Two overhead lines, each carrying tens of thousands of volts, were damaged in separate incidents.

A CE Electric UK engineer said vandals who daubed graffiti after climbing a pole supporting a 66,000 volt line near Ferryhill were lucky to be alive.

Rocks were also thrown at a transformer near Easington, leaving some customers without power early on Monday.

Paul Norton, head of safety for CE Electric UK, said: "There can be up to 400,000 volts of electricity flowing through electricity substations and overhead wires supported by pylons and wooden poles.

"We don't know the ages of the vandals who carried out these attacks, but all the equipment carries Danger of Death signs.

"I want to take this opportunity, in view of the ongoing school holidays, to ask parents or guardians to remind their children of the potential dangers of messing about near live electrical equipment."

Advice for children about how to stay safe near electricity supplies can be found on the company's website.