The chairman of the government's Energy Advisory Committee, Jimmy Kwok, on Friday said he expects HK Electric to be hit with a penalty for a voltage dip this week that caused traffic lights to malfunction and left some people stuck in lifts.

The firm has said the 45-minute outage in early hours of Wednesday was caused by a short circuit triggered by the accidental connection of a back-up cable during maintenance at an electrical station in Cyberport.

Kwok described the mishap as a "rookie mistake".

"I think the government will impose a penalty, but it will depend on the report submitted by HK Electric," he said.

"I think the company will be held responsible for the incident, and we have to see if there was sufficient supervision or not."

The government has told HK Electric to submit a detailed report within four weeks.