Abstract: A plug-in summation current transformer module for insertion into a module-overlapping installation space of a multi-pole rail-mounted device that is formed of a plurality of housing modules has a summation current transformer and primary conductors routed through the tubular summation current transformer. The primary conductors have primary conductor ends. A primary conductor holder for fixing the primary conductor ends is fastened to the summation current transformer and adopts a fixing position for holding the primary conductor ends in an assembling position for insertion into a module-overlapping installation space and a releasing position where the primary conductor ends move transversely into a releasing position allowing the primary conductor ends to join into their contacts in the modules.