Abstract: The difficulty in assessing economic loss of industrial processes caused by voltage sags is to quantify the degree of loss of different loss events. The concept of voltage sags loss event was introduced, which divided the voltage sags loss into process interruption loss and customer perceived loss,to quantify industrial process interruption and non-interruption loss respectively. Taking advantage of both equipment’s voltage tolerance curve（VTC） and parameter immune time（PIT）, a voltage sag economic loss assessment model based on the electrical characteristics, physical attribute and perceived loss of equipment was proposed. As a case study, one heating ventilation and air conditioning（HVAC） system in a factory with high cleanliness requirements has been used to validate the correctness and applicability of the proposed method.