Tuesday, May 15, 2012

Experts raise possibility of more cracks on third rail

SINGAPORE: Technical experts investigating last December's train disruptions have raised the possibility that there could be more cracks on the third rail in the current rail network which have not been discovered.

The panel of six experts shared their assessments and presented 33 recommendations at the ongoing Committee of Inquiry hearing into the breakdowns on Tuesday.

The court heard on Tuesday that cracks were discovered on the third rail in both the December 15 and December 17 disruptions.

While the crack on the December 17 was obvious at the time of the incident, the hairline crack in the December 15 incident was only detected weeks after the service disruption.

However, experts could not conclude what caused the cracks as investigations are still ongoing.

Professor David Ewins of Vibration Engineering from Imperial College London represented the experts.

He said they agreed that it was not possible to establish the actual sequence of events that caused the two disruptions.

He painted several scenarios.

For the December 17 incident, the experts were of the view that the crack in the third rail could have developed over several months.

This weakness made the structure more vulnerable to vibrations, causing the supporting rail claws to dislodge and third rail to sag.

As for the December 15 incident, it is not known if the crack on the third rail was pre-existing.

The experts said another possible cause of the first major disruption was defective materials used in the third rail assembly.

Professor Ewins said a defective fastener attached to a claw had broken.

After more trains passed by the incident site, two insulators that were of "low standard of quality" gave way and contributed to the dislodge of the claws.

The experts also believed there could be a link between the two disruptions.

In the second disruption, they said it was possible that a damaged current collector shoe on one of the trains that ran through the first incident went undetected.

The impact of the damaged shoe on the third rail could have caused two adjacent rail claws to drop.

The experts noted that the severity of the disruptions came about as two adjacent claws had dropped, causing the third rail to sag excessively.

And that there has been no known record of two claw-drops until last December.

Prof Ewins said that based on calculations, the probability of a claw dropping every day on the North-South and East-West Lines is as low as 1 in 500,000.

The experts had also conducted on-site measurements and lab tests.

They concluded that the type of train or how packed the trains are have no, or little effect, on the level of vibrations.

Tests revealed that in all the cases, except one, claws secured with cable ties did not dislodge with vibrations.

Overall, the experts believed it was not a single cause, but a combination of factors that caused the disruptions.

They gave a total of 33 recommendations that would "greatly reduce" a recurrence of the disruptions.

One of their recommendations was for tests to be done at joints of the third rail as soon as possible, with known problem areas to be given priority.

They also suggested that SMRT and the Land Transport Authority develop a more robust third rail assembly, as well as a way to detect rail sag due to a single claw-drop.

SMRT must further tighten its maintenance regime of the third rail by formalising procedures, while the interim solution of placing cable ties should be well-inspected and maintained.

- CNA/de

View the original article here

Source From Channel News Asia

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