Negative ion beam source test equipment, currently under construction at Institute of Plasma Physics, Chinese Academy of Sciences, is a prototype of a radio-frequency (RF) ion source for neutral beam injection. An extraction grid power supply (EGPS) for the RF ion source is utilized to extract negative ions. To acquire the output of the EGPS and assure its safe operation, an interlock protection system has been designed. The system meets the requirements of anti-interference signal transmission and management of fast protection in order to minimize adverse consequences. The system takes care of breakdowns occurring during normal operation. All the required functions are implemented by hard-wired circuits, so the response time of fault protection can be less than . This technical note describes the rationale of the isolated transmission and protection functions and presents the implementation of functions in the detailed design. System performance during the experimental tests is described, with emphasis on demonstration of the protection.