Generally electrical equipment that used in auxiliary control systems shown in bottom of pages. Therefore equipment (relay etc.) that usually installed in field area is directly connected to one of two polarity continuously, and other polarity connected to the other terminal of equipment via auxiliary contact of controller devices that shown on top of pages transitionally. Continuous connection must be negative, to prevent corrosion of equipment due to electrolytic action, because they usually installed in outdoor and corrosive area. Of course this recommendation related to battery system grounding way.For more information you can refer to Grounding Riddle No.20 .