Abstract: A three-pole apparatus to be installed in a cubicle of an electric substation, comprising: i) an air-insulated three-pole disconnector performing functions of isolating an electric line leaving the cubicle, and ii) a three-pole circuit breaker performing line protection functions, characterized in that: a) the three-pole apparatus further comprises a casing which is made of insulating material and houses internally, at least partially, said three-pole circuit breaker, wherein said casing is not sealingly closed and contains air; b) the circuit breaker comprises three vacuum circuit-breaker modules and the disconnector comprises three single-pole disconnector devices, wherein each of the single-pole disconnector devices in turn comprises a contact movable linearly from a service position into a disconnection position; c) wherein each of the contacts, when it is in its service position, projects at least partially from the casing; and d) the three-pole apparatus is configured for cooperating with an earthing