OIML R 46 is the international recommendation that defines the metrological requirements for active electrical energy meters used in trade measurement. It is the reference document behind national legal metrology approvals in over 60 countries that are OIML members.
What is OIML?
The International Organisation of Legal Metrology (OIML) is an intergovernmental organisation that develops international recommendations (R) and documents (D) for legal metrology — the measurement of quantities used in trade, health, safety, and environment. OIML recommendations are not directly binding law, but national metrology bodies (NMIs) adopt them as the basis for domestic type approval.
OIML R 46 Structure
OIML R 46 is published in three editions. The current edition (Ed. 3, 2012) is divided into two parts:
| Part | Title | Content |
|---|---|---|
| R 46-1 | Metrological and technical requirements | Accuracy classes, MPEs, influence quantities, type examination requirements |
| R 46-2 | Metrological controls and performance tests | Test procedures, test conditions, test points, acceptance criteria |
Accuracy Classes
OIML R 46 defines four accuracy classes:
| Class | MPE (active energy) | Typical Application |
|---|---|---|
| Class 0.2 | ±0.2% | Reference/laboratory meters, high-value industrial tariff points |
| Class 0.5 | ±0.5% | Large commercial and industrial (C&I) billing |
| Class 1 | ±1% | Commercial metering, small C&I |
| Class 2 | ±2% | Residential metering (most common class globally) |
Influence Quantities and Their Limits
The MPE applies at reference conditions. The standard defines how accuracy must be maintained under variations in influence quantities:
Voltage Variation
The meter must meet MPE from 0.7 × Un to 1.3 × Un (±30% of nominal voltage). In practice, most meters maintain accuracy from 0.5 × Un to 1.5 × Un.
Frequency Variation
MPE must be met from fn ± 2 Hz (49–51 Hz or 58–62 Hz).
Power Factor
Tests are conducted at unity power factor (PF=1.0), PF=0.5 inductive, and PF=0.8 capacitive. Many electronic meters now maintain Class 1 accuracy across the full PF range.
Starting Current (Ist)
The minimum current at which the meter must start registering. OIML R 46 defines Ist ≤ 0.4% of Imax for Class 1, ≤ 0.5% for Class 2. This is the low-flow equivalent for electricity — the starting current determines how well the meter captures very small loads.
Harmonic Distortion
OIML R 46 Ed.3 includes requirements for harmonic testing up to the 40th harmonic — reflecting the reality that modern loads (LED lighting, variable frequency drives, EV chargers) generate significant harmonic currents that older standards did not adequately cover.
Relationship to IEC 62053
OIML R 46 and IEC 62053 (accuracy classes for electricity meters) cover similar ground from different organisational perspectives:
| Aspect | OIML R 46 | IEC 62053 |
|---|---|---|
| Scope | Legal metrology (trade measurement) | Technical accuracy (any application) |
| Authority | OIML (intergovernmental) | IEC (technical standards body) |
| Enforcement | Via national NMI type approval | Via procurement specification |
| Class designation | 0.2, 0.5, 1, 2 | 0.1, 0.2, 0.5, 1, 2 (IEC 62053-11/21/22/23) |
| MID alignment | Reference basis for MID MI-003 | Referenced in MID technical standards |
Type Approval vs MID CE Marking
In countries outside the EU, meter manufacturers obtain OIML type approval from the national NMI (e.g. SASO in Saudi Arabia, SABS in South Africa, BIS in India). This is the primary market access mechanism. The certificate is country-specific.
In the EU, the MID CE mark (based on OIML R 46 principles) provides a single market-wide approval via a Notified Body.
OIML R 46 and Smart Meters
The 2012 edition addressed smart meter features:
- Software sealing: the metrological software must be protected against unauthorised modification (aligned with WELMEC 7.2)
- Time-of-use registers: requirements for TOU tariff accuracy
- Remote reading: the communication interface must not affect metrological performance
- Firmware updates: the metrological part must be re-verified after a firmware update that affects measurement functions
Further Reading
- OIML R 46 Edition 3 (2012) — oiml.org (free download)
- IEC 62053-21: Static meters for active energy (Class 1 and 2)
- IEC 62053-22: Static meters for active energy (Class 0.2S and 0.5S)
- WELMEC 7.2: Software guidance for measuring instruments