Liebert POWER PROTECTION STATIC TRANSFER SWITCH User Manual

This section discusses the options available for the STS2. The communications options are also dis-

cussed in 12.0 - Communication Interfaces.

The Programmable Relay Board (PRB) provides a means to trigger an external device when an event 

occurs in the STS2. Each PRB has 8 channels. Each channel has two sets of Form-C dry contacts, 

rated 1 Amp @ 30 VDC or 250 mAmp @ 125 VAC. 
Any alarm/event can be programmed to any channel or channels. Up to ten (10) events can be pro-

grammed to a relay. If multiple events are grouped to one relay, group the events logically to simplify 

troubleshooting when an event is triggered. The same alarm/event can be programmed to more than 

one channel. Up to two Programmable Relay Boards can be installed in the STS2 for a total of 16 

channels. Programming is performed through the touch screen display.
See Configuring the Programmable Relay Board Settings on page 97 for default settings and 

instructions for reconfiguring the relays. See Figures 21 and  22 for the location of the PRB. See 

Figure 24 for wiring details. Table 8 provides the PRB pinout.

When the STS2 is used as a primary-side switch—on the primary of an isolation transformer—opti-

mized transfer greatly reduces the transformer magnetization current during automatic transfers 

through a new transfer control algorithm. The Liebert algorithm (patent pending) optimizes the 

transfer timing so that the volt-seconds applied to the downstream transformer primary is balanced, 

minimizing peak saturation current.
The volt-second balance is achieved by directly computing the volt-second applied to the transformer 

during transfer events and determining the optimum time to turn on the alternate source SCRs in 

order to balance the volt-second within specified tolerance.
Optimized transfer also seeks to minimize the voltage disturbances while still maintaining trans-

former flux balance. This unique flux balance algorithm does not just wait for the balance point to 

occur, but will pulse fire the SCRs as soon as possible to minimize the voltage disruption. This results 

in far superior voltage waveforms applied to the load.

CH1

A

1-3

1

2

3

B

4-6

4

5

6

CH2

A

7-9

7

8

9

B

10-12

10

11

12

CH3

A

13-15

13

14

15

B

1-3

1

2

3

CH4

A

4-6

4

5

6

B

7-9

7

8

9

CH5

A

10-12

10

11

12

B

13-15

13

14

15

CH6

A

1-3

1

2

3

B

4-6

4

5

6

CH7

A

7-9

7

8

9

B

10-12

10

11

12

CH8

A

13-15

13

14

15

B

1-3

1

2

3

Key: N.O. = Normally Open; N/C. = Normally Closed; C = Common
Note: Pin 16 not used on TB1, TB2, and TB3.