From the manual. Maybe his outlet wasn't grounded?
3.14 Any installation of the 550/551-type KSU should be grounded and protected in accordance with the requirements in Section 518-010-105. The frame of the KSU power unit is grounded through the third, or "green," wire of its power cord. The local or circuit ground of the power unit, which is the positive side of the de output, should be connected to the same ground as the system protector. Do not connect circuit ground and frame ground together.
3.15 The 550/551C KSU has an external grounding terminal, labeled LOC GRD, on the front of the backboard above the interrupter. It is wired internally to the power unit ground circuit at the G± terminal of the connecting block. Connect this terminal to protector ground with a 14-gauge wire brought through one of the cable clamps below the connecting block. On 550/551A and B KSUs (MD), a multiple-post grounding terminal is located on the top of the connecting block.
BSP
