THIEVES have caused £40,000 worth of damage after tipping over a live electricity transformer and stripping it of copper.

The latest in a string of metal thefts around the Forest happened at Drybrook Quarry over the weekend.

The thieves forced a gate open and broke into a substation before tipping the 11,000 volt transformer on its side – possibly by attaching it to their vehicle.

"These were people who knew what they were doing, and they were prepared to take huge risks," said David Weeks, spokesman for quarry owners Hanson.

"They stole heavy duty cabling and destroyed a transformer that cost around £25,000. It was an organised operation."

The price of copper has more than doubled to £5,000 per tonne since the end of 2008 and police nationally have said they expect the number of thefts to get worse.

"Somebody is going to be killed doing this," said Steve Lewis, who manages the quarry site.

"The quarry is a particularly dangerous place to begin with and we are stepping up security at the site in response to the theft."

As well as stripping the transformer, cabling was also taken from another building at the quarry.

Drops of blood that were found by a broken window are being analysed by scenes of crime officers.

Police Superintendent Phil Haynes said there had been 10 significant thefts of metal in the Forest over the last month.

"The high value of certain types of metal is clearly having an influence on this area of crime" he said.

"We would ask members of the public to remain vigilant and report any suspicious activity to us.

"We had a similar situation two years ago and the number of thefts dropped when the price of the metals dropped."

The theft happened between 2.30pm on Saturday and 6.15am on Monday.