FIELD: electrical engineering; short-circuit relay protective gear for suspension and support insulators, circuit breaker and transformer bushings, etc. SUBSTANCE: junctions of respective bus system are disconnected in response to condition when total currents through all phases simultaneously exceeds resultant three-phase sequence of currents of respective bus system and total current of all resultant sequences of desired threshold levels. To this end only one current relay is used for each bus system that incorporates three-legged saturable current transformer and one selective threshold element. For all bus systems use is made of one starting threshold element. Three-legged saturable current transformers used for summing up three-phase sequence currents enable increasing currents through threshold elements without changing current transformer ratios, reducing number of relays and working points on saturation curve. EFFECT: reduced unbalance currents for protective gear operation. 6 cl, 2 dwg
