EN 61557-12 specifies the requirements for combined performance measuring and monitoring devices used in electrical distribution systems. These devices, which can be fixed or portable, measure and monitor electrical parameters in single and three-phase a.c. or d.c. systems with rated voltages up to 1,000 V a.c. or 1,500 V d.c. They are designed for both indoor and outdoor use. The standard excludes electricity metering equipment covered by IEC 62053-21, IEC 62053-22, and IEC 62053-23, as well as simple remote or monitoring relays. It is intended to complement IEC 61557-1, which outlines general requirements for measuring and monitoring equipment, and aids in verifying installation safety and performance. Combined performance measuring and monitoring devices (PMD) enhance distribution system performance by providing additional safety information, such as compliance with harmonic levels as specified in IEC 60364-5-52. PMD-A devices, meeting IEC 61000-4-30 class A, are used for power quality assessment.