedial actions that can safeguard the system against a recurrence", it added.

SMRT will continue its testing and investigations through the weekend by repeating the full cycle of checks.

Where possible, it will re-enact the conditions under which the series of power trips were observed on Tuesday (July 7), when both the North-South Line and the East-West Line were down for more than two hours.

It was the first time both major MRT lines were down, and the worst train breakdown since two major disruptions in 2011 triggered a public inquiry.

SMRT said it is working with external experts to review the design robustness of the power network, and to find ways to further segment it in order to avoid a similar power failure.

They have brought in a number of retired SMRT staff with experience in network power issues to help with investigations, and will hasten the acquisition of additional condition monitoring systems.

These systems can detect changes that give warning of an imminent component failure.

SMRT said it is looking into possible degradation in three areas: the insulation of a power substation, the power cable or the third rail, which provides electricity to each train.

It added that it was "highly unlikely" the disruption was caused by a defective train, as initially thought.

In the meantime, rectifications made on the night of July 7 have stabilised the system.

"However, until the root cause can be conclusively determined, SMRT is taking all possible precautionary and preventive measures to safeguard the system in terms of safety and service reliability," SMRT said.

It provided a more detailed account of what happened on the night of the breakdown.

From 6pm on July 7, repeated power trips were detected for an hour at multiple locations across the two lines, which total 100km in length.

Despite efforts to isolate the suspected source of the problem, the power trips persisted.

As the power trips were linked to the voltage protection circuit designed to safeguard commuter safety, SMRT made a deliberate decision to halt train services on both the North-South and East-West Lines.

Since then, SMRT has completed checks on all equipment that could have contributed to the disruption, with priority given to the higher probability 'hot spots'.

The Board chairman of SMRT, Mr Koh Yong Guan, said: "The SMRT Board apologises for the disruption that affected the entire North-South and East-West Lines on 7 July. The Board and Management of SMRT take full responsibility for the disruption.

"It is of great concern to us that so many commuters were affected by this disruption during the evening peak hours when many were making their way back home.

"In particular, we apologise to our Muslim commuters who were returning home to break fast.

"The Board gives its full support to the management and staff of SMRT to establish the root cause of the incident as quickly as possible, and to prevent a recurrence."