The IEC has found it most practical and convenient to distinguish four product categories for EMC standardization purposes:
Again in the EMC context, these are combinations of components, apparatus and systems assembled or erected in a given area, such as industrial plants, electricity substations or telecontrol systems for large areas.
If a set of units is dispersed over a wide area, this clearly may pose problems for emission measurement and immunity testing, and in practice it may not even be possible to test an installation as a whole unit.
If emission limits are set for the whole installation, as they sometimes are, measurements of harmonics or radio-frequency disturbances, for example, may have to be carried out in situ. In other cases, in particular for immunity, EMC requirements must be specified for each item and the suppliers of these items must then indicate the installation conditions for their products (such as earthing or wiring) that ensure the whole installation can function properly.