Liebert 100-1000A User Manual

The Liebert STS2 is designed for high reliability and high availability with a critical bus Mean Time 

Between Failure (MTBF) exceeding 1,000,000 hours. 
The Liebert STS2, Control/Power Module and all its options are designed, tested and certified, where 

applicable, to meet agency requirements as follows:

• UL 1008: Transfer Switch Equipment, for UL and ULc listing

• UL1459: Telephone equipment (Modem)
• NEC
• FCC Part 15 EMI emission limits for Class A computing devices.

The Liebert STS2 uses conservatively rated components, minimizing the need to go to maintenance 

bypass. All maintenance and repair work can be done through the front panel of the unit. This access 

makes repairs and maintenance easier.
Gating and control logic are partitioned so that failure of one source’s gating or sensing logic does not 

prevent the switch from transferring to the other source.
All control and logic components are mounted separate from the power components. The Liebert STS2 

is designed to minimize the exposure of hazardous voltages to allow safe servicing of the unit while 

the load is energized.
All electrical components requiring normal maintenance are replaceable without de-energizing the 

load, as long as one source is available. Solid-state switching devices are packaged to allow safe repair 

of those devices without having to de-energize the load. All non-automatic switches or circuit breakers 

are the plug-in or draw-out type to allow replacement without de-energizing the load. All solid-state 

power switching devices are rated to prevent hazardous device failure in power systems with 

available fault currents up to the defined limits, as listed in 15.1.5 - Electrical Requirements. 

Because improper installation can cause a system to fail, a Liebert Services service technician should 

thoroughly inspect the unit to ensure it is properly installed and its operating parameters are 

properly configured.
Once the Liebert STS2 is properly installed, you, as the on-site operator, can easily monitor the unit’s 

operation utilizing the touch screen or LEDs.
If you need assistance, or have questions, contact Liebert Services for technical support at 

1-800-LIEBERT (800-543-2378).

Under normal operating conditions, the Liebert STS2 routes power from the preferred input source to 

the output load. The unit monitors the voltage current and phase of each source to ensure that both 

are operating within set tolerances, and that the alternate source is available, should a transfer be 

required

Should the preferred source fail or be outside acceptable voltage limits, the Liebert STS2 transfers 

the input to the alternate source until the problem is rectified in the preferred source. The unit can be 

set to automatically transfer the input back to the preferred source, or only allow the transfer to be 

done manually.

For a transfer to be completed between input sources, the sources must meet certain parameters, or 

the transfer cannot take place. Uninterrupted transfer between sources is inhibited due to input 

source failure, sources out of sync, switch failure, or the unit is in bypass mode.