Electronics Guide

Test Levels

For the continuous field, five preferred test levels are defined, and the standard's informative Annex C relates each to a class of installation environment. Level 1 (1 A/m) is an environment in which devices that use electron beams, such as cathode ray tube monitors and electron microscopes, can operate. Level 2 (3 A/m) is a well-protected environment, such as a household, an office, or a hospital. Level 3 (10 A/m) is a protected environment, such as a commercial area, a smaller industrial plant, or the computer room of a high-voltage substation. Level 4 (30 A/m) is a typical industrial environment: heavy industrial plants, power plants, and the control rooms of high-voltage substations. Level 5 (100 A/m) is a severe industrial environment, typified by the switchyards of heavy industrial plants, medium- and high-voltage substations, and power stations. An open level, X, lets a product specification set any other value, above, below, or between these.

The standard also defines a short-duration field test that represents conditions during a power system fault, when line currents briefly rise far above their normal values and persist until protection operates, within a few milliseconds for a fuse and up to a few seconds for a protection relay. This test applies a higher field of 300 A/m (Level 4) or 1000 A/m (Level 5) for one to three seconds; it is not applicable at Levels 1 to 3. The continuous test confirms correct operation in the steady magnetic environment of normal service, whereas the short-duration test confirms that a transient fault-driven field surge causes neither damage nor unacceptable malfunction.

The level actually applied is set by the product standard for the equipment or, where none exists, by a generic standard: IEC 61000-6-1, for residential, commercial, and light-industrial locations, specifies 3 A/m, and IEC 61000-6-2, for industrial locations, specifies 30 A/m. Equipment destined for power generation or transmission facilities may need Level 5 or a special level set in its own product specification.

Test Methods

The test field is produced by an inductive coil, a loop of copper, aluminum, or another non-magnetic conductor, driven at the power frequency by a test generator, typically a variable AC source feeding the coil through a step-down transformer. Table-top equipment is usually tested in a single-turn square coil 1 m on a side, whose test volume, the region in which the field stays within about 3 dB of its central value, makers such as Teseq give as 0.6 m × 0.6 m × 0.5 m high. Floor-standing equipment such as racks needs a taller coil, and makers sell a 1 m × 2.6 m rectangular one for these tests. The coil factor, the field at the center of the coil for each ampere of coil current, turns a test level into a drive current. EMC Partner rates its 1 m square coil at 0.87 A/m per ampere, about 1.15 A for each A/m, and the Biot–Savart law gives a thin 1 m × 2.6 m loop about 0.68 A/m per ampere, close to 1.5 A for each A/m, so the 1000 A/m short-duration level drives more than a kiloampere through a single-turn coil. Multi-turn coils reduce the drive current, and two square coils in a Helmholtz arrangement, 0.6 m apart, stretch the test volume to 0.6 m × 0.6 m × 1 m high, provided the field over the equipment stays within ±3 dB.

In this arrangement, the immersion method, the equipment is placed at the center of the coil and operates normally while the specified magnetic field is applied. Because coupling depends on orientation, the coil is turned through 90 degrees, or the equipment reoriented, between exposures so that the field passes through the equipment along each of three orthogonal axes, identifying the most susceptible direction. At each orientation the field is held while the equipment's functions are exercised and its operation is compared with its expected performance.

Equipment too large for the standard coils is tested inside a coil built to suit its dimensions, or by the proximity method, in which a smaller coil is moved along the sides of the equipment to find particularly sensitive areas. A coil that encloses only part of a large unit can also be stepped along it, with the test repeated until the whole unit has been exposed. During development, small local coils placed near suspected sensitive components serve a related diagnostic purpose, locating the circuit that responds before formal testing.

Induced Voltages

According to Faraday's law, a time-varying magnetic field induces voltage in any loop it penetrates. The induced voltage is proportional to the field amplitude, the frequency, and the loop area. For a sinusoidal field of strength H and frequency f passing perpendicularly through a loop of area A in a non-magnetic environment, the induced voltage is

V = ωμ₀HA

where ω = 2πf is the angular frequency, and V and H are both RMS or both peak values. At power frequencies, even moderate loop areas can develop significant induced voltages when exposed to strong fields.

Circuit board loops, cable loops, and ground loops are common coupling paths. A 10 cm × 10 cm loop (0.01 square meter) exposed to a 10 A/m field at 50 Hz develops an induced voltage of approximately 40 microvolts, rising to about 0.4 millivolts at 100 A/m and 4 millivolts at the 1000 A/m short-duration level. While this seems small, 40 microvolts is about one least significant bit of a 16-bit converter with a 2.5 V input span, and a high-gain analog stage amplifies it along with the signal, so in precision measurement systems such interference can be significant.

Ground loops between equipment sharing a common ground reference are particularly problematic. Magnetic fields induce current flow around the ground loop, developing voltage differences between ground reference points. This ground loop voltage appears in series with single-ended signals referenced to ground, and as a common-mode voltage on differential ones, potentially corrupting measurements or causing control system errors.

Effects on Magnetic Components

Components that rely on magnetic properties respond to external magnetic fields, but the fields of the immunity standards are far too weak to saturate a closed transformer or inductor core. Even the 1000 A/m short-duration level corresponds to only about 1.26 mT in air, whereas manganese-zinc ferrites saturate at roughly 0.3 to 0.5 T and silicon steel at 1.5 to 2 T; saturation by an external field takes a far stronger source, such as a powerful permanent magnet placed against the component. At immunity-test levels the effect on magnetic components is instead the voltage the field induces in their windings, which matters most where the magnetic circuit is open or the signal is small: unshielded rod-core and drum-core inductors, and the input and audio transformers of low-level circuits, which are often enclosed in high-permeability cans for that reason.

Magnetic sensors including Hall effect devices, magnetoresistive sensors, and fluxgate magnetometers are inherently sensitive to external fields. When these sensors are used to measure fields other than power frequency (such as DC fields or higher-frequency AC), power frequency interference can overwhelm the desired signal or produce measurement errors through nonlinear detection.

Cathode ray tubes (though increasingly rare) are highly susceptible to magnetic field deflection, producing image distortion when exposed to even modest power frequency fields. Modern LCD and LED displays are generally immune, but their backlight drivers and control electronics may still be affected.

Effects on Electron Beams

Equipment using electron beams, including electron microscopes, electron-beam lithography systems, electron-beam welders, and certain vacuum tube devices, is extremely sensitive to magnetic field disturbances. Even very weak magnetic fields can deflect electron trajectories, affecting image quality, beam positioning, or tube performance. Such equipment often requires magnetic shielding or installation in low-field environments; the lowest IEC 61000-4-8 level, 1 A/m, is defined by the environment in which electron-beam devices can operate. Where the room itself is too noisy, active cancellation systems sense the ambient field and drive surrounding coils to oppose it.

Magnetic Shielding

Magnetic shielding requires materials with high magnetic permeability that provide a low-reluctance path for magnetic flux, diverting it around the protected region. Mu-metal, a soft nickel-iron alloy (roughly 80 percent nickel), is the most common choice for sensitive equipment; its initial relative permeability runs into the tens of thousands and its maximum permeability into the hundreds of thousands after a final anneal in a hydrogen atmosphere. Mu-metal saturates at a comparatively low flux density, about 0.75 T, so in strong fields it is often paired with a thicker outer layer of ordinary steel that handles the bulk of the flux. Its permeability is also sensitive to mechanical stress: shields are formed and machined before the final anneal, and a shield bent or dropped afterward must be re-annealed to recover its performance. A shield that has been magnetized by exposure to a strong field regains its low-field permeability after degaussing.

Multiple nested shields provide more effective attenuation than a single shield of equivalent total thickness, because each layer reduces the field incident on the layers inside it. A shield's performance is expressed as its shielding factor, S, the ratio of the field outside the shield to the field inside. For a long cylinder of diameter D, with a wall of thickness t much smaller than D, in a field perpendicular to its axis, the shielding factor is approximately

S ≈ μ_rt/D

where μ_r is the relative permeability of the wall, so a single wall improves only in proportion to its thickness. Two coaxial cylinders of diameters D₁ and D₂, with D₁ the smaller and individual shielding factors S₁ and S₂, combine as

S ≈ S₁S₂[1 − (D₁/D₂)²] + S₁ + S₂ + 1

Apart from the small additive terms, the product of the individual factors is therefore a ceiling that widely spaced layers approach, not something the spacing exceeds. Closely spaced layers fall well short of it, because the bracketed term shrinks toward zero as the gap closes, while an outer layer made very large loses part of its own factor, which falls as its diameter grows. With an assumed relative permeability of 20,000, a mu-metal cylinder 100 mm in diameter with a 1 mm wall has a shielding factor of about 200, and doubling the wall to 2 mm only doubles it, to about 400. The same material split into two 1 mm layers, 100 mm and 150 mm in diameter, gives individual factors of 200 and 133 and a combined factor of about 15,000, nearly forty times that of the single thick wall; with only 1 mm between the two layers, the combined factor would fall below 2,000. A finite cylinder shields a field along its axis much less effectively than a transverse one, and openings reduce both figures.

Openings and joints weaken a magnetic shield because they interrupt the high-permeability path that carries the flux around the protected volume. At 50 or 60 Hz every practical opening is tiny compared with the wavelength, so the waveguide-below-cutoff tubes and honeycomb vents that control radio-frequency leakage do nothing for a power frequency magnetic field, and a metallic cable run through a below-cutoff tube defeats it even at radio frequencies. A magnetic shield instead keeps its openings few and small and places them away from the most sensitive region, overlaps its seams rather than butting them, and extends any necessary opening with a sleeve of the same alloy, along which the field entering through the opening decays rapidly.

Loop Area Minimization

Reducing the loop areas in sensitive circuits directly reduces induced interference voltages. Signal and return paths should be routed together, preferably as twisted pairs or closely spaced traces on adjacent PCB layers; twisting also reverses the orientation of successive loops, so that their induced voltages largely cancel. Coaxial cable has an effectively zero loop area when its shield carries the return current, because that current is distributed symmetrically around the inner conductor. At power frequency, however, a shield grounded at both ends also carries ground-loop current, and the voltage that current develops along the shield's resistance appears in series with the signal.

Ground loop elimination reduces susceptibility to magnetically induced interference in the ground system. Single-point grounding, while sometimes creating high-frequency problems, eliminates ground loops that can pick up power frequency magnetic fields. Where multiple ground connections are necessary, optical isolators or isolation transformers can break the loop, and balanced differential inputs can reject the common-mode voltage it induces.

Circuit Design Approaches

Balanced differential circuits inherently reject common-mode magnetic interference. A field linking the large loop formed by a signal pair, its ground connections, and the ground induces the same voltage in both conductors, and a properly balanced input rejects that common-mode voltage. Only the field passing between the two conductors produces a differential voltage, and twisting the pair cancels most of it. High common-mode rejection ratio (CMRR) amplifiers and careful attention to circuit balance, including matched source and input impedances, maximize this rejection.

Filtering can attenuate magnetically induced interference when the signal occupies a different band from the power frequency. A low-pass filter with a cutoff well below 50 Hz suits slowly varying signals such as those from thermocouples and strain gauges, and a notch filter can remove the fundamental where the signal band is wider, at the cost of distorting signal content near the notch. Integrating analog-to-digital converters whose integration period is a whole number of power-line cycles, 20 ms for 50 Hz or 100 ms for both 50 and 60 Hz, place nulls at the power frequency and its harmonics. Adaptive cancellation, which measures the interference with a reference sensor and subtracts a scaled copy of it, is used in some precision instruments.