SBS Transit is aiming to restore full service to the North-East Line by 4.30pm.
A power fault had occurred at a stretch between HarbourFront and Outram Park stations at 9.51am Thursday morning.
In an update, SBS Transit said it decided to disrupt services between HarbourFront and Dhoby Ghaut so engineers could enter the tunnel and repair the stainless steel U-shaped bolt holding the cantilever arm of the overhead cables.
It currently has about 30 engineers on the ground working on repairs. Free shuttle bus services have also been activated to serve affected commuters.