AUTOMATIC cut-off valves for gas hotplates are being launched by the Hong Kong and China Gas Company Ltd to reduce the number of serious accidents involving domestic gas supplies. The decision was being considered before a tragedy in Tuen Mun this week in which one person was killed and eight other injured by an explosion on the 41st floor of an apartment building. Hugh Phillipson, Director of Electrical and Mechanical Services, said since the incident there had been a fresh impetus to try to minimise incidents. Called an excess flow valve, the gadget shuts off the gas supply when it exceeds a certain level. The valve has to be set high enough to allow for the normal operation of the hotplate but low enough so if there is a leak in the pipe it will shut the gas supply off. Initially the valve will be offered on newly installed equipment, but Hong Kong and China Gas will give customers the option of fitting it on existing equipment. The Electrical and Mechanical Services Department is to have talks with liquefied petroleum gas producers to introduce similar valves for their appliances. ''It is up to the gas producers to decide what they should do. If they say yes, then fine, but if they say no, we may have to take it further,'' said Keith Whittle, assistant director of the Gas Standards Office. But he warned that excess control valves were being offered as a deterrent against deliberate attempts to sabotage domestic supplies gas supplies. ''These valves are not the be-all and end-all because if people really want to interfere with the gas supply they will find some other way of doing it,'' he said. More than 11 per cent of gas leaks last year were due to tampering.