The ISTTOK tokamak operates routinely in AC mode, producing alternate current plasma discharges for up to 4 kA. In this way, the discharge length is increased from 25 ms (single discharge) up to 1.2 seconds. The longer discharge time is used to test liquid metals properties under these plasma loads. The operation of AC discharge demands control of external parameters such as vertical and horizontal fields and gas puff. Generation of fast electrons and runaways has been observed during the current rump-up after AC transition leading to the inefficiency of the formation of a plasma discharge which in turn interrupts the continues chain of AC plasma pulses. In addition, AC discharges for currents above 4 kA are very difficult to maintain full AC operation. Therefore, a series of experimental studies are being done in order to develop a better understanding of the AC transition in view of improving AC operation reliability at larger currents. In this line, the collaboration with Canberra plasma physics group aims at the development of a first order model with enough predicting capability such that the AC transition reliability can be improved via the control of the relevant external parameters. . The presentation will address the methods used to obtain from experimental data quantities that can be used to calculate equilibrium properties during the current transition. These data will be used as input for numerical equilibrium calculations at different time stamps in order to simulate the evolution of AC transition and indicate the type of external control required.