A rescue volunteer passed out while helping install a power line in Tham Luang cave on Friday afternoon, Chiang Rai governor Narongsak Osotthanakorn said.
The man fainted in the cave and was taken by ambulance to an army hospital. He was in a safe condition.
Mr Narongsak denied reports he was electrocuted.
The man was a rescue volunteer from Ubon Ratchathani, there to assist with the search for and rescue of the 13 missing footballers believed trapped in the cave, he said.
The incident occurred shortly before 3pm, according to the @Disaster_NewsTH Twitter account.