EMC frequently asked questions
Answers to common questions about EMC: what it is, who needs it, what tests are done, how to fix failures, pre-compliance, CE and FCC basics and more.
The basics
What is the difference between EMC and EMI? EMI (electromagnetic interference) is the disturbance. EMC (electromagnetic compatibility) is the goal: equipment that neither causes unacceptable interference nor is unacceptably affected by it. See What is EMC?
What is the difference between emissions and immunity? Emissions are what a product puts out; immunity is what it can withstand. Both are tested, and both are usually required.
Does every electronic product need EMC testing? Almost every market requires evidence of EMC compliance for most electrical and electronic products, but the route differs (declaration, certification, registration). Some products and categories are exempt or covered by other rules. See Regulations by region.
Is EMC only about radio? No. It also covers conducted noise on cables, electrostatic discharge, surges, voltage dips, harmonics, flicker and magnetic fields.
Testing
What tests are typically required? Radiated and conducted emissions, harmonics and flicker for mains-powered equipment, and immunity tests such as radiated fields, conducted RF, ESD, EFT/burst, surge and voltage dips. The exact list comes from the product or generic standard. See the standards map.
Why do radiated emission tests use 3 m or 10 m distances? Standards define limits at specific distances so that results are comparable. Limits differ with distance because the field falls off with distance.
What is a quasi-peak detector and why is it used? It weights disturbances by how often they repeat, to reflect their annoyance to listeners of broadcast radio. Emission limits below 1 GHz are defined for it. See Radiated emissions.
What is “pass” in an immunity test? The product standard defines performance criteria (A, B, C) for acceptable behaviour during and after the test. See Radiated immunity.
What does measurement uncertainty mean for a pass or fail? Each result has an uncertainty (computed per CISPR 16-4-2). Some standards say a product passes if the measured value is below the limit; others include the uncertainty in the decision. Check the standard and the regulator’s rule.
Do I need an accredited laboratory? It depends on the regulation. For EU CE marking for EMC no accredited laboratory is required by the Directive, though test reports are often expected from competent laboratories. Some regimes (for example certain US and Asian schemes) require recognised or accredited laboratories. Check the rules that apply to your product.
Failing and fixing
My product failed radiated emissions. What should I check first? Common-mode current on cables (use a current probe), the switching power supply, high-speed interfaces and enclosure seams. See Design for EMC.
Do ferrites fix everything? No. A ferrite on a cable can reduce common-mode current at some frequencies but it treats the symptom. Fixing the source or the return path is usually more robust.
What is a good first filter? A mains filter with common-mode and differential-mode stages placed at the inlet, bonded to the chassis. Performance depends on layout and impedance as well as on the filter itself.
Can I do pre-compliance testing myself? Yes. Near-field probes, a current probe, a spectrum analyser and a LISN find most problems early. Pre-compliance results are not a substitute for formal tests at a calibrated, validated site.
Regulations
Is the CE marking an EMC certificate? No. It is the manufacturer’s declaration that the product meets all applicable EU laws. For the EMC Directive the manufacturer assesses conformity; test reports are the evidence.
What is the difference between CE and UKCA? CE is the EU marking; UKCA is the marking for Great Britain. Both mean the manufacturer declares conformity with the applicable legislation of that territory. Check GOV.UK for where CE is still accepted.
Do FCC rules require immunity testing? No general immunity requirement exists in the FCC Part 15 rules for most products; compliance is about emissions. Other regulators (for example for medical devices) or customers may require immunity tests.
What is Class A versus Class B? Class A limits apply to equipment used in commercial and industrial environments; Class B limits are stricter and apply to equipment used in residential environments.
Are standards free? Standards are copyrighted publications and are sold by the IEC, ISO and national standards bodies. Laws that cite them are free.
Sector questions
Is automotive EMC different? Yes. The law is UN Regulation No. 10; test methods are CISPR 25, ISO 11452, ISO 11451 and ISO 7637, and manufacturers add their own specifications. See Automotive EMC.
Is medical-device EMC different? Yes. IEC 60601-1-2 defines immunity levels by use environment (professional healthcare facility, home healthcare) and links EMC to basic safety and essential performance.
What about wireless products? Products with intentional radio transmitters are covered by radio regulations (for example the EU RED) in addition to, or instead of, EMC rules; the radio standards deal with spectrum use and the EMC standard (for example ETSI EN 301 489 in the EU) with unintentional emissions and immunity.
Asking more
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