A GOVERNMENT expert told an inquest yesterday that the level of maintenance on a hydrogen generation plant was ''on the high side'' in the three months before an explosion killed two workers at a power plant. Mr Martin Wong, senior engineer of the Electrical and Mechanical Services Department, was giving evidence in the inquest on engineers Mr Wong Kwong-yu, 38, and Mr Yip Ka-pui, 40, who died in an explosion at Castle Peak on August 28 last year. Mr Wong checked through records supplied by China Light and Power on repairs and maintenance carried out between May 27 and August 27 last year. He said the number of defects was abnormally high. He said the document, which was signed by Mr Trevor Ferrer, maintenance superintendent of Castle Peak Power Station A, was trying to indicate that much of the servicing was for preventative maintenance rather than dealing with defects. Mr Wong said a series of alarms signalling a low level of lye supplying the electrolyser had been recorded on the alarm log between 5.14 pm and 5.49 pm on August 27, 1992. He said the series of alarms indicated that operators should top up the lye solution. ''I can see the hydrogen plant is not running as efficiently as it should be or was designed for. You should not have so many alarms. I don't know whether it's a regular thing,'' said Mr Wong. Although he could not be sure whether the series of alarms was an indication of low hydrogen supply to the system, he said it was something which any plant operator should pay attention to. He was told by senior shift charge engineer Mr David Ford during the investigation that a bang was caused by a dislodged gasket at the outlet of the low pressure compressor. Mr Wong said he suspected that there might have been a flammable mixture in the system since it required a tremendous force to dislodge a strongly-bolted gasket. The gasket was later replaced and that part was isolated after the bang, the court was told. ''I probably would investigate further because something out of the ordinary had happened,'' he said. The hearing before a jury continues.