1. The invention relates to a modular, expandable measuring device comprising an access-protected area.2.1 Systems having a variety of designs are known which offer protection against unauthorized manipulation by end users/customers, in particular comprising a sealable housing cover or an electronic security seal. The user is always the measuring facility operator or measurement service provider, who procures the resource counters, installs them at the customer's site, services and reads them; the same applies to communication installations. Lacking in practice, however, is a measuring device/meter which can be used universally, independently of other technical conditions, and which can be expanded/adapted in a simple manner and has at least one access-protected area/space in the housing.2.2. According to the invention, the measuring device, in particular modular energy meter (EZ), comprises: 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.2.3 The invention pertains to the field of measuring devices and systems for the combined recording of consumption data.