Conducted Emission Limits
Conducted emission limits define the maximum permissible levels of electromagnetic interference that electronic equipment may inject onto power lines, signal cables, and other conductors connected to external systems. These limits form the foundation of electromagnetic compatibility regulations worldwide, ensuring that electronic devices can coexist on shared electrical infrastructure without degrading power quality or interfering with other equipment.
Understanding conducted emission limits requires familiarity with multiple regulatory frameworks, measurement techniques, and the specific requirements that apply to different product categories and markets. This comprehensive guide covers the major international standards governing conducted emissions, from consumer electronics to industrial equipment, automotive systems, and military applications.
FCC Part 15 Subpart B
The Federal Communications Commission (FCC) Part 15 Subpart B establishes conducted emission limits for unintentional radiators in the United States. These regulations apply to digital devices and other electronic equipment that may generate radio frequency energy as a byproduct of their operation, covering the frequency range from 150 kHz to 30 MHz for conducted emissions measured at the AC power port.
Part 15 defines two classes of digital devices with different emission limits. Class A devices are intended for use in commercial, industrial, or business environments where interference to residential radio and television reception is less likely to occur. Class B devices are marketed for use in residential environments and must meet more stringent limits to protect nearby consumer electronics from interference.
Class A and Class B Limits
Class B limits are approximately 10 dB more stringent than Class A limits across the measured frequency range. For conducted emissions, Class B limits require the quasi-peak detector reading to remain below 48 dB microvolts from 150 kHz to 500 kHz, decreasing to 48 dB microvolts from 500 kHz to 30 MHz. Class A limits permit 60 dB microvolts from 150 kHz to 500 kHz and 60 dB microvolts from 500 kHz to 30 MHz using the quasi-peak detector.
Average detector limits are typically 10 dB below the quasi-peak limits for both classes. The dual-limit approach using both quasi-peak and average measurements ensures that both impulsive and continuous emissions are adequately controlled. Products must meet both sets of limits to achieve compliance, with the more restrictive limit determining design requirements at each frequency.
Measurement Requirements
FCC conducted emission measurements utilize a line impedance stabilization network (LISN) to provide a defined impedance at the equipment power port and to couple high-frequency emissions to the measurement receiver. The standard LISN presents a 50-ohm impedance to common-mode currents while providing isolation from variations in the actual power grid impedance. Measurements are performed on each power conductor (line and neutral) with the other conductor terminated.
The equipment under test operates in a configuration that produces maximum emissions, typically with representative peripherals connected and software executing typical operations. Multiple operating modes may require evaluation to ensure compliance under all conditions. Pre-scan measurements using peak detection identify frequencies of interest, followed by final measurements using quasi-peak and average detectors at each critical frequency.
CISPR 14: Household Appliances
CISPR 14-1 establishes emission requirements for household appliances, electric tools, and similar apparatus. This international standard addresses the unique emission characteristics of products containing motors, heating elements, switching controls, and other components that generate broadband and narrowband interference. The standard covers equipment operating from AC mains power in residential and commercial environments.
Unlike information technology equipment, household appliances often produce discontinuous interference synchronized with mains power cycles or mechanical operations. CISPR 14 includes provisions for measuring emissions from intermittent sources and click-producing devices, recognizing that brief disturbances have different interference potential than continuous emissions.
Emission Categories
CISPR 14-1 categorizes products based on their interference characteristics. Continuous disturbance sources include motor-driven appliances, electronic controls, and heating elements with electronic regulation. Discontinuous disturbance sources produce interference in bursts related to switching operations or mechanical cycles. Click-producing devices generate isolated disturbances during on-off switching or thermostat operation.
Each category has specific measurement procedures and limits tailored to its emission characteristics. Continuous interference is measured using quasi-peak and average detectors similar to other standards. Click measurements use a click analyzer or equivalent technique to assess the rate and amplitude of isolated disturbances. The applicable limits depend on both the disturbance category and the specific product type.
Frequency Ranges and Limits
CISPR 14-1 specifies conducted emission limits from 150 kHz to 30 MHz at the mains terminals. The limits vary by product type and interference category, with typical quasi-peak limits ranging from 66 dB microvolts at 150 kHz decreasing to 56 dB microvolts at 500 kHz, then remaining relatively constant through 30 MHz. Average limits are 10 dB lower throughout the frequency range.
Additional provisions address emissions on load and control terminals where applicable. Products with electronic controls or power electronics may require testing at terminals other than the mains input to ensure complete characterization of their emission behavior. Terminal voltage limits and injection current limits provide alternative approaches depending on the circuit configuration.
CISPR 15: Lighting Equipment
CISPR 15 addresses electromagnetic disturbances from lighting equipment, including fluorescent lamps, LED luminaires, high-intensity discharge lamps, and their associated control gear. The proliferation of electronic ballasts and LED drivers has increased the relevance of this standard, as these power conversion circuits generate conducted emissions similar to switching power supplies in other product categories.
The standard recognizes the unique operating characteristics of lighting equipment, including inrush currents during lamp starting, variations in emission levels with dimming, and the distributed nature of lighting installations where multiple luminaires may combine their emissions on shared circuits. Limits are structured to permit practical lighting products while protecting radio services from interference.
Product Categories
CISPR 15 covers several categories of lighting equipment with different applicable limits. Self-ballasted lamps (integrated electronic ballasts in lamp form factor) have specific limits reflecting their consumer nature and widespread residential use. Independent ballasts and LED drivers may be tested separately or as part of complete luminaire assemblies depending on how they are marketed and installed.
Specialized lighting including ultraviolet lamps for non-lighting applications and emergency lighting systems have additional provisions addressing their specific characteristics. Professional lighting equipment intended for broadcast, stage, or studio applications may apply different limits appropriate to their controlled operating environments.
Conducted Emission Requirements
Conducted emission limits in CISPR 15 extend from 150 kHz to 30 MHz with limit values that depend on the product category and lamp power rating. Limits generally decrease with increasing frequency, following patterns similar to other CISPR standards. Quasi-peak and average measurements are required, with products needing to comply with both sets of limits.
Special provisions address dimmed operation, where emission levels may increase significantly compared to full power operation. Products with dimming capability must meet limits across their dimming range, typically at settings that produce maximum emissions. Start-up transients are also considered, with allowances for brief excursions above steady-state limits during lamp ignition sequences.
EN 55011: Industrial, Scientific, and Medical Equipment
EN 55011 (harmonized from CISPR 11) establishes emission requirements for industrial, scientific, and medical (ISM) equipment that intentionally generates or uses radio frequency energy for purposes other than telecommunications. This standard covers a broad range of equipment including induction heaters, microwave ovens, medical diathermy equipment, industrial plasma systems, and scientific research apparatus.
ISM equipment presents unique challenges for emission control because it intentionally generates high levels of RF energy for process applications. The standard balances the need to limit interference with recognition that some emissions are inherent to the equipment function. Class A and Class B designations similar to other standards distinguish between industrial and residential use environments.
Group and Class Designations
EN 55011 classifies equipment into two groups based on their RF generation characteristics. Group 1 equipment does not intentionally generate RF energy for material processing but may contain internal RF sources such as microprocessors, power electronics, or ignition systems. Group 2 equipment intentionally generates or uses RF energy in the form of electromagnetic radiation, inductive coupling, or capacitive coupling for material treatment.
Within each group, Class A equipment is suitable for use in all establishments other than domestic and those directly connected to low-voltage power supply networks serving residential buildings. Class B equipment is suitable for use in domestic establishments and in establishments directly connected to residential power supply networks. The more stringent Class B limits apply to products like household microwave ovens intended for consumer use.
Mains Port Limits
Conducted emission limits at the mains terminal are specified from 150 kHz to 30 MHz for all equipment categories. Group 1 Class A limits are comparable to other industrial equipment standards, with quasi-peak limits of 79 dB microvolts from 150 kHz to 500 kHz decreasing to 73 dB microvolts from 500 kHz to 30 MHz. Group 1 Class B limits are approximately 13 dB more stringent.
Group 2 equipment has different limits reflecting the intentional RF generation inherent to its function. Class A Group 2 equipment in specific ISM frequency bands may have relaxed requirements recognizing that emissions in those bands are expected. Outside designated ISM bands, Group 2 equipment must meet limits similar to Group 1 equipment to protect radio services operating in adjacent frequency allocations.
Automotive Conducted Emissions
Automotive conducted emission standards address the unique electromagnetic environment within vehicles, where electronic systems must coexist with each other and with vehicle electrical systems on shared power distribution networks. CISPR 25 provides the primary international framework for automotive component EMC, while manufacturer-specific requirements often exceed these baseline standards.
The vehicle electrical system presents significant differences from utility power networks, including DC power distribution, variable voltage levels during engine cranking and charging, and high levels of conducted disturbances from ignition systems, motor drives, and switched loads. Automotive conducted emission requirements reflect these unique conditions and the need for robust EMC performance in the vehicle environment.
CISPR 25 Requirements
CISPR 25 establishes test methods and limits for conducted disturbances from components and modules installed in vehicles and boats with nominal 12V, 24V, or 48V electrical systems. The standard covers conducted emissions on power supply lines from 150 kHz to 108 MHz, extending to higher frequencies than typical mains-powered equipment to address vehicle radio receivers operating throughout the FM broadcast band and beyond.
Limits are specified in multiple severity levels (typically 1 through 5) allowing vehicle manufacturers to select appropriate requirements based on the component location and sensitivity of nearby electronic systems. Components mounted near antennas or sensitive receivers require more stringent limits than those in remote locations with greater physical separation from susceptible systems.
Measurement Methods
Automotive conducted emission measurements use artificial networks (AN) that present defined impedances to the equipment under test while coupling emissions to measurement receivers. The standard AN provides 50 ohms differential and common-mode impedance, representing typical vehicle wiring characteristics. Current probe methods provide an alternative approach for measuring emissions on wire harnesses.
The vehicle electrical environment simulation includes representative power source impedance, load connections, and ground references appropriate to the component installation location. Test setups replicate the electrical characteristics the component will encounter in the vehicle while enabling repeatable measurements comparable across different test facilities.
Military Conducted Emission Limits
Military conducted emission requirements, primarily defined in MIL-STD-461 for United States defense applications, impose more stringent limits than commercial standards to ensure electromagnetic compatibility in dense military electronic environments. Military platforms including aircraft, ships, ground vehicles, and portable equipment contain numerous sensitive receivers and high-power transmitters that require careful emission control to prevent mutual interference.
The operational environment for military equipment often includes intentional electromagnetic threats, extended frequency ranges, and mission-critical performance requirements that cannot tolerate interference. Conducted emission limits reflect these demanding conditions and the need for reliable operation in electromagnetic environments far more severe than typical commercial or residential applications.
MIL-STD-461 CE Requirements
MIL-STD-461 defines multiple conducted emission requirements designated by CE prefix codes. CE101 addresses conducted emissions on power leads from 30 Hz to 10 kHz, capturing low-frequency disturbances that could affect sensitive analog systems. CE102 covers the primary conducted emission range from 10 kHz to 10 MHz on power leads, with limits varying by platform type (aircraft, ship, ground vehicle, submarine).
Limits are expressed in dB microamps, measuring current rather than voltage as in commercial standards. This approach provides more consistent characterization across the varying source and load impedances encountered in military power distribution systems. Army, Navy, and Air Force platforms have different applicable limits reflecting their specific electromagnetic environments and operational requirements.
Platform-Specific Requirements
Aircraft installations face the most stringent conducted emission limits due to the close proximity of equipment, limited shielding effectiveness of composite airframes, and the critical nature of avionic system performance. Submarine requirements are similarly demanding due to the confined electromagnetic environment and the need for extremely low acoustic and electromagnetic signatures. Surface ship and ground vehicle limits are generally less stringent but still exceed commercial requirements.
In addition to standard limits, military programs may impose additional tailored requirements based on platform-specific electromagnetic analyses. Antenna-to-equipment coupling, safety-critical system protection, and mission equipment performance may drive requirements beyond baseline MIL-STD-461 levels. Electromagnetic compatibility control plans define the complete set of requirements for each military program.
Harmonic Current Limits: IEC 61000-3-2
IEC 61000-3-2 limits harmonic current emissions from equipment drawing less than 16 amperes per phase from public low-voltage supply systems. Unlike RF conducted emission standards that address high-frequency interference, this standard focuses on low-frequency harmonics of the mains power frequency (50 or 60 Hz) that can degrade power quality, cause heating in transformers and wiring, and interfere with power system operation.
Harmonic currents result from nonlinear loads including rectifier input circuits, switched-mode power supplies, variable frequency drives, and electronic ballasts. When large numbers of such loads are connected to power distribution systems, their combined harmonic currents can distort voltage waveforms, cause neutral conductor overheating, and reduce the efficiency of power delivery infrastructure.
Equipment Classes
IEC 61000-3-2 defines four equipment classes with different applicable limits. Class A includes balanced three-phase equipment and all equipment not classified in other categories. Class B covers portable tools, with limits 1.5 times the Class A values. Class C addresses lighting equipment, with limits expressed as percentages of fundamental current. Class D applies to equipment with special waveshapes (high peak factor) with input power between 75W and 600W.
The class distinctions reflect different harmonic emission characteristics and the relative numbers of each equipment type connected to power systems. Lighting equipment limits as percentages account for the wide range of lamp power levels while maintaining reasonable absolute emission levels. Class D captures equipment with particularly distorted current waveforms that produce disproportionate harmonic content.
Harmonic Limits
Class A limits specify maximum current for odd harmonics through the 39th and even harmonics through the 40th. The limits decrease with harmonic order, with the third harmonic limited to 2.30 amperes, the fifth to 1.14 amperes, and higher orders to progressively lower values. These limits apply to equipment with rated input power exceeding 75 watts.
Power factor correction circuits in modern power supplies help meet harmonic limits by shaping input current to approximate sinusoidal waveforms. Active PFC circuits can achieve power factors exceeding 0.99 with total harmonic distortion below 5 percent, easily meeting Class A requirements. Passive power factor correction using inductors provides a simpler but less effective approach suitable for lower power equipment.
Voltage Fluctuation Limits: IEC 61000-3-3
IEC 61000-3-3 limits voltage fluctuations and flicker produced by equipment with rated current up to 16 amperes per phase connected to public low-voltage supply systems. Voltage fluctuations occur when equipment draws varying current from the supply, causing voltage drops in source impedance that affect other loads on the same circuit. Rapid fluctuations can cause visible flicker in lighting, creating annoyance and potential health effects for building occupants.
The standard addresses both steady-state voltage changes and dynamic fluctuations. Sudden load changes from motor starting, heater cycling, or other intermittent loads can cause noticeable lighting flicker even when the resulting voltage change is relatively small. The human visual system is particularly sensitive to flicker at frequencies near 8.8 Hz, which corresponds to the maximum sensitivity of the flicker perception curve.
Flicker Assessment
Flicker severity is quantified using short-term (Pst) and long-term (Plt) flicker indices that weight voltage fluctuations according to their perceptibility. The Pst measurement uses a 10-minute observation period and models the human visual response to voltage fluctuations on incandescent lighting. A Pst value of 1.0 corresponds to the threshold of irritability where 50 percent of observers would find the flicker objectionable.
IEC 61000-3-3 limits Pst to 1.0 and Plt (calculated from successive Pst values over 2 hours) to 0.65. Equipment must demonstrate compliance through measurement using standardized test conditions or through analytical methods when the load characteristics permit calculation of expected flicker severity. Pre-compliance assessment helps identify potential problems before formal testing.
Voltage Change Limits
In addition to flicker limits, IEC 61000-3-3 restricts the magnitude of individual voltage changes. Relative steady-state voltage change (dc) must not exceed 3.3 percent under normal operating conditions. Maximum relative voltage change (dmax) including transient effects must remain below 4 percent for changes occurring more frequently than twice per hour, with higher limits permitted for less frequent events.
Inrush current limiting, soft starting, and power ramp controls help equipment meet voltage fluctuation requirements. Large loads such as motors, heating elements, and capacitive input power supplies may require current limiting during startup to prevent excessive voltage sags. The design tradeoff between fast operation and voltage fluctuation compliance influences equipment architecture for high-power applications.
Compliance Strategies
Achieving compliance with conducted emission limits requires attention to filter design, power supply architecture, grounding strategy, and cable management throughout the product development process. Common-mode and differential-mode filtering at the AC input prevents conducted emissions from reaching power lines, while proper circuit design minimizes emission generation at the source.
Multi-stage input filters combining common-mode chokes, differential-mode inductors, and X and Y capacitors provide effective attenuation across the regulated frequency range. Filter component selection must balance emission attenuation, component size and cost, safety requirements, and insertion loss effects on power supply efficiency. Simulation tools help optimize filter designs before hardware prototyping.
Pre-Compliance Testing
Pre-compliance conducted emission testing during development enables early identification and correction of emission problems. Bench-top LISNs and spectrum analyzers provide measurements comparable to formal compliance testing at lower cost and with greater flexibility for diagnostic investigation. Correlation to compliance measurement facilities helps engineers predict formal test results from pre-compliance data.
Near-field probing and current measurements on internal circuits help identify emission sources and evaluate the effectiveness of mitigation measures. Systematic testing of power supply and filter modifications enables optimization of the EMC design before committing to production tooling and final component selections.
Documentation and Testing
Formal compliance testing requires testing at accredited laboratories following standardized procedures documented in applicable standards. Test reports must include equipment configuration, operating modes, measurement equipment and calibration data, and complete measurement results across the required frequency range. Photographs, block diagrams, and equipment descriptions support the technical file required for regulatory submissions.
Self-declaration of conformity, third-party testing, and type approval processes vary by market and product category. Understanding the applicable conformity assessment procedures ensures that products can achieve market access efficiently while meeting all regulatory obligations. Maintaining compliance documentation throughout the product lifecycle supports ongoing market surveillance and any required re-evaluation for product modifications.
Summary
Conducted emission limits form a critical component of electromagnetic compatibility regulations that enable electronic equipment to share electrical infrastructure without mutual interference. From FCC Part 15 governing digital devices in the United States to CISPR standards harmonized internationally, from specialized requirements for household appliances and lighting to demanding military specifications, these limits define the electromagnetic performance required for regulatory compliance and market access.
Understanding the specific requirements applicable to a product category, designing effective filtering and emission control measures, and validating compliance through appropriate testing are essential competencies for EMC engineers and product developers. The standards continue to evolve as technology advances and new product categories emerge, making ongoing attention to regulatory developments an important aspect of professional practice in electronics design.