Emerson Static Transfer Switch User Manual

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).

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

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 K3

K2 K3

K2

K1

225A

225A

225A

225A

225A

225A

CB4

CB5

CB2

CB3

STS2

STS1

CB1

SOURCE 1

MICB2

MICB1

ISOLATION

TRANSFORMER

ISOLATION
TRANSFORMER