Abstract: The power plant generator and transformer’s relays are composed of both non-electrical and electrical protections.Engineers pay less attention to non-electrical protection on daily work.Many non-electrical protections are set incorrectly according to the specifications. In this paper,by analyzing an actual accident,compiling the regulations of non-electrical protection,analysis of non-electrical protection secondary circuit,we hope it will be helpful to non-electrical protection management work.

Key words: non-electrical protection specification, non-electrical protection accident, non-electrical protection analysis