INJURED & ANOTHER EE WERE ASSIGNED TO UNCRATE AND MOVE A NEW PLC CABINET(APPROX. 700-800 LBS). AFTER UNCRATING, THE CABINET WAS HOISTED & PLACED ON MACHINERY ROLLERS. AS CABINET WAS BEING MOVE D 1 OF THE ROLLERS JAMMED & STOPPED DUE TO AN ACCESS PLATE IN THE FLOOR. THIS CAUSED THE PANEL TO SLIDE OFF OF THE ROLLER & TIP OVER ON TOP OF ANOTHER EE. INJ. STRAINED BACK WHILE FREEING HIM