Abstract: When a gas unit is in peak regulation operation,the fire appears at the positive pole of generator collector ring,the stator grounding protection of the generator is acted,and the unit is completely stopped. By on-site inspection and analysis,the cause of the accident isthecontact of carbon brush being not good because of long-term overstandard of unit shafting,and eventually leads to local overheating. At the same time,it analysesthe defects in design and manufacture of the collector ring systematically. Further,according to the problems exposed,the corresponding improvement measures are put forward.