a three - pole apparatus , configured to be installed in a cubicle of an electric substation , comprises an air - insulated three - pole disconnector and a three - pole circuit breaker . the air - insulated three - pole disconnector is configured to isolate an electric line leaving the cubicle . the three - pole circuit breaker is configured to perform line protection functions . the three - pole apparatus comprises a casing comprised of insulating material and internally housing , at least partially , the three - pole circuit breaker . the casing is not sealingly closed and contains air . the circuit breaker comprises three vacuum circuit - breaker modules ; the disconnector comprises three single - pole disconnector devices . each of the single - pole disconnector devices in turn comprises a contact movable linearly from a service position into a disconnection position . each of the contacts , when it is in its service position , projects at least partially from the casing . the three - pole apparatus is configured for cooperating with an earthing switch which is not enclosed into the casing .