The cause of a service breakdown that affected almost a quarter of a million SmarTone subscribers earlier this month has still not been fully established. The mobile phone operator earlier blamed the breakdown of a back-up generator that restarted itself three times in a minute, causing a burn-out. But in an investigation report submitted to the telecommunications watchdog yesterday, the company said the actual cause of the generator breakdown on April 9 could not be identified 'even after a comprehensive inspection'. SmarTone also said it estimated 243,000 customers had been affected by the 2?hour stoppage, which occurred two hours after a power failure at a switchboard station triggered the back-up supply. It based the estimate on the number of active users in the five days before the incident. Charles Mok, founding chairman of the Internet Society Hong Kong, said he accepted that a cause could not be identified for every failure. 'It is most important to look ahead. Like the compensation issue, it is a good chance for SmarTone to turn crisis into opportunity and build a better image,' he said. Although it could not find the ultimate cause, the operator said it would implement a new control system, consisting of a voltage sensor and an isolator, which cut off the generator output in case of abnormal fluctuation of voltage, to prevent any electric current surges hitting the circuit breakers. It would also appoint an electrical and mechanical consultant to review the power systems of all three switchboard stations. The report said the three sudden restarts of the generator were caused by a malfunctioning control module. It should have placed the generator into manual-restart mode instead of restarting itself. SmarTone had replaced the control module with a device that would cut the power supply and turn the generator to manual-restart mode and switch to standby battery power in case of any stoppage. Following criticism of poor communication, the operator also pledged to inform the watchdog of any critical network outage within 15 minutes, and contact the media and its frontline staff within 30 minutes. A spokeswoman for the Office of the Communications Authority said the authority would study both the initial and full reports before deciding if any regulatory action was needed. It would also review the reliability of the power supply system and emergency response system of all other telecoms operators.