Service devices are used in the field of automation and normally form interfaces between programmable logic controllers (PLC) and the installations or installation components to be controlled. Service devices such as these generally have a multiplicity of electrical interfaces for additional modules, with the additional modules being connected via control lines to the PLC, to further service devices or to installation components. An electrical additional module for a service device, a combination of the additional module with the service device, and an additional module set having various additional modules which allow easier fitting and removal of the additional modules in the service device are disclosed. In at least one embodiment, an electrical additional module is proposed, having an appliance interface for fitting the additional module to a service device, with the service device having a stepped holding geometry with at least two steps on each of which a contact device is arranged for holding an electrical opposing contact device, with the appliance interface having step sections which are designed to be complementary to the at least two steps and on which in each case one of the electrical opposing contact device is arranged in order to make contact with the associated contact device, and with the at least two step sections being rigidly connected to one another when the additional module is not fitted.