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26 DC power system description
Single Point Ground (SPG) and Isolated Ground Bus (lGB) The
SPG
in
this grounding scheme
is
the AC Equipment Grounding bar in the AC
distribution panel
(ACEG).
If
this bar is insulated
from the
distribution panel,
as is recommended, then it is referred to as an IGB. In the following figures,
the preferred methods for using an ACEG and an IGB are illustrated.
Logic Return (LR) wiring and Logic Return Equalizer (LRE) location
When multiple columns are present or where links will be made to existing
SL-1 equipment, the LRs of the different coiurnns must be joined. Refer
to
System insfuflation procedures
(553-3001-210). When using a Distribution
Unit such as the
QBLlS,
QCA13 and most others, the existing Battery Return
(BR) bus will serve double duty as the LRE. The logic return wire must be
run in conduit to the LRE. A 6 AWG conductor then connects the BR (LRE)
to the ACEG in the AC panel.
This single point equalizing is required because communication between
modules that are not in the same physical line-up would put small amounts of
DC on the AC (green wire) ground. Residual AC voltages would be
‘-
superimposed (longitudinally) on the logic. There are a only a few volts of
common mode rejection capability on these lines. Refer to “System
grounding.”
120 V receptacles and cord or conduit items in the figures are used as
required.
-.-
Junction Box A junction box close to the pedestal may be required
depending on the wire size required between the pedestal and the distribution
point. See the chapter entitled “Power conductors.”
Power engineering
553-3001-152
s.
:
.
.