Focus on IEC 61557-12 standard

Increasingly, 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)".

The standard gives a list of the main requirements applicable to PMD with guidance about sensors to use (in case sensors are requested).

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.

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. S9).

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

Fig. S9 – 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
• overall system uncertainty: some information is provided about how to estimate uncertainty of a PMD operating with external sensors.

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 S10 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. S10 – Uncertainty limits for active energy at Power Factor = 1

Fig. S11 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. S11 – 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.

	Maximum uncertainty variation for active energy measurements according to table 9 of IEC 61557-12
Influence quantities	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%

Fig. S12 – Tests related to influencing quantities

Overall system uncertainty

Overall system uncertainty is the uncertainty including the instrumental uncertainty of several separated instruments (sensors, wires, measuring instrument, etc.) under the rated operating conditions.

In case sensors are embedded in the power meter, overall system uncertainty and operating uncertainty are the same.

In case sensors are external, it is recommended to use sensors with the same performance class as the power meter.