a testing and cut - off device for a telephone or computer line to connect two incoming strands to two outgoing strands , comprises a testing well inside which are located a first couple of terminals corresponding to the two incoming strands and a second couple of terminals corresponding to the two outgoing strands , and a pivotal cover for closing the well . these two couples of terminals are insulated from one another , when the cover is open . each terminal is accessible from the outside by a testing plug . the device further comprises electrical contacts in the cover for interconnecting the terminals when the cover is closed . said interconnection means in the pivotal cover also comprise means for galvanically linking the latter with an additional electrical component outside the well when the cover is closed . the additional component may be within or outside the cover . if outside it may be connected by wires joined to insulation displacement contacts attached to the cover .