Duty of Standby is a configuration scenario where one cooling system is referred to as the ‘Duty’ system and the other as the ‘Standby’. The choice between which device acts as the Duty is known as the service. In normal operation, the Duty air conditioner is running and the Standby is does not run. This type of configuration can suit server room environments using either single or three phase mains power supplies and where there is a need for redundancy (N+X) in the cooling operation. Server rooms with three phase mains power supplies can install either a three phase air conditioning system or single phase systems with one cooling unit per phase.