Power frequency magnetic field immunity testing is the electromagnetic-compatibility (EMC) test defined in IEC 61000-4-8 (adopted in China as GB/T 17626.8) that verifies whether electrical and electronic equipment continues to function correctly when exposed to the power-frequency (50 Hz or 60 Hz) magnetic fields generated by nearby current-carrying conductors. It is the continuous magnetic-field test of the 61000-4 family, distinct from the pulse and damped-oscillatory magnetic field tests, and judged against the same performance criteria (A/B/C/D) that govern every immunity test.
What Is Power Frequency Magnetic Field Immunity Testing?
The test addresses a specific physical environment. Wherever significant current flows at power frequency — in busbars, transformer windings, reactor coils, and overhead or cable conductors — a 50 Hz (or 60 Hz) alternating magnetic field surrounds that conductor. Equipment installed nearby is immersed in that field. If the equipment contains components sensitive to magnetic fields, the field can induce unwanted voltages, disturb sensing elements, or cause visible artifacts — and the test exists to verify that the equipment is immune to those effects at the field strength appropriate to its environment.
The single most important scope fact, and one that is rarely stated, is what the test does and does not affect. A power-frequency magnetic field couples to a circuit only through components that respond to magnetic flux. That means the test is primarily relevant to:
- Cathode-ray tubes (CRTs) — beam deflection distortion
- Hall-effect sensors and magnetometers
- Microphones and audio inductive pickups
- Reed relays and other magnetically actuated switches
- Transformers and inductive components
Most modern digital electronics — microcontrollers, memory, logic, power semiconductors — are essentially unaffected by power-frequency magnetic fields at the test levels, because they contain no magnetic-flux-sensitive elements. This is why, in practice, the test is specified mainly for measurement and instrumentation equipment, audio gear, and devices that include magnetic-field sensors, rather than for generic computing hardware. Knowing whether your product actually has a magnetically sensitive element is the first question before specifying this test.
The Three Magnetic Susceptibility Siblings
"Power frequency magnetic field immunity" is one of three related but distinct magnetic-susceptibility tests in the IEC 61000-4 family. They are frequently confused because they all use an induction coil and a magnetic field, but they model three different real-world phenomena with three different waveforms:
| Test | Standard | Phenomenon modelled | Waveform |
|---|---|---|---|
| Power frequency | IEC 61000-4-8 / GB/T 17626.8 | Continuous field from normal load current in conductors | Continuous sinusoidal, 50/60 Hz |
| Pulse | IEC 61000-4-9 / GB/T 17626.9 | Lightning strike to overhead lines / substation fault | Single pulse, 6.4/16 μs, 8/20 μs |
| Damped oscillatory | IEC 61000-4-10 / GB/T 17626.10 | Switching transients in HV/MV switchgear and isolators | Damped oscillatory, 0.1 / 1 MHz, repeating |
The three tests are not interchangeable. A product installed in an MV/HV substation control room is typically subjected to all three, because the continuous field from load current, the pulse from a lightning event, and the damped oscillatory wave from a switching operation are three separate disturbances that occur in that environment. This article addresses the power frequency (continuous) test only.
Which Standard Governs the Test?
| Document | Status | Role |
|---|---|---|
| IEC 61000-4-8:2009 | Current | Power frequency magnetic field immunity — test methods and levels |
| GB/T 17626.8 | Current in China (IDT) | Identical adoption of IEC 61000-4-8 |
| EN 61000-4-8 | European adoption | Same technical content, for EU CE marking |
| IEC 61000-4-1 | Current | Defines the performance criteria (A/B/C/D) used to judge pass/fail |
As with the rest of the 61000-4 family, the pass/fail judgment is not defined inside 61000-4-8 itself; it is defined generically in IEC 61000-4-1 and applied across the immunity family. GB/T 17626.8 is the identical Chinese adoption, so a domestic report cites GB/T 17626.8 and an international report cites IEC 61000-4-8 — same test, same levels, same coil geometry.
The Test Levels: Continuous and Short-Duration
The severity scale is the part most often mis-stated. IEC 61000-4-8 defines field-strength levels split into continuous field (applied for the duration needed to observe function, at normal steady load current) and short-duration field (applied for 1 to 3 seconds, simulating the high field that exists only during a short-circuit fault before protection clears it):
| Level | Continuous field (A/m, RMS) | Short-duration field, 1–3 s (A/m, RMS) |
|---|---|---|
| 1 | 1 | — |
| 2 | 3 | — |
| 3 | 10 | — |
| 4 | 30 | 300 |
| 5 | 100 | 1000 |
| X (open) | 300 / 1000 | — |
Two points that the published explanations routinely miss. First, the short-duration 300 A/m and 1000 A/m levels exist only at Levels 4 and 5 — lower levels are continuous-only, because the very high fields occur only under fault current, which is transient. Second, the continuous-field scale goes up to 1000 A/m (open class) for special applications, not stopping at 100 A/m. The applicable level is set by the product standard according to the equipment's environment:
| Continuous field | Typical environment |
|---|---|
| 1 A/m | Protected environment, away from transformers |
| 3 A/m | Residential / commercial |
| 10 A/m | Commercial / light industrial |
| 30 A/m | Industrial, near conductors |
| 100 A/m | Heavy industrial, near busbars / transformers |
| 300 / 1000 A/m | Special applications (open class) |
How Is the Test Performed?
The test places the equipment under test (EUT) inside an induction coil that generates a calibrated, uniform magnetic field at 50 Hz or 60 Hz. The setup and procedure:
- Induction coil — a coil of suitable geometry (a single square/round coil for one face, or a Helmholtz-like pair for thicker EUTs) sized so the EUT is immersed in the uniform central field. The coil dimensions must exceed the largest EUT dimension.
- Immersion method — the standard approach: the EUT is placed within the coil volume so the field threads through it. A "proximity method" exists for large fixed equipment that cannot be moved into a coil, using a small probe coil scanned over the surface.
- Three orthogonal orientations — because the field can arrive from any direction, the EUT is tested in all three mutually perpendicular positions (X, Y, Z) so every internal axis sees the field.
- Continuous application — at the chosen continuous level, for the time needed to exercise and observe the EUT's functions.
- Short-duration application — at Levels 4 and 5, an additional 1–3 s application at 300 or 1000 A/m, simulating the fault-current field.
- Field verification — the coil is first calibrated to produce the target field strength (measured by a field probe at the center) before the EUT is introduced.
The EUT is operated in its normal mode during exposure, because the test verifies operational immunity — the equipment must keep working while the field is applied.
How Is Pass/Fail Judged?
Judgment uses the generic performance criteria of IEC 61000-4-1. The criterion a product standard assigns depends on whether the field is continuous or short-duration:
| Criterion | Meaning | Typical assignment |
|---|---|---|
| A | Normal performance, no degradation | Required for the continuous field |
| B | Temporary degradation or function loss that is self-recovering when the field is removed | Permitted for the short-duration (fault) field |
| C / D | Operator-intervention / non-recoverable | Generally a fail |
The logic is that a device must operate normally in the steady ambient field it lives in (continuous → Criterion A), but a brief degradation during a fault-current transient that self-recovers is acceptable (short-duration → Criterion B). The test report states, for each EUT function, the criterion required by the applicable product standard and the criterion actually observed.
What Failures Does the Test Reveal?
Because the test only couples through magnetically sensitive elements, the failures it produces are specific:
- CRT beam distortion — wobble and color fringing on the raster (now mainly of historical interest).
- Hall-sensor / magnetometer error — incorrect current, position, or field readings, or false triggering of magnetic proximity sensing.
- Audio interference — hum pickup in microphones and inductive pickups at exactly 50/60 Hz (and harmonics).
- False relay operation — a reed relay chattering or pulling in under the test field.
- Induced voltage in measurement circuits — offset or noise in high-impedance analog measurement paths.
A digital control board with none of these elements will typically show no effect at any level — which is itself a valid (and common) test outcome, not a sign the test was done wrong.
Our Testing Capabilities
Beijing ZKGX Research conducts power frequency magnetic field immunity testing to IEC 61000-4-8 / GB/T 17626.8:
- Standards covered: IEC 61000-4-8:2009 and GB/T 17626.8 (continuous); the pulse test IEC 61000-4-9 / GB/T 17626.9 and the damped oscillatory test IEC 61000-4-10 / GB/T 17626.10 are available as the related siblings for substation-grade equipment.
- Test levels: continuous field 1 to 1000 A/m (Levels 1–5 and open class); short-duration 300 and 1000 A/m at Levels 4–5.
- Frequency: 50 Hz (and 60 Hz where the product's market requires it).
- Setup: calibrated induction coil, immersion method, three orthogonal orientations; proximity method for large fixed equipment.
- Judgment: functional monitoring during exposure against performance criteria A (continuous) and B (short-duration) per IEC 61000-4-1 and the applicable product standard.
- Sample types: measurement and instrumentation equipment, audio equipment, devices with Hall/magnetic sensors, industrial control equipment, and substation-grade equipment requiring the full magnetic-susceptibility suite.
- Deliverable: a test report stating the standard, coil geometry, test level (continuous and short-duration), orientation results, and the performance criterion required versus observed for each EUT function.
If you have equipment requiring power-frequency magnetic field immunity verification, contact our testing team to scope the applicable product standard, the level, and whether the pulse/damped-oscillatory siblings are also required.
Frequently Asked Questions
What is the power frequency magnetic field immunity test?
It is the EMC immunity test of IEC 61000-4-8 / GB/T 17626.8 that exposes equipment to a continuous 50/60 Hz magnetic field (1 to 1000 A/m) generated by an induction coil, to verify it keeps functioning in the field produced by nearby current-carrying conductors. It is the continuous test of the magnetic-susceptibility family.
Which equipment needs this test?
Equipment that contains magnetically sensitive components — CRTs, Hall sensors, magnetometers, microphones and inductive pickups, reed relays, transformers. Most modern digital electronics (microcontrollers, memory, logic) are not affected at the test levels, so the test is mainly specified for measurement, instrumentation, and audio equipment, and for devices with magnetic-field sensors.
What are the test levels for IEC 61000-4-8?
The continuous field runs from Level 1 (1 A/m) through Level 5 (100 A/m), with an open class extending to 300 and 1000 A/m. A short-duration field of 300 A/m (Level 4) and 1000 A/m (Level 5), applied for 1 to 3 seconds, simulates the high field present only during a short-circuit fault. Lower levels are continuous-only.
What is the difference between IEC 61000-4-8, -4-9, and -4-10?
IEC 61000-4-8 is the continuous power-frequency field from normal load current. IEC 61000-4-9 is the pulse field from a lightning strike or substation fault. IEC 61000-4-10 is the damped oscillatory field from switching transients in HV/MV switchgear. They model three different phenomena with three different waveforms and are normally specified together for substation-grade equipment.
What is the GB/T equivalent of IEC 61000-4-8?
GB/T 17626.8 (电磁兼容 试验和测量技术 工频磁场抗扰度试验) is the identical (IDT) adoption of IEC 61000-4-8. Its siblings are GB/T 17626.9 (pulse) and GB/T 17626.10 (damped oscillatory).