Thieves targeted eight electricity substations in one small area of Northumberland over a few days, CE Electric UK has revealed.
During the most serious incident, 140 properties in Ashington were left with no power after copper components were stolen from a compound on Thursday.
Electricity has since been restored to most of them.
A spokesman for CE Electric UK said the thieves were risking their own and other people's lives.
'Danger of death'
The Thursday night break-in at the substation on North Seaton Industrial Estate, in Ashington, was one of eight in a two mile (3.2km) radius over the past few days.
The others resulted in damage but did not cause any power cuts.
In all cases thieves ignored "danger of death" signs and security measures.
Peter McCormick, from CE Electric UK, said: "No doubt the thieves were lured by the prospect of making a quick buck, but by breaking in they're lucky they haven't killed themselves.
"To the untrained and uninitiated substations are potentially fatal and in the past our staff have discovered charred bodies after similar incidents."