Disclosed is a modular, expandable measuring device that includes an access-protected area. In one embodiment, the measuring device is a modular energy meter (EZ), which includes a two-part housing (SA, U) having a detachable connection for holding together the housing parts; at least one internal module (Z, K, IO, IR) arranged in a protected area; at least one external module (EO, ER, EK, EKR) arranged outside of the protected area; and at least one optical interface (OSE, OSI) which is arranged in the internal module (Z, K, IO, IR) and/or in the external module (EO, ER, EK, EKR) and is used for communication, including the detection of the opening of the protected area.