The introduction of Distributed Energy Resources (DER) onto distribution networks presents a number of constraints on the management of the protection scheme, among which the well-known “blinding phenomenon” of protection relays. Indeed, DER can reduce the short-circuit current measured by feeder protection relays which blind the relays. For conventional synchronous and asynchronous generators, their short-circuit contribution can easily be calculated and the blinding evaluated. On the other hand, with the Inverter-based resources, the short-circuit contribution depends on the regulation laws of the generators that is usually not known precisely. In this paper, the maximum blinding of overcurrent protection relays caused by Inverter-based Distributed Energy Resources is addressed by formulating a nonconvex optimization problem. This optimization problem is reformulated as a Semi Definite Program to allow a resolution with convex optimization tools. The program is implemented and compared to another similar method based on random draws, on a real MV/LV microgrid.