it nothing but an normal 3 phase rectifier circuit , in this i have choose the value of relay such that when it is conducting taking reference as b phase as b phase can be off using s/w in btwn there will be no available voltage for relay to operat and hence it will trip the relay giving a signal the only thing is that this circuit should be provided with a timer circuit which can easly be made using 555 timer ic hence in the absence of any phase it will give the indication now u can add and circuit after this circuit to monitor the single phasing ( aabsence of any phase in three phase system)
