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IEC PAS 63151: Measurement Method for Electromagnetic Field Strength of Wireless Power Transfer Systems
Standardized EMF measurement techniques for WPT systems from consumer electronics to electric vehicle charging
Scope and Technical Background
Wireless Power Transfer (WPT) technology has moved rapidly from laboratory concepts to mainstream commercial deployment, powering everything from smartphones and wearable devices to electric vehicles and industrial equipment. As WPT systems proliferate, concerns about human exposure to electromagnetic fields (EMF) have intensified. IEC PAS 63151 addresses this critical need by establishing a standardized measurement methodology for determining the electromagnetic field strength produced by WPT systems. This Publicly Available Specification (PAS) provides the technical community with a consensus approach to EMF characterization before the development of a full international standard.
IEC PAS 63151 is applicable to WPT systems operating in the frequency range from 1 kHz to 10 MHz, covering the vast majority of inductive and resonant WPT technologies in use today. This includes systems based on the Qi, AirFuel, and SAE J2954 standards.
The fundamental challenge in measuring WPT EMF is the combination of near-field effects, high power densities, and the presence of both magnetic (H-field) and electric (E-field) components. Unlike far-field RF measurements, where plane-wave assumptions simplify calculations, WPT systems operate in the reactive near-field region where the field distribution is highly non-uniform and strongly dependent on coil geometry, alignment, and load conditions. The PAS specifies measurement probes, positioning systems, and data processing methods specifically designed to handle these near-field conditions.
| Parameter | Specification | Notes |
|---|---|---|
| Frequency range | 1 kHz – 10 MHz | Covers inductive (10–150 kHz) and resonant (100 kHz–MHz) WPT |
| Field types measured | H-field (A/m) and E-field (V/m) | Both components required for compliance assessment |
| Measurement distance | 0 cm (contact) to 50 cm from device surface | Multiple distances to characterize decay characteristics |
| Probe type | Isotropic E/H field probes | Tri-axial design for orientation-independent measurement |
| Maximum uncertainty | ± 3 dB (k=2) | 95% confidence level for compliance decisions |
Measurement Setup and Probe Positioning Protocol
The PAS defines a rigorous measurement setup that minimizes environmental interference while ensuring reproducibility. The WPT device under test (DUT) is placed on a non-conductive, low-permittivity support structure (typically Rohacell or dry wood) at least 1 meter from any large metallic objects. The measurement probe is mounted on a computer-controlled positioning system capable of 3-axis movement with 1 mm positional accuracy. A key innovation in the standard is the virtual grid scanning method: the probe traverses a predefined 3D grid around the DUT, recording field strength at each point, and the data is interpolated to generate a complete field distribution map.
Proper probe calibration is essential. The standard requires that isotropic probes used for WPT measurements be calibrated in a known reference field at the specific operating frequency of the DUT, with calibration traceable to national metrology institutes. Using probes calibrated only for 50 Hz power-frequency fields will introduce measurement errors exceeding 20 dB at WPT operating frequencies.
The positioning protocol defines a reference coordinate system centered on the geometric center of the WPT charging surface. Measurements are taken along three orthogonal axes (X, Y, Z) at intervals of 5 mm or less in regions where the field gradient is steep (within 5 cm of the surface), expanding to 10 mm intervals at greater distances. For each measurement point, both the RMS field strength and the peak field strength are recorded, with a minimum averaging time of 6 seconds to capture any modulation effects from the power control loop.
| Region | Distance from Surface | Grid Spacing | Measurement Points | Purpose |
|---|---|---|---|---|
| Near-field (high gradient) | 0 – 5 cm | 5 mm | ~3000 | Capture peak field regions |
| Transition region | 5 – 20 cm | 10 mm | ~1500 | Characterize field decay |
| Far-field (low gradient) | 20 – 50 cm | 20 mm | ~500 | Verify compliance boundaries |
Data Processing and Compliance Assessment
Once the raw field measurements are collected, IEC PAS 63151 specifies a data processing pipeline that includes background field subtraction, probe factor application, spatial averaging over the human body cross-section (for comparison with ICNIRP exposure limits), and uncertainty analysis. The spatial averaging algorithm is particularly important because exposure limits (such as ICNIRP 2020 and IEEE C95.1) specify averaging over a 200 cm² surface area representing the human torso or head. The PAS provides a reference implementation of the averaging algorithm using a sliding 200 cm² window applied to the interpolated field grid.
From an engineering design perspective, the PAS offers valuable insights for WPT system developers. The measurement data can be used to validate electromagnetic simulation models, optimize coil shielding designs, and determine the minimum safe separation distance between the WPT charger and human body. The standard specifically notes that ferrite shielding on the back side of WPT coils can reduce stray magnetic fields by 15–25 dB, but the effectiveness is highly dependent on ferrite material composition (Mn-Zn ferrites with permeability >2000 at operating frequency are recommended).
For electric vehicle WPT systems (3.7–22 kW), compliance with IEC PAS 63151 measurement methodology has enabled manufacturers to demonstrate that at a distance of 30 cm from the vehicle charging pad, the magnetic field strength is typically below 27 μT (RMS), which is well within the ICNIRP 2020 general public exposure limit of 30.7 μT at 85 kHz operating frequency.
Frequently Asked Questions
Q1: Is IEC PAS 63151 mandatory for WPT product certification?
As a PAS, it is not a normative standard and is not mandatory. However, regulatory authorities in several regions (EU, China, South Korea) are referencing it as the preferred measurement method for WPT EMF compliance assessments. Manufacturers adopting the PAS methodology can streamline their certification process significantly.
As a PAS, it is not a normative standard and is not mandatory. However, regulatory authorities in several regions (EU, China, South Korea) are referencing it as the preferred measurement method for WPT EMF compliance assessments. Manufacturers adopting the PAS methodology can streamline their certification process significantly.
Q2: How does this standard differ from IEC 62233 (household appliance EMF)?
IEC 62233 addresses EMF from household appliances at power frequencies (50/60 Hz) and uses a different probe positioning approach optimized for larger devices with diffuse field distributions. IEC PAS 63151 is specifically designed for the higher frequencies and highly localized fields characteristic of WPT systems, with much finer spatial resolution requirements.
IEC 62233 addresses EMF from household appliances at power frequencies (50/60 Hz) and uses a different probe positioning approach optimized for larger devices with diffuse field distributions. IEC PAS 63151 is specifically designed for the higher frequencies and highly localized fields characteristic of WPT systems, with much finer spatial resolution requirements.
Q3: Can the measurement method be applied to bidirectional WPT systems?
Yes, but additional considerations apply. For bidirectional WPT (vehicle-to-grid), measurements must be performed in both power transfer directions, and the higher power levels (up to 22 kW for passenger vehicles) may require extended measurement grids with additional safety precautions for the test personnel.
Yes, but additional considerations apply. For bidirectional WPT (vehicle-to-grid), measurements must be performed in both power transfer directions, and the higher power levels (up to 22 kW for passenger vehicles) may require extended measurement grids with additional safety precautions for the test personnel.
Q4: What is the expected evolution of this PAS toward a full standard?
IEC PAS 63151 is currently being used as the baseline document for the development of IEC 63151 (full international standard), expected for publication in 2027–2028. Key areas of refinement include expanded frequency coverage up to 30 MHz, improved uncertainty budgets for multi-coil WPT arrays, and alignment with the upcoming ICNIRP 2025 guidelines for intermediate frequencies.
IEC PAS 63151 is currently being used as the baseline document for the development of IEC 63151 (full international standard), expected for publication in 2027–2028. Key areas of refinement include expanded frequency coverage up to 30 MHz, improved uncertainty budgets for multi-coil WPT arrays, and alignment with the upcoming ICNIRP 2025 guidelines for intermediate frequencies.
