Liebert POWER PROTECTION 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 STS2, the neutrals 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 effectively 

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 11 or as is typical with a double-

ended substation).

No control wiring is needed on the standard STS2. Only certain options require external control wir-

ing. See 6.0 - Options on page 13 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.