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askST: What happens when lightning hits an MRT train?

Reader Koek Kai Bin wrote in to ask about the measures to protect MRT trains from getting struck by lightning during a thunderstorm.

Transport reporter Adrian Lim answers.

Lightning strikes on the MRT viaducts are rare, with an average of one or two incidents in recent years, said operator SMRT in a blog post last year.

Still, experts say commuters in a train cabin are safe, similar to how passengers in a plane or car are protected.

If lightning strikes a train, the steel frame of the train rises in voltage, but the current would flow around the metallic walls of the train car.

"In the process of doing so, because the wall is electrically conducting, there is very little voltage difference generated," Professor Liew Ah Choy, a lightning expert, told The Straits Times previously. A significant voltage difference is what causes electrocution.


Grab poles in the train cabins are attached to the train's interior and not connected to the external frame, according to SMRT in a blog post last year. Even if a commuter is holding onto one when lightning hits, he would be unaffected. PHOTO: ST FILE

Even if a commuter is holding onto a metallic pole in the train when lightning hits, he would be unaffected.

"Say, your hand is holding the pole and your feet are on the (train cabin) floor. The voltage on the pole is roughly the same as the voltage on the floor," said Prof Liew, who is from the National University of Singapore's electrical and computer engineering department.

SMRT also said in its blog last May, following a lightning incident, that the grab poles are attached to the train's interior and not connected to the external frame.

The operator added that when a train is struck by lightning, the current flows around the outer shell - not through the cabin - and passes through the wheels to the track.

The protection offered in a train cabin is based on the concept of a "Faraday cage", named after 19th century scientist Michael Faraday. Other examples of Faraday cages are cars and planes.

A version of this article appeared in the print edition of The Straits Times on March 25, 2017, with the headline 'What happens when lightning hits an MRT train?'. Print Edition | Subscribe