National type approval may have been granted separately to liquid fuel meter and system accessories, such as a self-service machine, parallel display or price and volume pre-setting mechanisms. The Declaration of Conformity in accordance with MID granted by the manufacturer covers the whole measuring system and its accessories.
If there are new distribution meters which meet the MID requirements installed in the distribution substation, national type approved accessories can be connected to a system which meets the MID requirements if all equipment works in a reliable manner and if sub equipment essentially meets the MID requirements. Respectively, an accessory which meets the MID requirements can be connected to a nationally approved distribution meter. The manufacturer of the equipment installed is responsible for the compatibility of the equipment in accordance with WELMEC instructions 8.8 "Guide on the General and Administrative Aspects of the Voluntary System of Modular Evaluation of Measuring instruments".