Gary - The below may be a little more than you bargained for, but here goes. My apologies, if this is more basic than your question.
Each of the drawings in Figures NAB.1 and NAB.2 show grounding electrodes at the bottom. In Figure NAB.1, the electrode is used to earth the centralized dc power system at the batteries, similar to what one would find in an ac power system. In this scenario, the -48V return must be isolated from chassis, same as the neutral in a ac system. In NAB.2, there is a special power system earthing allowed at the equipment, sometimes referred to as a "grounding window." In this scenario, the -48V return is allowed to be connected to the chassis of utilization equipment located within the defined area of the grounding window (usually an area in a single room). Equipment so connected and located within the grounding window must have provision for connecting to a grounding electrode, collocated in the defined area, plus have the markings and other construction features identified in Annex NAB.3. In neither case is the power system floating. In Figure NAB.1, equipment grounding conductors are pointed to in the right-hand portion of the drawing. In Figure NAB.2, the equipment grounding conductors' label is nestled among the primary and secondary power boards. For US installation code reference, see NEC Section 250-22(b). In a cursory review, I found no CEC exception to Rule 10-202, though I believe there may be one, since Annex NAB is supposed to be harmonized between the US and Canada. Perhaps someone more familiar with the CEC can comment. Regards, Peter L. Tarver, PE [email protected] -----Original Message----- From: Gary McInturff [mailto:[email protected]] Sent: Friday, October 13, 2000 8:47 PM Confused and missing something pretty basic here, I think. As I look at the diagrams in annex NAB for UL 1950 or UL60950 I am pretty confused. What it appears to show me is a floating system -48 and -48 reference lines into the power supply system. Do I really have to float my circuit boards etc, (ungrounded SELV?), and use the -48 reference line as the return path rather than chassis and its bonding wire (grounded SELV). Logic and chassis are not tied togeather until after a considerable distance down the line? Am I stairing into the obvious and missing the point? Heavy sigh Gary
