Understanding the IEC 61557-12 Standard That Makes Meter Comparisons Easier – Part 2

As I described in my previous post, the International Electrotechnical 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 one-to-one basis. The standard defines a number of performance classes, based on the type of energy being measured. My last post provided a general overview of the parameters on which these classes are based. In this post, I’ll be covering one of those parameters, guaranteed accuracy, in greater depth and helping to decode device markings.

Uncertainty over a measuring range

As I mentioned in my previous post, 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 controlgear), 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.

Figure 1. Uncertainties for “sensor operated PMD/Sx” (working with external sensors) and for “directly connected PMD/DD” (working with embedded sensors)

IEC 61557-12 also provides a way to calculate the overall system uncertainty of PMD systems that incorporate external sensors. The standard’s Table 4 provides useful information related to accuracy between meters embedding sensors and meters requiring external sensors.

Table 4

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 related measuring ranges by PMD performance class are described in the standard’s Table 5 and described in its Figure 8, which follows.

Table 5

Figure 8

Understanding product markings

The standard also defines a standard marking scheme for easier comparison and specification when evaluating multiple manufacturers’ products. This code describes installation options, operating-temperature ranges and accuracy class. You can see this code applied to Schneider Electric’s PowerLogic PM8000 Power Meter, below.

If you are looking for more details, you can check out the following documents:

You can find information for ordering a copy of IEC 61557-12 on the IEC website. For more information about standards and access to additional tools, resources and product information you can register for our dedicated Consulting Engineer portal site.

How helpful are you finding IEC 61557-12 to be in your work identifying and specifying the right PMDs for your projects? Add your experiences in the comment section, below.

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