Emerson AC4 User Manual

The Liebert AC4 is ideal for coordinated control of systems with redundant equipment, such as 

multiple environmental units or pumps. When the Liebert AC4 controller detects an alarm in an 

operating device, the Liebert AC4 enables a standby device and controls the device in alarm as 

configured, either leaving that device operating or disabling it. 
The Liebert AC4 controller can also balance usage of devices by rotating units through Operating and 

Standby modes according to a user-defined schedule. This helps keep redundant equipment operating 

efficiently and only when needed. The Liebert AC4 can perform routine testing of standby devices and 

alert personnel if a standby device fails an operating test and requires attention.
Another capability of the Liebert AC4 is monitoring the status of connected devices and keeping 

personnel apprised through local alarming. 
The Liebert AC4 controller can interface with anything that closes an electrical contact. To improve 

process efficiency and troubleshooting, the controller tracks data in two types of logs: alarm and 

event. The Liebert AC4 has a local LCD interface and an RS232 interface.
When an alarm condition arises, the Liebert AC4 displays alarm information, sounds an audible 

alarm, changes the state of the common alarm relay and turns connected devices on or off according to 

user configuration. See 1.5 Typical Configuration for a more detailed example.

The Liebert AC4 features access through two interfaces that allow users to configure the unit, silence 

alarms, back up logs, and perform many other functions to keep a large or small operation running 

smoothly and safely.

• LCD—liquid crystal display on the front of the panel
• Service Terminal Interface—accessible via a computer connected directly to an RS232 port

These interfaces permit easy access and configuration of the Liebert AC4, allowing users to view data, 

silence alarms and have full access to information stored in the controller, including logs of alarms 

and events.

The two types of data logs—alarm history and event history—can be viewed on the LCD on the front 

of the Liebert AC4 or the Service Terminal Interface. The logs may also be downloaded to a computer 

through the Service Terminal Interface.

The alarms available with the Liebert AC4 are:

• Discrete input alarm (for each digital input)
• Loss of power
• No standby available
• Standby device failed
• No restart available