An explosion occurred within an urban heating facility, at the time of performing recalibration operations. Four plant employees plus a subcontracted staff member died on the spot; 2 others succumbed 2 and 5 days later, while another employee suffered serious burns. The explosion, caused by the rupture of a pipe containing overheated vapour at 550°C, was not responsible for any consequences beyond installation boundaries. Nine days before the accident, a certified inspection body had performed a repeat test on this specific part of the circuit at a pressure below that specified in the testing protocol, and this certification would have apparently been forged. The unit involved, placed into service in 1982, was equipped with a capacity of 492 tonnes/hr.