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AC power system description 13
General notes for AC figures
Permanent connection to Branch Circuit
When conduit or a raceway is
used it should be metal and regardless of the type, it must contain an insulated
ground wire (green), 6 AWG or larger.
Cord connection to Branch Circuit
A separate safety ground is always
required when removable line cords are used for the following reasons:
-
a telephone wire could contact AC elsewhere in the building while the cord
is unplugged (during installation)
-
to minimize hazards from lightning transients when unplugged
-
to minimize the effects of stray grounds (pedestal to floor RS-232 links to
monitors) during normal operation
The additional safety ground must be 6 AWG or larger, preferably insulated
and must be connected from the pedestal Frame Ground to the Service Panel
Ground Bus
. .
One 30 Amp circuit per pedestal is required. Isolation as required by NEC
250-74 and 384-27 (Exception 1) is preferred.
Single Point Ground (SPG) and Isolated Ground Bus (IGB) The SPG in
this grounding scheme is the Equipment Grounding Terminal Bar in the AC
distribution panel
(ACEG).
If this bar is isolated from the distribution panel,
as is recommended, then it is referred to as an IGB. In the following figures,
the preferred method using an ACEG is 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 columns must be joined. See
System
installation procedures
(553-3001-210). An LRE (a bar or plate) in the
immediate vicinity allows the LRs to be joined to a single equalizing point. A
6 AWG conductor then connects the LRE to the IGB in the AC panel.
For these multiple column applications, the LRE is typically located in a
nearby rack, overhead trough or under a raised floor, It must be insulated
from its support structure.
Power engineering 553-3001-l 52