MID Electricity Meters: MI-003 Requirements and Software Assessment

Introduction

Active energy meters serve as core measurement devices for electricity trade settlement, and their software compliance directly impacts measurement accuracy and trade fairness. The European Union’s Measuring Instruments Directive (MID) 2014/32/EU establishes comprehensive legal metrology requirements for electricity meters through Annex V (MI-003), while WELMEC Guide 7.2 provides detailed technical implementation guidance for these requirements.

Based on section 11.4 of WELMEC Guide 7.2 (2023 edition), this article systematically analyzes the specific software requirements for active energy meters, focusing on technical implementation points under Class C risk level, providing practical compliance guidance for electricity meter manufacturers, certification bodies, and related technical personnel.

Electricity Meter Technical Description

Hardware Configuration

The core hardware architecture of active energy meters includes three key modules:

1. Voltage and Current Measurement Inputs

Responsible for collecting electrical signals from the circuit under measurement, typically from voltage transformers (PT) and current transformers (CT) or directly connected low-voltage circuits. In modern electronic electricity meters, analog signals are digitized through analog-to-digital converters (ADC) and sent to the microprocessor for processing.

2. Active Power Calculation Unit

Based on the collected instantaneous voltage and current values, active power is calculated through integration. Under purely resistive loads, active power equals the product of RMS voltage and RMS current; under inductive or capacitive load conditions, the power factor must be considered.

3. Energy Integration Module

Integrates the calculated active power over time to generate cumulative energy consumption values. The integration process runs continuously, and results are stored in non-volatile memory to prevent data loss from power interruption.

Software Configuration

Electricity meter software configuration follows the main system requirements of WELMEC Guide 7.2:

• Type P: Uses embedded systems built specifically for measurement purposes, with software stacks designed for specific measurement tasks, offering higher security and predictability
• Type U: Uses general-purpose computing devices (such as personal computers or general-purpose microcontrollers) to implement measurement functions; if the operating system is used to meet MID basic requirements, Extension O requirements must also be applied

Measurement Principle Characteristics

Active energy meter measurement has non-repeatability—the same measurement cannot be repeated under exactly the same conditions because actual consumed energy is a monotonically increasing physical quantity. This characteristic places special requirements on software design:

• Reliable data storage
• Effectiveness of fault recovery mechanisms
• Integrity of audit trails

Fault Detection and Response Requirements

MID Annex V (MI-003) section 4.3.1 specifies the requirements that electricity meters must meet after interference:

1. Ability to resume normal operation and measure within the Maximum Permissible Error (MPE) range
2. All measurement functions must be protected, with no loss or degradation of measurement functions
3. All measurement data existing before interference must be recoverable
4. No energy changes greater than the Critical Change Value may be displayed

MI-003 Referenced Provisions

The core regulatory basis for electricity meter software assessment includes:

• MID Annex I Article 8.5: Prohibition of accumulated data reset
• MID Annex I Article 10.5: Measurement result reading requirements
• MID Annex V (MI-003) Article 4.3.1: Interference effects and fault recovery
• MID Annex V (MI-003) Article 5.2: Display digit requirements

Class C Software Specific Requirements (I3-1 to I3-9)

I3-1: Fault Recovery Requirements

Regulatory Basis: MID Annex V (MI-003) Article 4.3.1

Core Requirement: Software must be able to resume normal processing after interference, applicable to all risk levels (B, C, D).

Technical Implementation Points:

• Fault detection mechanism: Handle abnormal states caused by hardware faults, electromagnetic interference, power fluctuations, etc.
• Recovery measures: Restore the electricity meter to normal measurement state
• Log recording: Record information during fault periods using date stamped flags

Documentation Requirements:

• Brief description of fault recovery mechanism
• Description of relevant tests conducted by the manufacturer

Acceptable Solution: Use a hardware watchdog circuit that resets through a microprocessor subroutine executed periodically. If the microprocessor enters a deadlock, the watchdog will trigger after a preset time and reset the microprocessor.

I3-2: Impact of Non-Legal Software on Dynamic Behavior

Core Requirement: Non-legal software must not adversely affect the dynamic behavior of the measurement process.

Technical Understanding:

Non-legal software (user interface display, communication protocol handling, data logging, and other auxiliary functions) must not occupy excessive system resources, ensuring that the real-time performance of legal software is not affected. Resource guarantees include:

• Central processor time
• Memory resources
• Interrupt response opportunities

Documentation Requirements:

• Description of interrupt hierarchy
• Timing diagrams of software tasks
• Description of runtime duration limits for non-legal tasks

I3-3: Additional Function Requirements

Core Requirement: Additional functions such as prepayment or interval metering must not affect legal metrology functions.

Isolation Principle: The design and implementation of additional functions must follow the “isolation principle,” which can be achieved through software separation (referring to Extension S requirements) or other technical means.

Documentation Requirements: Refer to relevant requirements of Extension S (software separation).

I3-4: Backup Facility Requirements

Regulatory Basis: MID Annex V (MI-003) Article 4.3.1

Core Requirement: Electricity meters may be equipped with functions to periodically backup measurement data, which should be stored in non-volatile memory.

Backup Interval Calculation:

If backup facilities are used for fault recovery, the minimum backup interval should be calculated based on the principle of ensuring the critical change value is not exceeded:

Backup frequency ≥ Maximum Permissible Error / Measurement Rate

Documentation Requirements:

• Brief description of backup data content
• Timing of backup occurrences
• Calculation process for minimum backup interval

I3-5: Software Download Requirements

Core Requirement: During software installation, the total pause time of the measurement process must not exceed one minute.

Additional Measures: If software installation indeed requires more than one minute, additional measures need to be taken, such as:

• Installation during low energy consumption periods
• Using phased installation strategies

Regulatory Basis: Additional supplements on top of Extension D (D1 to D4).

Documentation Requirements: Refer to relevant requirements of Extension D, including:

• Authorization mechanism for software download
• Integrity verification of downloaded data
• Audit trail of download process

I3-6: Prohibition of Accumulated Data Reset

Regulatory Basis: MID Annex I Article 8.5 and MID Annex V (MI-003)

Core Requirement: For utility measurement instruments, displays of the total quantity of supply that serve as the basis for payment in whole or in part, or displays from which such totals are derived, must not be capable of being reset during use.

Protection Mechanisms:

• After type approval, illegal reset of accumulated data must be prevented through hardware seals or other protection means
• Reset operations must generate tamper evidence
• Accumulated registers of measuring instruments may be reset before conformity assessment procedures

Acceptable Solution: The total measurement register must be protected by hardware seals. Other registers (such as time-of-use tariff registers) may use the same protection method as parameters (referring to P7/U7 requirements), provided there is a total register protected by hardware seals.

I3-7: Measurement Result Reading Requirements

Regulatory Basis: MID Annex I Article 10.5

Core Requirement: Measurement results used as the basis for payment may come from different registers, which are activated through remote control, clock, or other means. The instrument should be able to display results for each register through the user interface periodically or on demand.

Multi-Tariff Metering Requirements:

• Electricity meters should be able to display the total quantity of each register on the display through the user interface (such as buttons on the meter)
• Display the currently active tariff register
• Allow different registers’ results to be displayed periodically or on demand on different displays
• Must clearly identify which display corresponds to which register, with no ambiguity

Acceptable Solutions:

• Use buttons to switch display of data from different registers
• Display results from each register sequentially through automatic polling

I3-8: Tamper Prevention Requirements

Scope of Application: Type P configuration electricity meters with mechanical counters

Core Requirement: Calculate checksums or use other alternative indicators to support software modification detection and display this information when needed for control purposes.

Exception Clause Conditions:

1. The user interface has no control capability to activate the display of checksums or software modification indicators on the display, or the display technically does not allow displaying these values (mechanical counter case)
2. The instrument has no interface for communicating software identification
3. The software cannot be changed after production, or can only be changed by simultaneously replacing the hardware or hardware components containing the software

Acceptable Solution: Under the premise of meeting the above three conditions, imprint the checksum or software modification alternative indicator on the instrument’s nameplate.

I3-9: Display Digit Requirements

Regulatory Basis: MID Annex V (MI-003) Article 5.2

Core Requirement: The number of digits displaying the total quantity must be sufficient to ensure that when the electricity meter runs continuously under full load conditions (current I=Imax, voltage Un, power factor PF=1) for 4000 hours, the displayed value does not return to the initial value.

Calculation Example:

For a three-phase electricity meter, under conditions of maximum current 60A, rated voltage 230V, power factor 1, and running for 4000 hours:

Maximum Cumulative Energy = 3 × 60A × 230V × 4000h / 1000 = 165600 kWh

This requires at least 6-digit numeric display.

Technical Points:

• Internal registers must have sufficient digits to store cumulative energy values
• Display digits only need to meet the requirement of not returning to zero after 4000 hours of full-load operation
• Internal storage digits should consider the amount of data that may be generated over longer operating cycles

Examples of Legal Metrology Related Parameters

Means of modifying software, settings, and/or parameters must be protected whenever these parameters affect the determination of measurement results. Typical legal metrology related parameters for electricity meters include:

1. Calibration Factors

Used to adjust the measurement accuracy of electricity meters, directly affecting the final measurement results. Calibration factors are typically determined during the type approval phase and should not allow arbitrary changes during subsequent operation.

2. Linearization Factors

Used to compensate for measurement errors of electricity meters in non-linear operating ranges, establishing correction curves to keep errors within allowable ranges throughout the measurement range.

3. Legal Metrology Related Register Configuration

Defines the storage method and format of energy data, including total registers, time-of-use tariff registers, demand registers, etc.

4. Transformer Ratios

Transformation ratio parameters when electricity meters are used with external current transformers, typically set during installation and commissioning.

5. Other Parameters

• Power factor compensation settings
• Voltage ratios
• Pulse constants

All these legal metrology related parameters need appropriate protection measures (hardware seals, software encryption, or a combination of both) to prevent unauthorized modifications.

Risk Level Assignment and Applicability

Class C Risk Level Definition

WELMEC Guide 7.2 divides software risk into six levels (A to F), but for measuring instruments within MID scope, actually levels B to D are used.

Requirement Level for Class C:

Aspect	Requirement Level
Software Protection	Medium level - Prevent intentional modification using common simple tools (such as text editors)
Software Verification	Documentation-based verification + Functional testing validation
Software Consistency	Binary code of legal metrology related software of individual instruments is consistent with type-tested software

Risk Level Assignment for Active Energy Meters

According to the provisions of section 11.4 of WELMEC Guide 7.2:

• Type P (Specialized Equipment): Applies Class C risk level
• Type U (General Equipment): Applies Class C risk level

The selection of Class C reflects the importance of active energy meters in trade settlement and the relative level of software risk—requiring both a higher level of software protection and considering the maturity of electricity meter technology and moderate software complexity.

Additional Requirements for Type U Configuration

For electricity meters with Type U configuration, if the operating system is used to meet MID basic requirements or may affect compliance, additional Extension O (General Operating System) requirements must also be met.

Compliance Implementation Recommendations

Based on the above analysis, the following compliance implementation recommendations are proposed for active energy meter manufacturers:

1. Software Architecture Design

Fully consider the isolation requirements between legal metrology functions and additional functions during the software architecture design phase:

• Adopt software separation design principles
• Implement data flow control between legal metrology related software and non-legal software through protected interfaces
• Meet requirements such as I3-3 from the architectural level

2. Data Backup and Recovery

Establish comprehensive measurement data backup and recovery mechanisms:

• Calculate minimum backup interval according to I3-4 requirements
• Ensure compliance with critical change value limits under any fault scenario
• Implementation of backup mechanisms should be supported by detailed technical documentation
• Conduct sufficient testing and verification

3. Accumulated Data Protection

Strictly implement accumulated data reset protection measures:

• I3-6 requirements are the bottom line for legal metrology of electricity meters
• Must ensure cumulative registers cannot be illegally reset through hardware seals or equivalent means
• Reset operations must generate tamper evidence

4. Software Consistency Management

Strictly control software consistency throughout the software lifecycle management:

• Class C risk level requires legal metrology related software of individual products to be completely consistent with type-approved software
• Establish comprehensive software version control and release management mechanisms

5. Certification Body Communication

Recommend that manufacturers communicate with certification bodies early in the product development process:

• Identify and resolve potential compliance risks in advance
• Pay close attention to the latest technical guidance regarding active energy meters published on the WELMEC website

Reference Standards

• MID 2014/32/EU: European Measuring Instruments Directive
• MID Annex V (MI-003): Technical requirements for active energy meters
• WELMEC Guide 7.2 (2023): MID Software Guide
• WELMEC Guide 11.3: Technical guidance on protection measures for active energy meters
• OIML R 46: International recommendation for active energy meters
• EN 50470-1: General requirements for AC electricity metering equipment

Conclusion

Software assessment for active energy meters is a multi-level, multi-dimensional comprehensive system, covering core content such as technical description framework, specific software requirements, legal parameter identification, and risk level assignment. Through deep understanding of the technical requirements of MI-003 standard and WELMEC Guide 7.2, manufacturers can ensure their products meet the legal metrology requirements of the EU market, providing reliable technical assurance for electricity trade settlement.