In this paper, a new voltage sag approach is proposed, analytically modelled and validated by both simulation and field measurements. The main aim of this approach is to propose a new voltage sag approach, which appears during unintentional islanding operations (IOs) [1]. The unintentional IO occurs when an induction motor is removed from the main utility following a circuit breaker (CB) fault clearing, transiently, the induction motors (IM) are acting as generators maintaining the affected distribution feeder with voltage until it is reconnected. The modelled voltage sag in the current article follows an exponential form, in fact, here it will be demonstrated that the proposed model reaches satisfactorily the field measurements and evidences the dependability of the model adopted. Furthermore, it is worthwhile to note that this novel power quality (PQ) event has not been investigated yet and enhances the current voltage sag studies. Lastly, it is crucial to point out that all recorded events and the large amount of data needed so as to validate this transient, has been measured in a distribution network (DN) located in Spain.