Technology of Electrical Automatic

Understanding the IEC 61557-12 Standard for Meter Comparisons

The International Electro-Technical Commission’s (IEC) Standard 61557-12, “Electrical safety in low voltage distribution systems up to 1000V a.c. and 1500V d.c. – Equipment for testing, measuring, or monitoring of protective measures – Part 12: Power Metering and monitoring Devices (PMD),” is a benefit for specifiers seeking to compare PMDs on a one-to-one basis. The standard defines a number of performance classes, based on the type of energy being measured.

Functional classification of IEC 61557-12

In Edition 2 of IEC 61557-12, three kinds of PMD were created based on their functionalities:

  • PMD-I is usable for energy efficiency or cost allocation purposes. Unless local regulation and provided they have a third-party certificate, these devices can also be used for bill checking or sub-billing purposes
  • PMD-II, usable for basic power monitoring
  • PMD-III, usable for advanced power monitoring or demand-side power quality
PMD type of IEC 61557-12
PMD type of IEC 61557-12

BlueJay’s products are mainly suitable for PMD-ll and PMD-lll

Uncertainty over a measuring range

IEC 61557-12 applies to PMDs with directly embedded sensors (PMD/DD) and those paired with external sensors (PMD/Sx). This is especially important in today’s market, in which many electrical devices – including protection relays, remote terminal units and many types of circuit breakers – feature embedded measurement functions. In addition to product standards relating to their primary task (like IEC 60497 for low-voltage switchgear and control gear), these devices also can reference their performance class under IEC 61557-12. This gives specifiers the information they need to ensure measurement accuracy lives up to expectations.


It’s important to note that the performance class for a PMD with an external sensor is calculated differently than that for a product with directly embedded sensors. This is to recognize the impact the external sensor’s accuracy will have on the combined system’s performance. As Figure 1, below, illustrates, the final performance class under IEC 61557-12 reflects the sensor’s accuracy class (as defined under IEC 61869-2) combined with the IEC 61557-12 performance class of the PMD, itself.


IEC 61557-12 also provides a way to calculate the overall system uncertainty of PMD systems that incorporate external sensors. The standard’s Table 2 provides useful information related to accuracy between meters embedding sensors and meters requiring external sensors.

Performance class of the PMD

So, as an example, it can be said that a class 1 “directly connected PMD” is expected to be at least equivalent to a class 0,5 “sensor-operated PMD” installed with a class 0,5 sensor.

The power meters produced by Chongqing Blue Jay Technology Co., Ltd. have the technical advantages of high-precision measurement. For example, the 194 series manufactured by Blue Jay, including 194E,194Z, 194L,194DR, and 194Y, all conform to the definition of IEC 61557-12, and the PMD measurement accuracy is 0.5s, while 194J and 194Q conform the definition of IEC 61557-12, the PMD measurement accuracy is 0.2s, which has higher accuracy advantages and can measure more advanced parameters such as maximum demand, harmonic analysis, etc., 194 series power meters can transmit remote data through RS485, which is convenient for customers to remote manage.