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Power and Control Wiring
12
5.3 System Grounding
Equipment grounding — Grounding is primarily for equipment and personnel safety, although
proper grounding also enhances equipment performance.
All input and output power feeds must include an equipment grounding means as required by the
NEC and local codes.
An insulated equipment ground conductor is recommended to run with each input and output power
feed. The equipment ground conductors should be at least the minimum size conductor per the NEC
based on the upstream overcurrent protection device.
4-Wire-Plus-Ground Systems — When 4-wire-plus-ground input feeds are utilized, the input power
sources must be properly grounded. Because the neutral is not switched by the Liebert STS2, the neu-
trals of the two power sources are solidly interconnected. The NEC prohibits grounding a power
source at more than one point. Connecting the neutrals of two grounded power sources together effec-
tively grounds each of the sources at more than one point, which allows neutral current to flow on the
ground system, defeats ground fault protection, creates a safety hazard, and violates the NEC.
Where possible, the two power sources should be located in close proximity and a single
neutral-to-ground bond made (as shown in Figure 4 on page 12 or as is typical with a double-
ended substation).
Figure 4 Typical one-line diagram of two PDUs and a static transfer switch
5.4 Control Wiring Connections
No control wiring is needed on the standard Liebert STS2. Only certain options require external con-
trol wiring. See 6.0 - Options on page 14 for details
!
WARNING
If conduit is used as a grounding means, adequate electrical continuity must be maintained at
all conduit connections. The use of isolating bushings with a metal conduit can be a safety
hazard and is not recommended.
SOURCE 2
K1 K3K2 K3
K2K1
225A 225A 225A
225A225A 225A
CB4 CB5
CB2
CB3
STS2
STS1
CB1
SOURCE 1
MICB2MICB1
ISOLATION
TRANSFORMER
ISOLATION
TRANSFORMER