Bulgaria's state-owned National Electricity Company, NEK, said on Friday that it has opened a 42 million levs ($25 million/21.5 million euro) tender for four-year contracts for 24-hour armed security services.The tender is divided into seven lots, each of which concerns different assets of the company, NEK said in a tender notice.Interested parties can file offers for more than one lot.The deadline for submitting offers is November 9.(1 euro = 1.95583 levs)