SINGAPORE - Multi-beam door sensors on lifts cannot guarantee that lift accidents will not happen, said the Building and Construction Authority (BCA) on Monday .

The authority was referring to lift doors with multiple infrared beams. These beams act as a safety feature and trigger the reopening of doors when they are obstructed.

"While the likelihood of the lift doors closing when there is an obstruction could have been reduced had there been multi-beam sensors, these sensors still have 'blind spots'," said the BCA in response to media queries.

"This is because the beams cannot detect objects that are smaller than the spacing between beams. This is why several manufacturers/suppliers of multi-beam sensors issue disclaimers, that 'objects thinner than the beam spacing may not be detected'."

This comes after an investigation report found that there was nothing wrong with the Tah Ching Road lift which severed an elderly woman's hand last month.

In citing the report that was submitted last week, the BCA said that all door protective devices were found to be working and could not be proven to have malfunctioned on the day of the incident.

On Oct 9, Madam Khoo Bee Hua lost her left hand after the lift doors closed on the dog leash that was looped around her left wrist. As the lift went up, the leash likely pulled her hand through the gap in the cabin doors. The 85-year-old's hand was crushed, severed and dropped to the bottom of the lift pit.

The lift, equipped with a single beam sensor, was not designed to detect anything thinner than 10mm, the investigation report said. Madam Khoo's dog leash was only 2mm thick.

National Development Minister Lawrence Wong said on Monday that he has asked the BCA to "give priority" to reviewing Singapore's lift safety framework, which includes protective devices in lifts.

"We have a strict safety regime for lifts in Singapore today, and we will continue to enhance our standards," Mr Wong wrote in a Facebook post.

The BCA said it regularly conducts reviews of lift safety standards, adding that the current review began last year and will be completed in mid-2016.

During such reviews, the authority consults industry players, such as lift manufacturers, lift contractors and building owners.

The BCA added that Singapore's door protective devices are on a par with international standards, such as those in America, Europe and Hong Kong.

Lifts in Singapore must be fitted with protective features such as beam sensors and door edges, which re-open doors when pushed back by a certain amount of force. Some beam sensors are "3D" and can even detect obstructions in areas around the doors.

But the BCA added that all sensors are deactivated when lift doors are almost closed.

"This is to enable the doors to completely close. Otherwise, they would sense the other side of the lift door and re-open," the BCA explained.

"Hence, even with multi-beam sensors, there is no guarantee that it would be absolutely foolproof in preventing an incident like that which happened at Tah Ching Road."

In the light of the Tah Ching Road incident, the authority will also be ramping up public education on the safe use of lifts through publicity activities and distributing of posters to town councils and building owners.

Mr Wong said: "In Singapore, we use lifts on a daily basis. They are an essential facility in our high-rise living environment. So we must continue to ensure that our lifts are safe, and not take things for granted."