Five workers were hurt in an explosion yesterday while working on an oil pipeline in Nea Peramos, outside of Athens, in an accident that their co-workers said was a result of safety standards not being met. Three of the men, suffering from burns and head injuries, were hospitalized but were not in a critical condition, authorities said. The other two were allowed to go home after being treated for mild injuries. The men were conducting repair work on a pipeline that belongs to petrol refiner Hellenic Petroleum. Their co-workers said the explosion was probably caused by a spark from one of the hammers the men were using when they should have been supplied with rubber hammers. Hellenic Petroleum said it is investigating the accident.