Aiming at the difficulty of evaluating the process immunity time(PIT)

a method to calculate PIT based on Gantt chart has been presented. Firstly

according to the definition of PIT and the basic theory of Gantt chart

the relationship between them is analyzed. Secondly

a concept of immunity time constant(ITC)is defined to help to analyze Gantt chart. Then

the components under different types of voltage sag and their connection relationships are described in Gantt chart. With the critical path in the Gantt chart

the PIT can be calculated. Finally

an experimental test platform has been established. The escalator is taken as the researching object. With the comparison between the calculation and experimental test results

the effectiveness of the proposed method is verified

which can make full use of Gantt chart and provides an effective tool to evaluate the PIT for complicated industrial process.