Digital Power Metering and monitoring Devices (PMDs) are fast replacing analog equipment, providing more accurate measurements and enabling remote data access. But specifying these devices could become problematic without common reference information describing such factors as, say, appropriate operating temperatures and the exact power parameters the device is capable of tracking. The International Electrotechnical Commission (IEC) has addressed this concern with a standard that establishes common set of reference requirements for electrical measurement.
Understanding what this standard covers and how to read related manufacturer information can make it easier for engineers and other electrical professionals to compare and select the right PMD for any given application. Its full name is a mouthful: IEC 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).” However, its impact is equally broad – enough to fill two separate posts. In this post, I’ll provide an overview of IEC 61557-12’s scope and requirements. Next, I’ll provide information on related PMD-testing requirements described by the standard.
IEC 61557-12 was developed to help specifiers select the right device for any electricity cost-management application. It also helps promote state-of-the-art electrical management on the demand side of the electrical network. To that end, this standard covers energy measurements, as well as measurement of many other critical electrical characteristics. In this way, IEC 61557-12 differs from standards for electricity metering equipment, which focus only on energy measurements.
In its section on PMD functions, IEC 61557-12 lists all possible electrical characteristics the devices might measure, along with related requirements – such as rated ranges of operation or allowable measurement techniques. The listed characteristics include:
- Active energy (with performance classes equivalent to the classes defined in IEC 62053-21 and IEC 62053-22)
- Reactive energy (with performance classes equivalent to the classes defined in IEC 62053-23)
- Apparent energy
- Active, reactive and apparent power
- Root-mean squared (RMS) phase and neutral current
- RMS voltage
- Power factor
- Voltage dip and swell
- Voltage interruption
- Voltage unbalance
- Harmonic voltage and distortion
- Harmonic current and distortion
- Maximum, minimum, peak, average and demand values
As a big bonus for specifiers, IEC-61557-12 establishes three performance classes for registered devices. The classes define how well a PMD operates across four specific parameters for every type of electrical measurement it is marketed to provide, as shown in Figure 1.
- Guaranteed accuracy refers to the limits of uncertainty in the results a PMD provides, over a specified measuring range and under reference conditions. I’ll be covering the topic of uncertainty limits in my next post.
- Defined measuring range specifies the minimum and maximum values of quantities between which limits of measurement uncertainty are defined. For current, measuring range is specified by manufacturers through:
- nominal current (In) and maximum current (Imax) for sensor operated PMDs (called PMD/Sx)
- base current (Ib) and maximum current (Imax) for directly connected PMDs (called PMD/DD)
- Influence quantities refers to environmental conditions, such as temperature and other climatic impacts and electromagnetic perturbations that might be encountered in switchboards or electrical cabinets. The standard specifies maximum permitted variations of accuracy due to those influence quantities.
- Zero-blind (Gapless) measurement – in other words, continuous (rather than intermittent) monitoring – is required for several capability parameters under the standard, particularly for energy measurements.
Not just for standalone devices
Importantly, IEC 61557-12 applies to PMDs embedded within other equipment as well as standalone devices, which is critical today, when PMD measurement capabilities are increasingly present in protection relays, feeder remote terminal units and a number of circuit breaker offerings. So, when selecting these products, specifiers can refer to the standard to define the required performance class for the embedded measurement function.
Testing criteria in the next post
In my next post on IEC 61557-12 I’ll be looking at the testing criteria that help define a PMD’s uncertainty limits – essentially, the limits of its accuracy. As you’ll read, these criteria look at devices’ performance both in isolation and as part of an overall system.
If you are looking for more details, you can check out the following documents:
|Power meters selection guide||White paper||Power Meter Selection Guide for Large Buildings|
|IEC 61557-12||White paper||Guide to using IEC 61557-12 standard to simplify the setup of an energy measurement plan.|
|Measurement applications||White paper||Guide to energy measurement applications and standards|
Also, for more information about standards and access to additional tools, resources and product information you can register for our dedicated Consulting Engineer portal site.
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