The current retirement system for federal employees, including USPS postal workers, is called the Federal Employees Retirement System, or FERS. Before 1987, the program was called the Civil Service Retirement System, or CSRS, and had slightly different rules. Both FERS and CSRS offer a disability retirement annuity for those who haven't reached retirement age but can't work due to a physical or mental disability. The rules for federal disability retirement are different than for Social Security disability, and in fact, federal employees are required to apply for Social Security disability before they apply for FERS or CSRS disability benefits. Here are the rules on eligibility, benefit amounts, how to apply, and more.