Nine electricity substations have been broken into during the last two days, with copper earthing materials stolen.
Central Networks, the firm which looks after electricity supply, said seven substation were broken into on Tuesday and two more on Wednesday.
A spokesman said some substations had just been repaired after previous thefts, and a a £1,000 reward for information is being offered.
He added this type of vandalism could cost someone their life.
Phil Wilson, customer operations manager for Central Networks, said he was amazed people are willing to put their lives in danger for the cost of scrap metal.
"We keep our substations locked and fenced off for good reason and by ignoring safety warnings these vandals risk being seriously injured or even killed," he said.
The reward for information leading to a conviction is being offered through Crimestoppers.