Emerson Static Transfer Switch User Manual

All power and control wiring should be installed by a qualified electrician. All power and control wir-

ing must comply with the NEC and applicable local codes. Unless otherwise labeled, use the recom-

mended tightening torque as shown in Table 24 - Torque tightening.
The busbars are accessible through the front of the Liebert STS2 and designed to allow one-handed 

tightening.
Cables can be installed through the top or bottom of the unit through removable conduits plates. 
See Figures 5 through 10 for wiring entrance locations.
Input wiring also can be installed under a raised floor to optional input junction boxes and cables, 

when used.

If the unit is furnished with input junction boxes, the input power connections are made to the input 

power terminals located in the junction boxes. Input power cables, furnished with the input junction 

boxes, are shipped unattached to the unit for connection between the junction boxes and the bus bars 

provided inside the unit. Input junction box installation is discussed in 5.2 - Input Junction Box 

Installation—Optional on page 10.
If input junction boxes are not furnished, the input power connections are made to the bus bars pro-

vided inside the unit. These bus bars are accessible through the front of the unit.
Output power connections also are made to the bus bars provided inside the unit. These bus bars are 

accessible through the front of the unit. See Figures 14 to 19 for details on the bus bars. 

The two input power feeds (sources) to the Liebert STS2 should be from two independent sources to 

avoid a common source failure.
To ensure proper operation of the Liebert STS2, the two input sources must be the same nominal volt-

age level and phase rotation. 
For uninterrupted automatic transfer, the two input sources should be synchronized within 15 degrees.

The Liebert STS2 is designed for operation with 3- or 4-wire, solidly grounded sources only. 
For 4-wire operation, the common source neutral must be connected to the Liebert STS2. See 

Figure 3 on page 10 for a typical one-line diagram. Refer to Figures 14 to 19 for electrical field con-

nections on all units, with both single and dual output breakers.

Risk of electric shock
Verify that all input power and control circuits are de-energized and locked out before making 

connections inside unit.

Risk of unit damage.
The input sources to the static switch must be grounded-wye sources. Input sources other 

than solidly grounded-wye sources may cause damage to the switch.