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.