A POWER plant engineer told an inquest he had never experienced a drop in hydrogen purity similar to the one that had occurred before a fatal explosion in Castle Peak. Mr Andy Cheung, shift charge engineer with China Light and Power at the plant, was testifying yesterday at the inquest into the death of fellow engineers Mr Wong Kwong-yu, 38, and Mr Yip Ka-pui, 41. They died in an explosion at the plant at about 10.05 am on August 28 last year. A board of inquiry concluded the explosion was caused by a mixture of air and hydrogen in two high pressure hydrogen storage tanks. The court heard that at about 12.30 am on August 28 an indicator showed that hydrogen purity in the casing of generator two in plant B had dropped to the indicator's minimum reading of 85 per cent. The level should have been more than 95 per cent. Mr Cheung, who has been with the company since 1972, said he had never experienced a 15 per cent impurity in the generator casing before. ''Whether or not it was serious does not only depend on the 85 per cent impurity. Other matters had to be taken into consideration,'' he said. He added he did not know what the impurity was. The court heard the purity level rose to slightly more than 85 per cent after the generator was purged with hydrogen shortly before 7.15 am on August 28. Mr Cheung said he expected the hydrogen purity level to improve gradually. He finished his duty at about 8 am and handed over to the next shift. The hearing before Coroner Mr Warner Banks and a jury continues. Although a chemist had taken gas samples from the high pressure vessels, the results of the analysis were not available by that time.