Focus on IEC 61557-12 standard

Digital equipment is replacing analog equipment in electrical installations. It supports more accurate measurement of new values and is able to make these available to users at both local and remote locations.

Devices intended to perform monitoring have various characteristics which require a shared reference system. This system must allow users to make easier choices in terms of performance levels, dependability and to interpret different measured parameters.

All these various measuring devices (referred to as "PMD" for "Power Metering and Monitoring Device") have to meet the requirements of international standard IEC 61557‑12: "Electrical safety in low voltage distribution systems up to 1000V a.c. and 1500 V d.c. – Equipment for testing, measuring or monitoring of protective measures – Part 12: Power Metering and monitoring devices (PMD)".

Nowadays, increasingly, devices such as circuit breakers, protection relays, PLCs, are embedding measurement, because there is a need to monitor energy usage as close as possible to the loads and at all levels of the power distribution system.

The measurement data provided by these new devices need to be consistent (measurement range, measurement method, accuracy, ...) with the data provided by dedicated PMD devices, to be able to aggregate them all together.

This is why a new concept of EPMF ("Equipment embedding Power Metering and Monitoring Functions") was added, in IEC 61557-12 amendment 1, published in 2021. It is applicable to devices complying both with their own product standard AND complying additionally with the measuring functions of IEC 61557-12.

Selecting PMDs and EPMFs that comply with IEC 61557-12 ensures that measured data can be compared and can then be aggregated in a coherent way.

PMD - Power Metering and Monitoring Devices compliant to IEC 61557-12 standard

PMD functions

All the possible electrical parameters to be measured are listed. For each parameter, a list of requirements is specified, such as the rated range of operation, the range of influence quantities, the measurement techniques, etc.

The considered electrical parameters are given here:

• Active energy (classes are equivalent to the classes defined in IEC 62053-21 and IEC 62053-22),
• Reactive energy (classes are equivalent to the classes defined in IEC 62053-23)
• Apparent energy,
• Active, reactive and apparent power,
• Frequency,
• r.m.s. phase and neutral current,
• r.m.s. voltage,
• Power factor,
• Voltage dip and swell,
• Voltage interruption,
• Voltage unbalance,
• Harmonic voltage and distortion,
• Harmonic current and distortion,
• Maximum, minimum, peak, average, demand and values.

Classification of Power Monitoring Devices (PMD)

Table 1 of IEC 61557-12 is defining 3 categories of PMD, depending on the application need:

• PMD-I can be used in several applications, including basic Energy Efficiency applications
• PMD-II can be used in basic power monitoring applications and advanced Energy Efficiency applications
• PMD-III can be used in advanced power monitoring applications and for network performance

Fig. S11 – Functional classification of PMD with minimal required functions (IEC 61557-12)

Functionalities symbol	PMD type
	PMD-I Energy efficiency	PMD-II Basic power monitoring	PMD-III Advanced power monitoring /network performance
P		☑	☑
Q		☑	☑
S		☑	☑
Ea	☑	☑	☑
Er		☑	☑
Eap			☑
f		☑	☑
I		☑	☑
IN			☑
U and/or V		☑	☑
PF		☑	☑
THDu and/or THDv and/or THD-Ru and/or THD-Rv			☑
THDi and/or THD-Ri			☑

Marking

According to this standard, devices have a code denoting their installation options, operating temperature range and accuracy class. As a result, it has become significantly easier to select and identify these devices (see Fig. S12).

PowerLogic PM8000 series power meter complying with IEC 61557-12 as:
PMD/SD/K70/0,2 and PMD/SS/K70/0,2

Fig. S12 – Identifying measuring devices in accordance with IEC 61557-12

Uncertainty over a measuring range

The notion of performance classes (e.g. class 1 for active energy measurement) specified by IEC 61557-12 is much more than a requirement related to uncertainty at nominal current.

• intrinsic uncertainty: compliance covers performance under two sets of reference conditions
• operating uncertainty: compliance covers performance under 12 environmental and electromagnetic influence quantities which typically affect PMD operation

Intrinsic uncertainty

Intrinsic uncertainty is the uncertainty of a measuring instrument when used under reference conditions (e.g. at 23°C) for different Power Factor values. In this standard, it is a percentage of the measured value (readings).

Figure S13 specifies intrinsic uncertainty limits for class 1 and class 0,2 active energy measurement at Power Factor = 1, according to Table 8 of IEC 61557-12.

Fig. S13 – Uncertainty limits for active energy at Power Factor = 1

Figure S14 specifies intrinsic uncertainty limits for class 1 and class 0.2 active energy measurement at Power Factor = 0.5 inductive and 0.8 capacitive, according to Table 8 of IEC 61557-12.

IEC 61557-12 also specifies requirements about no-load conditions and starting current.

Fig. S14 – Uncertainty limits for active energy at Power Factor = 0.5 inductive and 0.8 capacitive

Operating uncertainty (based on variations due to influence quantities)

Operating uncertainty is the uncertainty under the rated operating conditions (including drifts related to temperature, frequency, EMC, ….)

IEC61557-12 specifies tests and uncertainty maximum variation of uncertainty due to various influence quantities such as ambient T°, frequency, unbalance, harmonics and EMC.

Fig. S15 – Tests related to influencing quantities

Influence quantities	Maximum uncertainty variation for active energy measurements according to table 9 of IEC 61557-12
	Conditions	for class 1	for class 0.2
Ambient T°	PF = 1	0.05% / °K	0.01% / °K
	PF = 0.5 Ind	0.07% /°K	0.02% / °K
Aux Power supply	24 Vdc +/-15%	0.1%	0.02%
Voltage	PF = 1 ; 80% / 120% Un	0.7%	0.1%
	PF = 0.5 Ind ; 80% / 120% Un	1%	0.2%
Frequency	49 Hz 51 Hz / 59 Hz 61 Hz PF = 1	0.5%	0.1%
	49 Hz 51 Hz / 59 Hz 61 Hz PF = 0.5	0.7%	0.1%
Reversed phase sequence		1.5%	0.05%
Voltage unbalance	0 to 10%	2%	0.5%
Phase missing	One or 2 phase missing	2%	0.4%
Harmonic in current and voltage	10% Un 5th 20% Imax 5th	0.8%	0.38%
	Odd harmonic in current	3%	0.6%
	Odd harmonic in tension	3%	0.6%
Common mode voltage rejection		0.5%	0.2%
Permanent a.c. magnetic induction 0.5 mT		2%	2%
Electromagnetic RF fields		2%	0.98%
Conducted disturbances induced by RF fields		2%	0.98%

EPMF - Equipment embedding Power Metering and Monitoring Functions compliant to IEC 61557-12 standard

Equipment embedding Power Metering and Monitoring Functions (EPMF) are devices which both:

• comply with their product standard in terms of safety, EMC, climatic, mechanical and functional requirements
• embed additional power metering and monitoring functions, and thus also comply with annex H of IEC 61557-12 (published in 2021 in amendment 1), in terms of measurement performance

For example, Masterpact MTZ is:

• a circuit breaker compliant with its product standard IEC 60947-2 (safety, EMC, climatic ...)
• an EPMF compliant with annex H of IEC 61557-12, in terms of measurement performance

Note that Masterpact MTZ is both an EPMF and a PMD (e.g. additional compliance to IEC 61010 and IEC 61326-1 ...)

Fig. S16 – Masterpact MTZ is a circuit breaker compliant with its product standard IEC 60947-2, an EPMF compliant with annex H of IEC 61557-12, and also a PMD fully compliant to IEC 61557-12