A 34-year-old Roxbury man who is a Boston EMT is facing charges stemming from his alleged assault on a domestic partner, Suffolk District Attorney Daniel F. Conley’s office said. Juan Rivas allegedly committed the assault when he was off duty, prosecutors said. Rivas was arraigned Tuesday morning in Dorchester Municipal Court and pleaded not guilty on three counts of assault and battery and one count of rape. His bail has been set at $50,000, with his next court appearance to be on March 6, said Conley's office.Globe correspondent Jennifer Smith contributed to this report. Martin Finucane can be reached at firstname.lastname@example.org.