In many cases it is advantageous that equipment is modular so that the equipment comprises a body device and plug-in units installed in the body device. The equipment can be, for example but not necessarily, telecommunication equipment such as an internet protocol “IP” router, an Ethernet switch, an Asynchronous Transfer Mode “ATM” switch, and/or a MultiProtocol Label Switching “MPLS” switch. A commonly used construction is such that the telecommunication equipment comprises a frame and plug-in units which are installed in plug-in unit slots of the frame. In this case, the frame represents a body device for receiving the plug-in units. Electrical connectors in a plug-in unit make galvanic contacts with corresponding electrical connectors in the frame when the plug-in unit is inserted in the plug-in unit slot of the frame. The frame may have wirings such that plug-in units installed in the frame form a full mesh network or such that plug-in units installed in the frame are connected to a central element which may comprise one or more plug-in units installed in the frame or which may be a functional component integrally built in the frame.
Care is needed in the installation of a plug-in unit in a body device because galvanic contacts between the electrical connectors of the plug-in unit and corresponding electrical connectors of the body device may be poor, or even non-existent, if the plug-in unit is not properly installed in its place. In one prior-art solution, pins of some poles in the electrical connectors between the plug-in unit and the body device are shorter than pins of other poles in these electrical connectors. These shorter pins are part of a test circuit for detecting whether the plug-in unit is installed in such a way that the shorter pins make galvanic contacts in the electrical connectors. If the shorter pins make the galvanic contacts, there is a good certainty that also the longer pins in the electrical connectors make adequate galvanic contacts. The above-described prior-art solution for indicating correct installation of a plug-in unit is suitable in cases where some poles of an electrical connector can be made such that galvanic contacts between these poles constitutes a sufficient guarantee of correct galvanic contacts between other poles of the electrical connector. In conjunction with some electrical connector structures this may be, however, challenging. Therefore, there is a need for other technical solutions for indicating correct installation of a plug-in unit.