The utility model relates to a secondary circuit 3U0 broken line alarm apparatus comprising a voltage transformer LHL and a voltage transformer LHN which is connected to an L phase, an S phase and an N3 phase of a main system and which is connected to an L phase and an N3 phase. A current relay LJL and a current relay LJN are respectively arranged between secondary sides of the voltage transformer LHL and the voltage transformer LHN to the L-phase and the N3 phase of a relay protection chamber cable. A load resistor 1FR and a load resistor 2FR are respectively connected between the L phase and the S phase and between the S phase and the N3 phase. Normally, a voltage between the L phase and the S phase is 100V, and a voltage between the S phase and the N3 phase is also 100V, and the current relays are in action states. Only when loss of phase of the L phase, or the S phase or the N3 phase occurs, returning will result, and alarming will be given through a control circuit. The advantages of the secondary circuit 3U0 broken line alarm apparatus of the utility model are that loss-of-phase alarming is realized; occurrence of misoperation is effectively prevented; and examination and repair are facilitated.