Synchronous machines with brushless excitation have the disadvantage that the field winding is no accessible for the deexcitation. This means that, despite the proper operation of the protection system, the slow de-excitation time constant may produce severe damage in the event of an internal or external short circuit. In this paper the test in a 15 MVA of a novel high-speed deexcitation system for brushless synchronous machines is presented. The de-excitation is achieved by inserting a resistance in the field circuit, obtaining a dynamic response similar to that archived in machines with static excitation. This system has been validated through experimental tests on a 15 MVA industrial type generator, with satisfactory results.