A man suffered serious burns while trying to tamper with or steal live cables from a building site near Tredegar in Blaenau Gwent, say police.
The 35-year-old is being treated in hospital but his injuries are not said to be life-threatening.
Gwent Police said the incident happened at the site on the Heads of the Valleys road at about 02:00 BST on Friday.
Officers have warned about the dangers of tampering with live cables as people are putting their lives at risk.
Ch Insp Matthew Williams said: "We are continuing our efforts to raise awareness of the dangers of tampering with live cabling.
"Those who tamper with, or attempt to steal cable or wiring are putting their lives at serious risk.
"Anyone who thinks that stealing metal or cables is easy money, should think about whether it is worth risking their life for, because that is a very real possibility," he added.