Conducted emissions: LISN method and CISPR + FCC limits

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

Guide - EMC methods

Conducted emissions are the mains-side counterpart of radiated emissions. Any product connected to the electrical grid injects part of the high-frequency energy it generates internally onto its power leads, and that pollution can disturb other equipment sharing the same network. The European and US EMC regimes regulate the phenomenon across the 150 kHz to 30 MHz band, requiring measurement at the LISN (Line Impedance Stabilisation Network). This page details the LISN architecture, CISPR 32 and FCC Part 15 limits by class, the quasi-peak and average detectors, the current-probe method as an alternative, and the mitigation mechanisms using passive components. It is written for product engineers preparing an EMC campaign and project managers who need to understand why pre-compliance does not guarantee the final lab pass.

What conducted emissions measure

Every power converter, every digital clock, every fast switching event inside a product generates broadband electromagnetic noise. Part of it propagates by direct radiation (covered by radiated tests from 30 MHz to 1 GHz and beyond), the other part conducts through power leads and interface cables back to the mains. The regulated quantity is the disturbance voltage at the power port, expressed in dB(microvolt).

The 150 kHz to 30 MHz band is not arbitrary. The lower bound, 150 kHz, sits just above the upper edge of the long-wave broadcast band and marks the start of medium-wave AM receiver sensitivity. The upper bound, 30 MHz, corresponds to the threshold above which cables start to radiate efficiently: above it, the radiated test takes over. The continuity between the two campaigns is ensured by CISPR 32, which covers both phenomena in the same product standard.

Common mode and differential mode

On a two-wire lead (line plus neutral, or positive plus negative), two propagation modes coexist.

• Differential mode (DM) corresponds to a current flowing in one direction on one conductor and in the opposite direction on the other. This is the normal power-transfer mode. DM noise typically comes from the switching currents of a switched-mode power supply.
• Common mode (CM) corresponds to a current flowing in the same direction on both conductors, returning through earth via parasitic capacitance. This is the hardest mode to control because it depends on the geometry and the quality of the ground. Most overshoots seen in the lab are common-mode problems.

The DM-CM distinction matters because mitigation is different. An X capacitor filters DM but not CM ; a Y capacitor and a common-mode choke filter CM but not DM. A product that fails by 6 dB in the high band (10 to 30 MHz) almost always has a common-mode problem.

Why the measurement is so sensitive to the setup

The impedance seen at the power port varies by several orders of magnitude between 150 kHz and 30 MHz depending on the local network: a rural outlet, an urban outlet behind a distribution board, an industrial outlet near a variable-frequency drive all have different characteristics. The LISN solves this dependency by enforcing a reference impedance, the same across labs and campaigns.

§ § §

LISN architecture

The LISN, or Line Impedance Stabilisation Network, is defined in CISPR 16-1-2. The most common model is the V-network 50 ohms / 50 microhenrys plus 5 ohms, applicable to the 9 kHz to 30 MHz range in its standard configuration.

Block diagram and role of each element

                +--------------------+
   AC mains  ---|      50 uH         |-- EUT output (line or neutral)
   (input)      +--------------------+
                         |
                         |
                         +--- 5 ohms ---+
                         |              |
                       50 ohms          |
                         |              |
                         +--- 1 uF -----+ to ground
                         |
                       50 ohm RF port
                       (to receiver)

Each branch of the LISN duplicates the above chain for line and for neutral. The measurement switch selects which of the two branches feeds the receiver, the other being terminated on 50 ohms.

Reading the elements:

• 50 microhenrys in series on the line: blocks the high-frequency currents from the mains back into the product. At 150 kHz, about 47 ohms ; at 30 MHz, about 9.4 kohms.
• 5 ohms in parallel with the choke: stabilises the low-frequency impedance where the choke alone would be nearly transparent.
• 1 microfarad in series to ground: high-frequency coupling capacitance, low impedance across the band.
• 50 ohms to the RF port: standard measurement impedance into the EMI receiver.

For the EUT, the LISN therefore behaves as a source impedance of 50 ohms in parallel with 50 microhenrys plus 5 ohms, stable across the full 150 kHz to 30 MHz range. This is the reference impedance for the CISPR and FCC limits.

LISN classes per CISPR 16-1-2

CISPR 16-1-2 defines several artificial networks according to rated current and frequency target.

Type	Current range	Frequency range	Use case
V-network 50 ohms / 50 uH plus 5 ohms	up to about 100 A typical	9 kHz to 30 MHz	Single-phase and three-phase products, AC mains, most common case
V-network 50 ohms / 50 uH plus 1 ohm	up to several kA	9 kHz to 30 MHz	Industrial high-power
Delta-network	variable	150 kHz to 30 MHz	Symmetric-input products with no neutral
T-network	variable	150 kHz to 30 MHz	Telecom lines, some cellular cases
AAN / ISN	variable	150 kHz to 30 MHz	Telecom ports, Ethernet lines, etc.

The V-network 50 ohms / 50 microhenrys plus 5 ohms is the default for IT, multimedia, industrial and consumer products. The choice of an AAN or ISN for telecom ports is specific to emission measurement on communication lines (xDSL, Ethernet, telephone). A high-current industrial LISN is needed above roughly 16 A nominal, and mandatory above 100 A.

Ground plane and bonding

LISN performance depends on the quality of the ground plane. CISPR 16-1-2 requires a reference conductive plane of at least 2 m x 2 m, on which the LISN and the EUT are placed, with a direct low-impedance bond between the LISN enclosure and the plane. A missing screw and the measurement can drift by 5 to 10 dB in the high band. The EUT sits on an insulating support typically 10 cm above the plane, except for floor-standing equipment. The distance between the EUT power port and the LISN is 1 m measured along the cable.

CISPR 32 (EN 55032) limits, Class A and Class B

CISPR 32 and its EN 55032 transposition set the limits for multimedia equipment connected to the mains, covering a large portion of IoT, IT and consumer products.

LISN voltage method

The limits are expressed in dB(microvolt) measured at the RF port of the LISN, with quasi-peak and average detectors applied jointly on the same sweep.

Frequency band	Class A QP (dB(uV))	Class A AVG (dB(uV))	Class B QP (dB(uV))	Class B AVG (dB(uV))
150 kHz to 500 kHz	79	66	66 to 56 (log)	56 to 46 (log)
500 kHz to 5 MHz	73	60	56	46
5 MHz to 30 MHz	73	60	60	50

Reading: in the low band 150 kHz to 500 kHz, the Class B limit decreases logarithmically from 66 to 56 dB(uV) quasi-peak. The analytical formula is given in CISPR 32 Table 11. Above 500 kHz, the limits become piecewise constant, with a Class B step up starting at 5 MHz that follows the typical spectral profile of switched-mode supplies.

The gap between the two classes is 13 dB in the low band, 17 dB in the mid band, 13 dB in the high band. This is a wide gap, which translates at design time into very different filter choices.

Current probe (alternative)

For products with no mains port or non-interceptable leads, CISPR 32 admits the disturbing current method with a calibrated current probe, with limits in dB(microampere) calibrated to give equivalent protection.

Frequency band	Class A QP (dB(uA))	Class A AVG (dB(uA))	Class B QP (dB(uA))	Class B AVG (dB(uA))
150 kHz to 500 kHz	53 to 43 (log)	40 to 30 (log)	40 to 30 (log)	30 to 20 (log)
500 kHz to 30 MHz	43	30	30	20

The current probe is not a systematic substitute for the LISN: it is a complementary method when the LISN is not applicable. The test report has to justify the choice.

CISPR 11 (EN 55011) limits for industrial and medical

CISPR 11 covers industrial, scientific and medical (ISM) equipment. It distinguishes group 1 (no intentional internal RF energy) and group 2 (intentional RF energy: microwave oven, induction welder). For medical equipment connected to the mains, CISPR 11 applies instead of CISPR 32, with group 1 Class B limits practically identical to CISPR 32 Class B across 150 kHz to 30 MHz. The distinction sits in the associated immunity levels (IEC 60601-1-2) and the technical-file documentation. The choice between CISPR 11, CISPR 32 or another is recorded in the technical file. See CE tests.

FCC Part 15 Subpart B limits

The FCC regulates conducted emissions of unintentional radiators (digital devices) in 47 CFR section 15.107. The limits are structurally aligned with CISPR 32 and historic CISPR 22, but with differences in detail and detectors.

FCC detectors

The FCC retains quasi-peak and average as the main detectors across 150 kHz to 30 MHz, like CISPR. The difference shows up above 5 MHz for some configurations: the FCC may add a peak detector, more severe than QP on short transients. ANSI C63.4 gives the procedural detail.

Section 15.107(a) and (b) limits

Frequency band	Class A QP (dB(uV))	Class A AVG (dB(uV))	Class B QP (dB(uV))	Class B AVG (dB(uV))
150 kHz to 500 kHz	79	66	66 to 56 (log)	56 to 46 (log)
500 kHz to 5 MHz	73	60	56	46
5 MHz to 30 MHz	73	60	60	50

The values are identical to CISPR 32 in the vast majority of cases, which is not an accident: the FCC aligned its limits with CISPR 22 (precursor to CISPR 32) in 2000 to ease mutual recognition of test reports. A product that passes CISPR 32 Class B conducted almost always passes FCC Part 15 Class B, provided it was measured with a LISN compliant with both regimes.

See CE vs FCC EMC for the full comparison of the two regimes.

Quasi-peak and average detectors: why both?

The detectors are not a receiver option but a metrological specification defined in CISPR 16-1-1, which fixes the time constants, the resolution bandwidth (9 kHz RBW across 150 kHz to 30 MHz) and the weighting factor.

Quasi-peak (QP)

The QP detector integrates the signal with a charge time constant of 1 ms and a discharge time constant of 160 ms (CISPR 16-1-1). It gives the same reading as a peak detector for a continuous signal but penalises isolated pulses: an isolated pulse of the same amplitude as a continuous carrier reads lower in QP. The historical idea is to simulate the annoyance perceived by an AM radio receiver. A spectrum made of very brief, very spaced pulses can pass QP despite a high peak.

Average (AVG)

The AVG detector gives the mean value of the signal after envelope detection. On a continuous carrier, QP and AVG read the same. On a pulsed signal, AVG is lower than QP. AVG is used to catch pure carriers, typically the spectral lines of a switched-mode power supply. A product that meets QP but exceeds AVG almost always has a dominant spectral line (switching carrier, clock harmonic).

Detector summary table

Detector	Charge time	Discharge time	Behaviour on signal	Regulation
Peak	minimal	maximal	Captures instantaneous crest	FCC complement, CISPR pre-compliance
Quasi-peak (QP)	1 ms	160 ms	Weighted by repetition rate	CISPR + FCC, main conducted limit
Average (AVG)	equal	equal	Mean value	CISPR + FCC, complementary conducted limit
RMS-average	variable	variable	Trade-off for modern signals	CISPR, optional

In practice, a product must meet the QP and the AVG limits at the same time, on the same sweep. Exceeding either fails the test.

LISN voltage method vs current-probe method

Both methods measure the same phenomenon but with different physical principles.

Feature	LISN voltage method	Current-probe method
Quantity measured	Conductor-to-earth voltage, dB(uV)	Cable-induced current, dB(uA)
Band	150 kHz to 30 MHz	150 kHz to 30 MHz, 30 MHz to 1 GHz for CM
Reference network	V-network 50 ohms / 50 uH + 5 ohms	None (direct measurement)
Ground plane	Mandatory, 2 m x 2 m minimum	Recommended
Use case	Product with AC mains port	Battery, interface cables

The current probe is typically used to measure common mode on interface cables (USB, Ethernet, remote sensors), or the conducted emissions of a battery product without a mains port. Procedure in CISPR 16-2-1, limits associated with telecom ports in CISPR 32.

Mitigation with passive components

Most overshoots seen at the lab are resolved by appropriate mains-input filtering. Three component families cover the essentials.

X capacitors (differential-mode filtering)

X capacitors sit between line and neutral, downstream of the fuse and upstream of the bridge rectifier. They filter differential-mode noise. Classes X1 (2.5 kV crest, direct inlet on high-energy-transient networks), X2 (2.5 kV crest, residential and commercial), X3 (1.2 kV crest, downstream of other filtering). Typical value: 100 nF to 470 nF, with a 1 to 4.7 megohm discharge resistor in parallel to avoid charge retention at disconnect.

Y capacitors (common-mode filtering)

Y capacitors sit between line-earth and neutral-earth. They filter common-mode noise. Classes Y1 (8 kV crest, double insulation, medical and SELV), Y2 (5 kV crest, double protection), Y4 (2.5 kV crest, low voltage only).

The Y trap is the legal limit on leakage current: IEC 60950-1 then IEC 62368-1 cap the total leakage current at 0.25 mA (residential) or 3.5 mA (industrial), which bounds the Y value to a few nanofarads (typically 2.2 nF to 4.7 nF per conductor). Beyond that, the product fails electrical safety tests even if it would pass EMC by a wide margin.

Common-mode chokes

A common-mode choke (CMC) uses two windings on the same ferrite core, in opposite senses: DM currents cancel magnetically (transparent in DM) and CM currents add (high CM impedance). Typical mains values: 1 mH to 10 mH per winding, rated up to 30 A. Manganese-zinc ferrite for the low band 150 kHz to 5 MHz, nickel-zinc ferrite for 5 MHz to 100 MHz.

Integrated fuse-filter modules

Fuse-filter modules combine the AC inlet, the fuse, the optional switch and a pre-wired LC filter in an IEC C14 housing. Typical vendors: Schaffner FN series, Schurter DD/FMW, TDK ZAB series. Faster design at a higher per-unit cost than a custom filter.

§ § §

Pre-compliance vs full compliance

A pre-compliance measurement is an informal test on a development bench, without a standardised ground plane, with a simplified LISN and a department spectrum analyser. A full-compliance measurement is run in an ISO/IEC 17025 accredited lab, with a calibrated LISN, a compliant ground plane, a dedicated EMI receiver, and a documented protocol.

Typical gaps between the two

Four main sources:

1. Ground plane: a bench measurement without a conductive plane can underestimate common mode by 5 to 15 dB in the high band.
2. LISN calibration: a low-end LISN shows drifting impedance with frequency, especially in the low band.
3. Detector: a peak measurement overestimates QP by about 5 to 15 dB depending on duty cycle, but underestimates when the signal is continuous.
4. Cable arrangement: the exact layout (length, looping, distance to plane) changes parasitic capacitance and common-mode coupling.

A useful pre-compliance targets margins and trends, not lab reproduction: calibrated compact LISN (Schwarzbeck, Tekbox, Com-Power) on a 50 x 50 cm ground plate minimum, peak measurement with a conservative empirical correction to estimate QP, a 6 to 10 dB margin against the final limit. See CE pitfalls for documentation pitfalls.

Common pitfalls in conducted-emissions campaigns

Five mistakes recur in first-pass lab reports.

1. LISN saturation by inrush current. A switched-mode supply with no inrush-current limit can saturate the LISN's 50 microhenry choke for the first tens of milliseconds after power-on. The impedance presented to the EUT collapses, and the steady-state measurement is biased. Fix: add a soft-start (NTC, or active MOSFET-based) at the mains input, or use a high-power LISN.

2. Wrong LISN class. Testing a 32 A three-phase industrial device with a single-phase 16 A LISN is observable, but the reverse is more subtle: testing a single-phase 6 A device with a 100 A industrial LISN gives pessimistic measurements in the low band because of the higher parasitic inductance. The rule is to pick a LISN matched to the product's rated current.

3. Insufficient ground bonding. A LISN connected to the ground plane by a 50 cm wire instead of a 5 cm copper strap has non-negligible HF impedance above a few MHz. The emissions then leak back into the LISN housing instead of going to the RF port, and the measurement can drift by 3 to 8 dB in the high band.

4. Undocumented cable arrangement. The looping, position and length of power and interface cables change the parasitic capacitance to earth and the common-mode coupling. CISPR 16-2-1 imposes a repeatable setup procedure: 1 m exactly between EUT and LISN power lead, looped if longer, peripherals laid out per a defined pattern. A campaign without a documented arrangement is not reproducible.

5. Configuration change between QP and AVG runs. The final test is two sweeps: one in QP, one in AVG. If lab temperature, product orientation or cable position move between the two, the runs are no longer comparable. The report has to record identical configuration between the two.

See FCC pitfalls for the US-specific pitfalls.

Articulation with other EMC tests

Conducted emissions are one of the seven axes of the full EMC campaign of a mains product. The others: radiated emissions (30 MHz to 1 GHz and above), current harmonics (IEC 61000-3-2, IEC 61000-3-3), the immunity battery (ESD, surge, EFT, radiated RF, conducted RF, voltage dips).

The typical lab session runs conducted emissions in the morning (fast setup, immediate debug possible), moves to radiated in a semi-anechoic chamber in the afternoon, and reserves immunity for the following days. For radio products (Wi-Fi, BLE, cellular), the campaign adds RED Article 3.1(b) tests with a modulated carrier, on top of the generic EMC tests rather than instead of them. See RED tests for the detail.

Key takeaways

• Conducted emissions cover 150 kHz to 30 MHz at the power port, measured at the LISN (Line Impedance Stabilisation Network, 50 ohms / 50 microhenrys plus 5 ohms).
• CISPR 32, CISPR 11 and FCC Part 15 share the same numeric thresholds in residential Class B, allowing a single design path on the mains input for EU plus US.
• Two detectors apply jointly: quasi-peak and average. Exceeding either fails the test.
• The current-probe method is an alternative for battery products or interface cables, with its own dB(microampere) limits.
• Ground plane, LISN bonding and cable arrangement are the three main sources of the pre-compliance to full-compliance gap.
• X capacitor (DM), Y capacitor (CM) and common-mode choke filtering covers most cases, with the Y leakage-current legal cap at 0.25 mA in residential.

For implementation on the CE side, see CE tests. On the US side, see FCC tests. For term definitions, see the Glossary.

Working on sizing the mains filter and preparing a conducted-emissions campaign for a real product? AESTECHNO can audit your design and certification plan. Get in touch →

Sources & references

1. CISPR 16-1-2:2014+A2:2017, Artificial networks for the measurement of conducted disturbances , IEC webstore.iec.ch/publication/61808
2. CISPR 32:2015+A1:2019 / EN 55032, Multimedia equipment, emission requirements , IEC webstore.iec.ch/publication/26241
3. CISPR 11:2024 / EN 55011, Industrial, scientific and medical equipment, emission requirements , IEC webstore.iec.ch/publication/67721
4. 47 CFR Part 15, Radio frequency devices, Subpart B emission limits , FCC / eCFR www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15
5. ANSI C63.4-2014, Methods of measurement of radio-noise emissions , IEEE / ANSI standards.ieee.org/ieee/C63.4/5536/
6. Directive 2014/30/EU on electromagnetic compatibility , EUR-Lex eur-lex.europa.eu/eli/dir/2014/30/oj