Upon an intentional or emergency disconnection from the main grid, a microgrid is expected to continue working in islanded mode. Thus, (at least) one resource needs to act as slack and compensate for power variations to keep the power balance, and ensure the security of supply. Although several resources might be eligible to become slack, some are more suitable than others (energy storage systems in particular) depending on the state of both the resources and the grid before the islanding transition. In this paper, we validate a recently proposed method to select in real-time the best slack-candidate using an abstract representation of the internal state of the available resources. The same method can be used to actively switch the slack during islanded operation to accommodate the intrinsic stochastic nature of the microgrid’s resources. Our main contribution is the validation of the method in a real-scale microgrid, including a discussion of implementation and deployment aspects. To support our findings, we present extensive experimental results in different operating conditions.