Union workers have launched a blistering attack on National Grid, saying the giant electric company has failed to keep its grid system safe for its employees. The local head of the Utility Workers Union and the Brotherhood of Utilities Workers Council, which represent about 1,000 employees, said recent rate hearings have revealed that National Grid has thousands of overloaded pole transformers and faulty line insulators that have led to a few serious injuries. “They are scared,” Gary Sullivan, president of UWU Local 369 and the council, said of union members who have to work on electric lines throughout the state. Attorney General Martha Coakley’s office confirmed yesterday that it has similar worries about the transformers and insulators that unions are complaining about. Coakley’s office, which often battles with utilities, is now contesting National Grid’s request for a 5.5 percent distribution rate increase for its one million customers. “We share the unions’ concerns regarding worker safety and the company’s record on maintaining its distribution system,” Coakley’s office said in a statement yesterday. National Grid fired backed that “safety is a priority at National Grid and allegations that the system is unsafe are simply untrue.” The company has worked closely with unions in the past to make upgrades, the statement read. “The union appears to be using this misrepresentation as leverage to accomplish a separate agenda,” National Grid said. “We are disappointed in this behavior. Our safety record is impeccable; safety is a top priority at National Grid.” Unions and companies often spar over issues while in contract negotiations. But Sullivan said yesterday his union contracts don’t expire until 2011 and employees have “no ulterior motive” for the criticism aimed at National Grid