Why test equipment against a 50 Hz magnetic field?

Power transformers, asynchronous motors, distribution switchgear and high-current chargers continuously generate magnetic fields at the mains frequency (50 Hz in Europe, 60 Hz in North America). Those fields penetrate metallic enclosures through apertures and cabling, and couple into PCB current loops. A poorly shielded Hall sensor, a relay coil, a legacy CRT display or a sensitive audio amplifier can be disturbed. IEC 61000-4-8 verifies that the equipment maintains its function under continuous exposure, a typical condition for installations near a transformer substation or an electrical cabinet.

How does IEC 61000-4-8 differ from 4-9 and 4-10?

IEC 61000-4-8 covers continuous power-frequency magnetic field (50 or 60 Hz, optionally 16.67 Hz for railway or 400 Hz for avionics). IEC 61000-4-9 covers pulsed magnetic field, simulating the effect of a short-circuit current or a nearby lightning strike: 6.4/16 microsecond pulse, levels up to 1000 A/m peak. IEC 61000-4-10 covers damped oscillatory magnetic field, simulating high-voltage breaker and switch operations: 100 kHz or 1 MHz oscillation, levels up to 100 A/m. The three tests are complementary and all three appear in EN 61000-6-2 for the severe industrial environment.

Which level applies to which environment?

Level 1 (1 A/m) corresponds to a residential environment far from industrial sources. Level 2 (3 A/m) covers typical residential and commercial. Level 3 (10 A/m) is the common requirement for industrial and tertiary buildings, with moderate power-equipment presence. Level 4 (30 A/m) targets the severe industrial environment near transformers or high-current chargers. EN 60601-1-2 requires 30 A/m for medical devices in critical care environments (ICU, operating rooms, CCU) where MRI proximity and defibrillators create a high ambient field.

Continuous field or short-duration field, which procedure?

The standard distinguishes two modes by phenomenon type. The continuous field is applied for at least 1 to 3 seconds after stabilisation, long enough to observe steady-state EUT behaviour; this is the case for the generic levels 1 to 4. The short-duration field (transient test) applies the level for 1 to 10 cycles of the mains frequency, i.e. 20 ms to 200 ms at 50 Hz; this mode simulates short-circuit phenomena or motor inrush. The product standard specifies which of the two modes applies, and some require both in sequence.

Why test on three orthogonal axes?

A magnetic field has a direction in space. The sensitive structures inside the EUT (PCB current loops, relay coils, Hall sensors) respond to the field according to their own orientation. Testing a single axis leaves the other two orientations untested, and a product can pass on X yet fail on Y or Z. The standard therefore requires three orthogonal orientations, obtained by rotating the EUT 90 degrees after each sequence (or by switching the coil orientation, which is equivalent). The report documents all three orientations and the observed behaviour for each.

Which performance criterion applies?

Criterion A is the only one applicable for IEC 61000-4-8, because the phenomenon under test (power-frequency field) is permanent in the real world. The EUT must therefore maintain its function without degradation during exposure and after. A temporary degradation that clears at the end of the test is not acceptable, in contrast to transient tests (ESD, EFT, surge) where criteria B and C are admissible per function. The argument is physical: in the real environment the field does not disappear, so any temporary failure becomes a permanent failure on site.

How is the induction coil sized?

The standard describes two accepted geometries: the 1 metre square single coil and the double Helmholtz coil, which produces a more uniform field in its central volume. For small EUTs the single coil suffices. For larger EUTs, enlarged geometries (1 m by 2.6 m or two-turn coils in series) are specified. The coil manufacturer supplies the calibration constant that ties injected current to the magnetic field produced at centre, in A/m per ampere of excitation. The calibration is verified annually against a traceable field probe.

Which campaign mistakes drive first-pass failures?

Three pitfalls recur in failure reports. First, EUT positioning outside the uniform field volume of the coil (typically the central 50 percent): the field there is weaker, the lab applies an apparent level but the EUT does not receive the required level, and post-campaign verification is harsh. Next, omission of one of the three orthogonal orientations, often the vertical axis on flat-lay EUTs: the report must show all three rotations. Finally, EUT in standby instead of active operation: equipment tested outside its nominal regime passes trivially, because no sensitive function is exercised.

IEC 61000-4-8: power-frequency magnetic field immunity

Author Hugues Orgitello Last updated 27 May 2026 ~14 min read

Guide - Low-frequency EMC immunity

Power-frequency magnetic field immunity testing per IEC 61000-4-8 is an often overlooked horizontal EMC test, because it addresses a permanent phenomenon rather than a transient one. The 50 or 60 Hz field produced by transformers, motors, distribution cabinets and high-current chargers penetrates enclosures and couples into PCBs. EN 55035 (multimedia), EN 55024 (information technology equipment) and above all EN 60601-1-2 (medical, with a 30 A/m requirement for critical-care environments) reference this test. This page lays out the standards framework (Edition 2.0 of 2009), severity levels from 1 to 30 A/m, the geometry of induction coils, the test procedure on the three orthogonal axes, the single criterion A, the connection to IEC 61000-4-9 (pulsed field) and 4-10 (damped oscillatory field), and the campaign pitfalls.

Why a low-frequency magnetic immunity test

Electromagnetic compatibility splits phenomena into three families: high-frequency fields and currents (radiated and conducted tests above 150 kHz), fast transients (ESD, EFT, surge), and low-frequency fields close to mains. The last family is covered by IEC 61000-4-8 and its neighbours 4-9 and 4-10.

The magnetic field at 50 or 60 Hz is present wherever a significant alternating current flows: a distribution transformer cell typically generates 5 to 50 A/m at one metre, a 10 kW motor produces 1 to 5 A/m at the same distance, an automation cabinet with busbar reaches 10 to 30 A/m at 0.5 metre. These levels correspond to the real environment of industrial or medical equipment installed near electrical infrastructure.

Why a low-frequency field disturbs electronics

At 50 Hz the wavelength is 6000 km. No antenna or resonant-cavity coupling is at play. The coupling mechanisms are quasi-static. The field penetrates metallic enclosures through apertures (vents, ventilation slots, discontinuous gaskets), because the skin depth in mild steel at 50 Hz is roughly 0.8 mm: a 1 mm sheet attenuates by only a few decibels.

Once inside, the field couples into PCB current loops. A 10 cm square loop exposed to 10 A/m at 50 Hz picks up roughly 50 microvolts peak, which sounds modest but is enough to disturb a 24-bit measurement sensor, a class AB audio amplifier, a Hall sensor or a sensitive relay. Legacy CRT displays were the first victims (beam deflection visible at 1 A/m), but the topic stays current for electron microscopes, precision balances, biomedical signal amplifiers and resistive current sensors.

Standards framework: IEC 61000-4-8 Ed 2.0:2009

The edition in force is IEC 61000-4-8 Ed 2.0:2009, superseding Edition 1.0 of 1993 and its 2000 amendment. Edition 2.0 clarified accepted coil geometries, extended the frequency domain (16.67 Hz to 400 Hz beyond the 50/60 Hz mains), and tightened the performance criterion.

Product standards that reference IEC 61000-4-8

Product standard	Required level	Criterion	Justification
EN 55035 (multimedia)	1 A/m (residential) or 3 A/m (commercial)	A	Audio, video, IT consumer equipment
EN 55024 (IT equipment, legacy pre-CISPR 35)	1 A/m	A	Office IT equipment
EN 61000-6-1 (generic residential)	3 A/m	A	Generic residential/commercial standard
EN 61000-6-2 (generic industrial)	30 A/m	A	Severe industrial environment
EN 60601-1-2 (medical)	3 A/m generic, 30 A/m ICU/CCU/OR	A	Critical care, MRI and defibrillator proximity
EN 50121-4 (fixed railway)	30 A/m at 16.67 Hz and 50 Hz	A	Traction substations

EN 60601-1-2 pushes the 30 A/m requirement furthest, because critical-care environments accumulate sources: patient-side transformers, defibrillators, portable MRI, motor-driven beds, infusion pumps. A poorly protected patient monitor can show ECG artifacts or SpO2 drift nearby, with direct clinical risk.

Frequencies covered

The standard test uses the local mains frequency: 50 Hz in Europe and most of the world, 60 Hz in North America and parts of Japan. When specified by the product standard, the standard also covers:

16.67 Hz for rolling stock on 15 kV traction networks (Germany, Austria, Switzerland, Sweden, Norway).
400 Hz for avionics and military equipment on 400 Hz networks.

See the glossary for the EMC acronyms, and CE tests for the general EMC framework in the EU.

Field-intensity levels

The standard defines five severity levels. The applicable level is set by the product standard.

Level	Continuous field intensity	Short-duration intensity	Typical environment
1	1 A/m	-	Residential far from industrial sources
2	3 A/m	-	Typical residential and commercial
3	10 A/m	100 A/m (1 to 3 s)	Moderate industrial
4	30 A/m	300 A/m (1 to 3 s)	Severe industrial, medical critical care
X	Specified by product standard	Specified	Special case

The short-duration level (column 3) applies in addition to the continuous field at levels 3 and 4, and simulates transient phenomena associated with a short-circuit or a motor start. It runs for 1 to 3 seconds, or in a variant 1 to 10 cycles of the mains frequency (20 to 200 ms at 50 Hz).

Quick read for the designer

For a consumer product certified to EN 55035, the requirement is typically 1 A/m: a permissive level, reachable without special precautions. For an industrial product certified to EN 61000-6-2, the requirement rises to 30 A/m: this calls for magnetic shielding considerations, orientation of sensitive loops, and filtering on analogue sensors. For a medical device intended for critical care (EN 60601-1-2 in a professional healthcare environment), 30 A/m is mandatory and requires a robust architecture from the PCB upward.

Test hardware and configuration

An IEC 61000-4-8 campaign relies on a high-power mains-frequency generator, a calibrated induction coil and an EUT-positioning system.

The generator

The generator supplies alternating current at the test frequency (50, 60, 16.67 or 400 Hz per product specification). To produce 30 A/m at the centre of a 1 m by 1 m coil, typically 24 to 30 A are needed in a single-turn coil. Frequency stability and harmonic content are controlled: THD below 8 percent is required by the standard. The reference manufacturers are EM TEST (MIG 5060 and MIG OS6), Teseq/Ametek (ECOS and NSG 1007).

The induction coil

Three geometries are specified:

Single square coil 1 m by 1 m, single thick-section turn on a non-magnetic frame. Typical calibration constant: 0.87 A/m per ampere injected at centre.
Single rectangular coil 1 m by 2.6 m, for larger EUTs. Different constant, supplied by the manufacturer.
Two-element Helmholtz coils, two parallel coils separated by their radius, producing a very uniform field in the central volume. More accurate but bulkier.

The coil holds an annual calibration certificate giving the field map inside its usable volume (typically the central 50 percent of the bounding volume), with the uniformity tolerance per the standard (plus or minus 3 dB).

Configuration diagram

                    +-------------------------------+
                    |  Induction coil (1 turn)      |
                    |  geometry 1 m x 1 m           |
                    |                               |
                    |  +-------------------+        |
                    |  |                   |        |
                    |  |   EUT energised   |        |
                    |  |   centred in the  |        |
                    |  |   uniform volume  |        |
                    |  |                   |        |
                    |  +-------------------+        |
                    |        |                      |
                    |        | 90 deg rotations     |
                    |        | X, Y, Z              |
                    |        v                      |
                    +-------------------------------+
                            |          |
                            |          |
                       +------------+  |
                       | Generator  |  |
                       | 50/60 Hz   |  |
                       | I = 0 to 30A| |
                       +------------+  |
                            |          |
                            +--- GRP ---+
                                  |
                                Earth

  Orientations to test (3 orthogonal axes):
    X horizontal       Y horizontal           Z vertical
    [EUT]              [EUT rotated 90 deg]   [EUT rotated 90 deg]

Ambient conditions and EUT setup

The EUT sits on a non-conductive wooden table or non-magnetic support, centred in the uniform volume of the coil. The test room must be away from ambient-field sources above 1 A/m, which excludes immediate proximity to HV-to-LV transformers or unshielded distribution panels. The report logs the ambient field before and after the test.

The EUT runs its typical application software during the test (simulated ECG trace, sensor bus traffic, network packet stream). The observation setup must detect any degradation, even brief.

Procedure: the three orthogonal axes

The test consists of applying the field at the specified intensity in the three orthogonal orientations in turn, observing the EUT during and after each sequence.

Standard sequence for a continuous level

For a level 4 test (30 A/m) the procedure is:

EUT stabilisation. The EUT is energised, its application software runs, and the observation is verified as nominal for at least 1 minute.
Orientation 1 (X horizontal). The field is established at 30 A/m, held for 1 to 3 seconds after stabilisation, then cut. The observation is verified during and after.
90 degree rotation. The EUT is rotated about a vertical axis to expose Y to the field direction.
Orientation 2 (Y horizontal). The cycle repeats: field established, held, cut, observed.
90 degree rotation about a horizontal axis. The EUT is tilted to expose Z (vertical) to the field direction.
Orientation 3 (Z vertical). The cycle repeats.

A common variant: instead of rotating the EUT, the coil is rotated, or a three-orthogonal-coil system is used and the excitation is switched between them. The physical result is equivalent.

Sequence for the short-duration level

When the product standard specifies the short-duration level (levels 3 or 4), a second sequence follows the continuous sequence: the field is applied at the transient amplitude (100 or 300 A/m) for 1 to 3 seconds, or for 1 to 10 cycles of mains frequency (20 to 200 ms at 50 Hz), per the product specification. This mode simulates a nearby short-circuit or a high-power motor start.

Observation and acceptance

The observer verifies the EUT's primary function during each application of the field and during the 1 to 3 seconds after the cut. Any degradation, even brief, is logged with the orientation, the level and the nature of the observation (communication loss, measurement artifact, analogue output drift).

Criterion A: why not B or C

Unlike transient tests (ESD, EFT, surge) where criteria B (automatic recovery) and C (recovery by user intervention) are admissible per function criticality, IEC 61000-4-8 accepts criterion A only for continuous phenomena.

The argument is physical. A power-frequency magnetic field is not a transient in the real environment: equipment installed near a transformer remains exposed continuously for years. A temporary degradation in the lab would become a permanent degradation on site, hence a lasting loss of function. Criterion A is the only consistent choice.

Exception for short-duration mode

The short-duration mode (levels 3 and 4 with 100 or 300 A/m for 1 to 3 seconds or 1 to 10 cycles) may accept criterion B per the product standard, because it simulates a transient (short-circuit, motor start) which is not permanent. EN 61000-6-2 for example specifies criterion A for the 30 A/m continuous and criterion B for the 300 A/m short-duration.

The magnetic family: 4-8 vs 4-9 vs 4-10

Three IEC 61000-4-x tests cover magnetic phenomena. Their relationship is summarised below.

Test	Simulated phenomenon	Levels	Duration	Criterion	Typical product standard
IEC 61000-4-8	Continuous power-frequency magnetic field (50/60 Hz)	1 to 30 A/m	Continuous, 1 to 3 s	A	EN 55035, EN 60601-1-2, EN 61000-6-2
IEC 61000-4-9	Pulsed magnetic field (nearby lightning strike, short-circuit)	100 to 1000 A/m peak	6.4/16 microsecond pulse	B	EN 61000-6-5 (substations), EN 61000-6-2
IEC 61000-4-10	Damped oscillatory magnetic field (HV switching)	10 to 100 A/m peak	100 kHz or 1 MHz oscillation	B	EN 61000-6-5, severe industrial EN 61000-6-2

IEC 61000-4-9, pulsed field

The 4-9 test simulates the magnetic effect of a very high fault current (distribution short-circuit, nearby lightning). The pulse has a 6.4 microsecond front and 16 microsecond half-duration, with peaks up to 1000 A/m at level 5. Criterion B applies, the phenomenon being transient. Mainly relevant for HV substations and severe industrial environments.

IEC 61000-4-10, damped oscillatory field

The 4-10 test simulates HV breaker or disconnector switching, which generates a damped oscillation at 100 kHz or 1 MHz. Peak 100 A/m at level 5. Criterion B applies. Mainly relevant for substation equipment.

For neighbouring EMC tests, see ESD per IEC 61000-4-2, surge and burst per IEC 61000-4-5 and 4-4, and mains dips and interruptions per IEC 61000-4-11.

The medical case: 30 A/m in critical-care environments

EN 60601-1-2 is the standard that formalises the highest level for IEC 61000-4-8. Table 9 of EN 60601-1-2:2015 (and its A1:2021 amendment) specifies:

3 A/m for equipment intended for the home healthcare environment.
30 A/m for equipment intended for the professional healthcare facility environment, which includes intensive care units (ICU), cardiac care units (CCU), operating rooms, imaging rooms, and zones near MRI installations.

The performance criterion applied to each function of the medical equipment is derived from a clinical risk analysis per ISO 14971. If loss of function under magnetic-field exposure can cause patient harm (stopping an infusion pump, drift on an ICU ECG monitor, loss of an alarm function), the applicable criterion is A with no alternative possible. The justification appears in the risk-management file and in the test report.

Why 30 A/m in a hospital

Three main sources generate significant fields in critical-care environments:

MRI scanners, with static fields of the order of one tesla and gradients that extend into adjacent rooms. The 50 Hz field coupled to the magnetisation can reach 10 to 50 A/m at 2 or 3 metres from the magnet.
Defibrillators and HF electrosurgical units in operating rooms, generating strong transient fields picked up by surrounding equipment.
High-power medical supplies for motorised beds, parallel infusion pumps and resuscitation carts, producing cumulative 50 Hz fields.

The 30 A/m requirement of EN 60601-1-2 is sized to guarantee operation of vital equipment in this cumulative environment.

Common campaign pitfalls

Six errors recur in first-pass failure reports for the 4-8 test.

EUT positioned outside the uniform volume. The coil produces a uniform field in only the central 50 percent of its bounding volume, with a plus or minus 3 dB tolerance. An EUT placed near the coil edge receives a weaker field than expected, and the lab may wrongly conclude the EUT is compliant. The test plan must specify positioning and the report must photograph it.
One of the three orientations omitted. The classic pitfall on flat-lay EUTs is to forget the vertical axis, which requires a rotation about a horizontal axis that is not always practical. Testing X and Y without Z passes an EUT that would have failed on Z. The report must photograph all three rotations.
Coil calibration out of date. The coil's A/m-per-ampere constant drifts if the frame deforms, if the turn is displaced, or if a magnetic mass has been added nearby. Annual calibration is mandatory, and absence of ferromagnetic mass within 2 metres is documented.
EUT in standby or out of nominal mode. Equipment tested in standby exercises few sensitive functions and often passes trivially. The test plan must specify the operating mode (measurement loop, simulated ECG capture, active network transmission) and the observation criterion.
High generator THD. A generator with THD above 8 percent injects harmonics that couple differently than pure 50 Hz. The standard requires THD below 8 percent at rated current; the measurement is logged in the report.
Ambient field not measured. The test room must have an ambient field below 1 A/m. A room near an unshielded distribution transformer can exceed that. Measuring the ambient field before and after the test is mandatory. See CE pitfalls for general EMC reporting anomalies.

Robust design for low-frequency magnetic fields

Compliance with 4-8, especially at the 30 A/m level, is decided upstream by PCB and enclosure design.

Reduce sensitive current loops

Any closed current loop on a PCB picks up an electromotive force proportional to the area, the field intensity, and the frequency. At 50 Hz and 30 A/m, a 100 square cm loop picks up 80 microvolts peak. Reducing the loop area (signal return routed close to the forward path, continuous ground plane) reduces the induced parasitic voltage proportionally.

Orientation of sensitive components

Components with a strong magnetic coupling factor (relay coils, measurement transformers, Hall sensors, moving-coil microphones) have a sensitive direction. Orienting them so their sensitive axis is perpendicular to the most likely orientation of the external field reduces coupling. This optimisation targets a specific axis, but the three-orthogonal-axes test verifies that the EUT survives in any orientation.

Magnetic shielding

Mild-steel enclosures attenuate poorly at 50 Hz (0.8 mm skin depth, a few dB per millimetre). For significant shielding, high-permeability materials are needed: mumetal, permalloy, low-frequency ferrites. A 0.5 mm mumetal shield can reach 30 to 40 dB attenuation at 50 Hz, but the material is expensive, fragile, and loses its properties after bending if not annealed. A full mumetal envelope is typically reserved for very-high-precision instruments.

Filtering and software integration

On analogue sensors, a low-pass filter with cutoff below 50 Hz attenuates the induced parasitic. On ECG or audio measurements, a 50 Hz notch is standard. In software, averaging over 20 or 40 ms windows (one or two mains periods) removes the residual magnetic component.

What to remember

IEC 61000-4-8 Ed 2.0:2009 evaluates immunity to a continuous magnetic field at mains frequency (typically 50 or 60 Hz, optionally 16.67 Hz or 400 Hz).
Five severity levels: 1, 3, 10, 30 A/m and X (product-specific). A short-duration mode supplements levels 3 and 4 (100 or 300 A/m for 1 to 3 seconds or 1 to 10 cycles).
Three orthogonal orientations mandatory, by EUT rotation or by orthogonal-coil switching.
Criterion A only for the continuous field, because the phenomenon is permanent in the real world. Criterion B is admissible for the short-duration mode per product standard.
EN 60601-1-2 requires 30 A/m for the professional healthcare facility environment (ICU, CCU, OR, MRI proximity), with criterion A on critical functions justified by ISO 14971.
IEC 61000-4-9 and 4-10 complete the magnetic family: 4-9 pulsed (lightning, short-circuit), 4-10 damped oscillatory (HV switching). Both accept criterion B.
Robustness comes from design: smaller sensitive current loops, oriented relay coils and Hall sensors, mumetal shielding for extreme cases, low-pass and 50 Hz notch filtering on sensitive sensors.

For practical certification, see CE tests for the EU EMC framework, RED tests for radio, and the glossary.

Sources & references

IEC 61000-4-8 Ed 2.0:2009, power-frequency magnetic field immunity test , IEC webstore.iec.ch/publication/4189
EN 60601-1-2:2015+A1:2021, medical devices, electromagnetic compatibility , IEC webstore.iec.ch/publication/67383
EN 55035:2017+A11:2020, multimedia equipment, electromagnetic immunity , CENELEC standards.cencenelec.eu/dyn/www/f?p=205:110:0::::FSP_PROJECT,FSP_ORG_ID:67027,1258635
EN 55024:2010+A1:2015, information technology equipment, immunity , CENELEC standards.cencenelec.eu/dyn/www/f?p=205:110:0::::FSP_PROJECT,FSP_ORG_ID:35375,25
IEC 61000-4-9 Ed 2.0:2016, pulsed magnetic field immunity test , IEC webstore.iec.ch/publication/4191
Directive 2014/30/EU on electromagnetic compatibility , EUR-Lex eur-lex.europa.eu/eli/dir/2014/30/oj