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Currently, under-voltage trip devices are widely installed in circuit breakers of low-voltage distribution networks and end users' sides. These protection devices can prevent equipments from damage when voltage sag or power supply interruption occurs. Although relay protection devices can act correctly and clear the fault as soon as possible when a ground or inter phase fault occurs in the nearby high-voltage transmission network, instantaneous action of under-voltage trip devices may lead to widespread loss of load, fluctuation of frequency, even worse, cascading failure and collapse of the power system. The widespread load loss accidents arising from under-voltage trip devices happened in recent years are summarized in this paper. Two typical load loss accidents happened in Beijiao substation are introduced. The effects of under-voltage trip devices on power system are discussed base on EMS accident recall, PMU, fault recorder and other tools. Finally, suggestions on management of under-voltage trip devices and accident precautions are proposed.