What's more, the other unit within the system only took over briefly before failing due to a bad contact between the unit and its batteries.
The UPS was last activated on May 20, due to a fluctuation in the main power supply, and performed successfully, according to an HKEx official.
A complete report detailing what exactly went wrong between May 20 and Tuesday's failure will be available next week.
The exchange has commissioned an independent electrical and mechanical consultant to review the UPS action plan. Tests are scheduled for the weekend to ensure the system functions properly in the future.
The consultant will also advise on the performance of the current UPS supplier, which was appointed in August. The exchange said that it would consider introducing future improvements in light of the consultant's recommendations.