Transformer Safety: IEC 61558 Insulation and Testing

Transformer safety under IEC 61558 depends on the complete insulation system, operating conditions and verification tests, not on an isolation-voltage figure alone.

This article explains insulation classes, creepage, clearance, winding construction and partial-discharge assessment for engineers designing or selecting power transformers and switch-mode power supplies.

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It develops the topics covered in the Würth Elektronik presentation by Antwi Nimo and Markus Häpe, with particular attention to the relationship between component requirements and final-equipment approval. [1]

Key Takeaways

IEC 61558-1 provides general transformer safety requirements; SMPS transformers require the applicable particular requirements of IEC 61558-2-16 as well. Final-equipment requirements remain relevant.
Functional insulation does not provide electric-shock protection. Double insulation combines basic and supplementary insulation; reinforced insulation provides equivalent protection through a qualified insulation system.
Clearance is measured through air; creepage follows an insulating surface. Select each using the applicable standard, voltage characteristics, insulation function and environmental conditions rather than a universal millimetres-per-volt rule.
Fully insulated winding wire and triple-insulated wire can support compact constructions, but wire names and layer counts do not establish the insulation rating of a finished transformer.
Dielectric withstand and partial-discharge tests assess different aspects of insulation. Apply the test requirements for the particular construction, voltage and standard rather than a blanket rule for every reinforced transformer.
Component selection must preserve the documented insulation construction, winding-to-core separation, thermal limits and PCB interface. A component assessment does not replace safety evaluation of the assembled equipment.

1. Transformer Safety Standards: Scope and Definitions

Transformer safety begins by identifying the component’s function and the safety standard applicable to the equipment containing it. The following guidance concerns power transformers and transformers used in SMPS; signal transformers also require application-specific bandwidth, signal-integrity and interface assessments.

1.1 IEC 61558 overview: scope and structure

IEC 61558-1:2017 addresses electrical, thermal and mechanical safety for the dry-type transformers, reactors, power supply units and combinations within its scope. It covers encapsulated and non-encapsulated windings, but not transformers forming part of the electricity distribution network. The relevant particular part supplements or modifies the general requirements for a specific transformer or power-supply application. [2]

IEC 61558-2-16:2021 covers switch-mode power supply units and transformers for SMPS and is used together with IEC 61558-1:2017. The 2021 revision includes definitions of voltage characteristics and an alternative clearance-dimensioning method; partial-discharge requirements and construction descriptions were moved to Part 1. Corrigendum 1 was published in 2023 and is included in the IEC’s corrected publication. [3, 4]

Standard	Application area	Role in transformer assessment
IEC 61558-1	Transformers, reactors and power supply units within its scope	General electrical, thermal, mechanical, construction and test requirements
IEC 61558-2-16	SMPS and transformers for SMPS	Particular requirements used with IEC 61558-1
IEC 62368-1	Audio/video, information and communication technology equipment	Equipment-level safeguards and acceptance requirements for the transformer in its application
IEC 60601-1	Medical electrical equipment	Equipment-level safety requirements; a general transformer rating does not establish medical suitability
IEC 61010-1	Measurement, control and laboratory equipment	Equipment-level safety requirements for the intended circuits and operating conditions

Table 1. Relationship between transformer standards and common equipment safety standards. The applicable edition and national adoption must be identified for the product assessment. [1–3]

1.2 Insulation levels and design impact

Insulation terminology describes a protective function, not simply a number of physical layers. Basic and supplementary insulation have different roles; double and reinforced insulation are alternative ways of providing the required level of protection. [1, 5]

Functional insulation is needed for correct operation but does not provide protection against electric shock.
Basic insulation provides basic protection against electric shock.
Supplementary insulation is independent insulation added to basic insulation to provide protection if the basic insulation fails.
Double insulation comprises both basic and supplementary insulation.
Reinforced insulation is an insulation system providing protection equivalent to double insulation. It is not defined universally as three layers of wire insulation.

Insulation arrangement	Possible construction approach	What must be demonstrated
Basic insulation	A qualified barrier, wire construction or combination of insulating materials	Compliance with the applicable basic-insulation construction, distance and test requirements
Supplementary insulation	An independent barrier additional to basic insulation	Independence from the basic insulation and compliance with supplementary-insulation requirements
Double insulation	Basic and supplementary insulation in the complete winding assembly	Both protective functions, including relevant winding-to-core paths
Reinforced insulation	A qualified wire, barrier or combined insulation system	Protection equivalent to double insulation under the applicable standard

Table 2. Insulation functions and construction approaches. Enamel colour, nominal layer count and a wire trade name are not substitutes for construction assessment. [1, 5]

FIW means fully insulated winding wire. FIW and triple-insulated wire, also called TIW, can reduce the space required by some insulation arrangements, but their permitted use depends on the wire specification, insulation grade, winding combination, mechanical protection, terminations and applicable standard. IEC 61558-1:2017 introduced FIW-specific tables and ageing tests; FIW is therefore a defined construction route, not a generic substitute for every tape-insulated design. [1, 2]

2. Core Technical Safety Factors

Electrical spacing and insulation materials must be selected against the actual stresses at each insulation boundary. Mechanical layout, contamination, temperature and voltage waveform interact, so changing one design input can invalidate a previously acceptable construction.

2.1 Mechanical, environmental, thermal and electrical factors

Mechanical: specify bobbin material, winding geometry, margin or shelf tape, barriers, sleeves and terminal routing. Check that winding movement, lead placement and soldering do not reduce the intended separation.
Environmental: determine the local conditions around the insulation, including dust, moisture and condensation. An enclosed transformer and exposed PCB terminals may experience different local environments.
Thermal: assess winding and core heating in the final assembly. Higher power density can increase hot-spot temperature and insulation ageing even when the nominal ambient rating appears adequate.
Electrical: specify rated supply voltage, RMS and peak working voltage across each barrier, relevant transient stresses, waveform and frequency. Current and losses determine thermal loading; voltage stresses determine much of the insulation-coordination assessment.

Include maximum operating altitude in the specification because reduced air density affects clearance requirements. The presentation identifies operation above 2,000 m as an input requiring particular attention; use the applicable standard’s method rather than applying an arbitrary spacing increase. [1, 2]

2.2 Overvoltage categories and pollution degrees

Overvoltage category describes the expected transient environment associated with the circuit’s position and protection within an installation. It is not a general product-quality rating and cannot be assigned merely because a device is handheld, industrial or battery-powered. Category selection must follow the applicable equipment and installation requirements. [1]

Overvoltage category	Typical context	Design interpretation
IV	Origin of the low-voltage installation, such as service-entry equipment	Highest transient exposure among these categories at a given system voltage
III	Fixed-installation distribution and switching equipment	Higher transient-withstand requirements than Category II at a given system voltage
II	Energy-consuming equipment connected to the installation, including many plug-in products	Apply the category and supply conditions established for the equipment
I	Circuits with measures limiting transient overvoltages to the specified lower level	Requires a justified transient environment; not synonymous with handheld electronics

Table 3. Overview of overvoltage-category contexts. Category primarily informs transient withstand and clearance selection; it does not independently determine creepage. [1]

Pollution degree concerns contamination at the insulation surface and its effect on conductivity. The presentation distinguishes the following conditions, which are more useful than assigning a degree solely from the building or industry name. [1]

Pollution degree 1: no pollution, or only dry non-conductive pollution that has no influence on insulation.
Pollution degree 2: normally non-conductive pollution, with temporary conductivity possible because of occasional condensation.
Pollution degree 3: conductive pollution, or dry non-conductive pollution that becomes conductive because condensation is expected.

Do not assume that a laboratory is automatically pollution degree 1 or that every transformer in industrial equipment is pollution degree 3. Determine the local environment after considering the enclosure and any qualified sealing or encapsulation.

2.3 Clearance and creepage: selection in practice

Clearance is the shortest distance through air between conductive parts. Creepage is the shortest distance along an insulating surface between conductive parts. Distance through solid insulation is a separate construction parameter; an acceptable air gap or surface path does not establish the adequacy of a solid barrier. [5]

Design input	Clearance assessment	Creepage assessment
Voltage characteristics	Relevant transient, temporary, recurring and peak-voltage stresses under the selected method	Working-voltage quantity specified by the applicable table or method
Insulation function	Basic, supplementary or reinforced requirements	Basic, supplementary or reinforced requirements
Environment	Applicable pollution conditions and operating altitude	Pollution degree at the insulating surface
Insulating material	Geometry and the applicable field and construction assumptions	Material group derived from comparative tracking index, where applicable
Waveform and frequency	Relevant recurring stress and frequency-dependent provisions	Frequency-dependent provisions and specified voltage measure
Finished assembly	Shortest air path, including terminals, core and PCB interface	Shortest permitted surface path, including bobbin, barriers and terminations

Table 4. Inputs to clearance and creepage selection. The applicable standard determines how these inputs are combined; this table is not a numerical spacing calculator. [1, 5]

Comparative tracking index, or CTI, characterises a material’s resistance to surface tracking under a specified test. The associated material group can affect creepage requirements. CTI does not establish the dielectric strength, thermal endurance or mechanical suitability of the complete insulation system. [1, 5]

For SMPS, determine the actual voltage across the insulation boundary, including DC offsets and repetitive switching excursions. Record RMS and peak values and the relevant frequencies. The converter’s input voltage or a winding’s output voltage alone may not describe the stress between primary, secondary and core. [1]

IEC 61558-2-16:2021 includes an alternative clearance-dimensioning method, and the presentation highlights rated supply voltage in the revised clearance approach. This does not mean that rated supply voltage universally replaces working voltage for both clearance and creepage. Apply the voltage definitions and tables belonging to the selected method consistently. [1, 3]

A statement such as “400 V requires 7 mm creepage” is incomplete without the standard, insulation function, pollution degree, material group and relevant waveform conditions. Document the selected table, any permitted interpolation, the required minimum and the manufactured minimum rather than carrying an isolated distance into a new design.

3. Transformer Construction and Testing

Compliance depends on preserving the intended insulation system in a manufacturable assembly. Wire selection, barriers, terminations and winding-to-core separation must remain consistent with the construction that is assessed and tested.

3.1 Practical insulation solutions

Conventional enamelled wire can be used with suitable bobbins, insulating barriers, tapes and sleeves. Ordinary magnet-wire enamel must not be assumed to provide the required safety insulation merely because it passes a winding continuity or resistance test. FIW and TIW offer other construction routes, but their use must follow the relevant construction requirements. [1]

Margin or shelf tape: establishes controlled winding margins, but the finished surface path depends on the actual winding, tape and termination geometry.
Extended-rail and multi-section bobbins: provide separation and longer paths where the geometry supports the required insulation function.
FIW and TIW: require the specified insulation grade, mechanical protection and winding combination. Contact with other conductors and wire crossings must follow the applicable construction rules.
Terminations: control stripped-wire length, insulation burnback, lead retention and sleeving so that the intended barrier continues to the connection point.
Core insulation: assess primary-to-core and secondary-to-core paths as well as direct primary-to-secondary separation. Toroidal and bobbin-based constructions require different mechanical solutions.

Potting and encapsulation can protect insulation from contamination and support compact assemblies, but applying resin or gel does not automatically justify reduced distances or a lower pollution degree. The qualified construction must address the actual seal and material system. Potting-process shrinkage or expansion can also create incomplete seals or mechanical damage, so the process requires validation. [1, 6]

3.2 Dielectric withstand and partial-discharge assessment

A dielectric withstand test, often called a hipot test, applies a specified voltage for a specified time to check the insulation against the defined acceptance criteria. It does not, by itself, establish a safe continuous working voltage or the complete insulation rating. Partial-discharge testing measures localised discharge activity that can occur without complete breakdown of the insulation barrier. [1, 6]

The published change summary for IEC 61558-1:2017 specifically identifies the introduction of partial-discharge testing above 750 V for FIW constructions. IEC 61558-2-16:2021 states that partial-discharge requirements and construction descriptions were moved to Part 1. These statements do not establish a universal test rule for every TIW transformer or every reinforced-insulation design. Determine applicability from the construction clauses and the voltage definitions used in the selected standard. [2, 3]

A partial-discharge test specification needs more than a threshold voltage. Define the winding connections, conditioning and measurement voltages, waveform, timing, measurement arrangement and permitted apparent charge. Keep qualification tests and production acceptance tests distinct, and use the procedure required for the assessed construction. [1, 6]

A catalog datasheet illustrates why the conditions matter. Würth Elektronik part 750320974 lists reinforced insulation at 500 V RMS and 700 V peak with an operating main-bus frequency not exceeding 700 kHz, but basic insulation at 1,000 V RMS and 1,400 V peak with that frequency not exceeding 200 kHz. Both statements specify overvoltage category II and pollution degree 2. These are conditional ratings for that component, not general IEC spacing rules. [6]

The same datasheet separately lists a 4,000 V AC insulation test and a partial-discharge limit of 10 pC maximum under a sequence of 1,850 V peak for 5 s and 1,480 V peak for 15 s. The insulation grade, continuous working-voltage conditions and test voltages must therefore remain separate entries in a component specification. Do not copy this test sequence to a different construction without justification. [6]

3.3 Thermal, mechanical and production verification

Electrical testing is only part of the assessment. IEC 61558-1 also addresses thermal and mechanical safety, and its revision summary includes FIW ageing tests, temperature measurement, short-circuit and overload protection, and toroidal-core construction requirements. A satisfactory hipot or partial-discharge result does not replace those requirements. [2]

Validate the transformer in the intended equipment because winding temperature depends on electrical losses, ambient temperature, airflow, mounting and nearby heat sources. Control the safety-critical materials and construction dimensions in the manufacturing documentation, and define acceptance tests for the released part rather than relying on a prototype description. [2, 6]

4. Harmonisation with Other Standards

Transformer and equipment standards address related hazards, but their requirements and acceptance routes are not interchangeable. Establish the intended product markets, equipment standard and component-documentation route before freezing the winding construction.

4.1 IEC 62368-1 compatibility

IEC 61558 and IEC 62368-1 use related concepts such as insulation functions, creepage, clearance and dielectric testing. However, acceptance in audio/video or ICT equipment depends on the provisions of the applicable equipment standard and the transformer’s actual construction. The presentation identifies additional concerns such as mechanical protection at wire crossings, retention of winding end turns and restrictions on contact between different wire constructions. [1]

Do not describe an IEC 61558 assessment as automatic IEC 62368-1 approval. Confirm whether a component certificate, test report or additional construction evaluation is required, and check the applicable national adoption and deviations with the assessment laboratory.

4.2 Medical, laboratory and regional approval paths

Medical electrical equipment and measurement, control or laboratory equipment require assessment under their respective equipment standards. A transformer described as reinforced under one set of conditions is not automatically suitable for every patient-protection or measurement application. Specify the actual insulation function and application conditions rather than relying on an industry label. [1]

Keep thermal insulation-system recognition separate from electric-shock protection. A recognised temperature class describes a thermal capability within its documented scope; it does not independently establish reinforced insulation, allowable working voltage or equipment approval. Similarly, “designed to comply” is not the same statement as an independently issued certificate for the exact part. [1, 6]

5. Transformer Safety Design and Selection Checklist

Compare candidate transformers against a written operating envelope and evidence package. Catalog parts are useful when their documented conditions fit the application; custom parts are appropriate when the required winding arrangement, electrical performance or insulation construction cannot be met by a standard component.

5.1 Define the application before comparing parts

Identify the equipment standard, applicable IEC 61558 particular part, national adoption and intended product markets.
Specify topology, supply range, power, switching-frequency range, voltage waveforms and peak and RMS winding currents.
Define the required insulation function at each boundary, including primary-to-secondary, winding-to-core and isolated secondary-to-secondary paths where relevant.
Record rated supply voltage, working-voltage RMS and peak values, overvoltage category, pollution degree, altitude and ambient conditions.
Set thermal limits and cooling conditions, including the intended enclosure and neighbouring heat sources.

5.2 Compare datasheet parameters and construction evidence

Check turns ratio, inductance and tolerance, permissible volt-seconds, winding resistance, leakage inductance and interwinding capacitance with their stated measurement conditions.
Compare the working-voltage rating and insulation grade under the actual frequency, waveform, pollution-degree and overvoltage-category conditions.
Check dielectric withstand and partial-discharge specifications separately, including test connections, voltage, duration and acceptance criteria.
Confirm creepage, clearance, solid-insulation construction, wire specification, winding-to-core insulation and termination details.
Preserve the required separation at the PCB land pattern, nearby copper and mounting interface; component dimensions alone do not define every assembled path.
Verify the exact scope of certificates and test reports, including part identification, construction revision, standard edition and any conditions of acceptance.

5.3 Qualification, purchasing and change control

For a custom transformer, release a controlled drawing covering winding order, wire types, insulation materials, safety-critical dimensions, electrical limits and acceptance tests. For a catalog transformer, retain the applicable datasheet revision and supporting safety documentation. In both cases, establish traceability and review supplier changes affecting the approved construction. [6]

Do not approve an alternate solely because turns ratio, inductance and package dimensions match. Recheck the insulation system, voltage and frequency conditions, thermal behaviour, parasitics, assembly process and documentary evidence. A wire, tape, bobbin or potting substitution can require renewed assessment even when nominal electrical values are unchanged.

Conclusion

IEC 61558 compliance is an insulation-system and construction assessment supported by electrical, thermal and mechanical verification. Correct creepage and clearance selection requires defined voltage stresses, environmental conditions, insulation functions and an identified standards method.

FIW, TIW, engineered bobbins and encapsulation provide useful design options, but none removes the need to assess terminations, core paths, temperature and manufacturing control. Select transformers using conditional working-voltage ratings and relevant test evidence, then confirm that the assembled equipment preserves the required protection.

FAQ

These questions address common transformer specification, insulation and approval decisions.

What is IEC 61558 and which part applies to SMPS transformers?

IEC 61558 is a series addressing safety of transformers, reactors and power supply units within its scope. IEC 61558-1 provides general requirements, while IEC 61558-2-16 provides particular requirements for SMPS and their transformers. The relevant parts are used together, and the final equipment must also meet its applicable safety requirements.

What is the difference between basic, double and reinforced insulation?

Basic insulation provides basic protection against electric shock. Double insulation combines basic insulation with independent supplementary insulation. Reinforced insulation provides protection equivalent to double insulation through a qualified insulation system. These terms describe protective functions and requirements, not a universal number of wire layers, enamel colours or tape wraps.

How do overvoltage category and pollution degree affect transformer spacing?

Overvoltage category describes the expected transient environment and informs transient withstand and clearance selection. Pollution degree describes contamination and conductivity at the local insulation surface and affects spacing assessment. Neither parameter should be assigned from a product label alone. Creepage also depends on factors including working voltage, material group and insulation function.

When is partial-discharge testing required for a transformer?

Applicability depends on the standard, voltage definitions and insulation construction. The IEC 61558-1:2017 change summary identifies partial-discharge testing above 750 V for FIW constructions, but this is not a blanket rule for every TIW or reinforced transformer. The assessment must establish the required test sequence and acceptance criteria for the particular design.

Can conventional enamelled wire provide a compliant transformer construction?

Yes. A transformer using conventional enamelled wire can meet the required insulation function through suitable bobbins, barriers, tapes, sleeves and controlled winding geometry. Ordinary enamel alone must not be assumed to provide the required safety insulation. Compliance concerns the complete assembly, including terminations, winding-to-core paths, thermal performance and the applicable tests.

Does a high hipot rating prove reinforced insulation or medical suitability?

No. A hipot rating states a dielectric test condition, not the complete continuous working-voltage or insulation rating. Reinforced insulation also requires the relevant construction, spacing and verification evidence. Medical suitability depends on the equipment requirements and intended insulation function. Review the exact component documentation and final application rather than relying on a test-voltage headline.

How to Specify and Validate Transformer Insulation

This procedure connects application requirements to a controlled transformer specification. It supports design and approval planning rather than replacing the applicable standard or final-equipment assessment.

Specify and validate a transformer insulation system from application requirements to controlled production documentation.

Define application and environmental conditions
Identify the equipment standard, relevant transformer standard, intended markets and operating environment. Specify overvoltage category, local pollution degree, maximum altitude, ambient conditions and the required insulation function at each boundary.
Establish voltage and thermal stresses
Record rated supply voltage, RMS and peak working voltage, waveform and frequency across each insulation boundary. Include transient conditions, winding currents, losses and cooling conditions so the electrical and thermal assessments use the intended operating envelope.
Select wire and insulation construction
Choose conventional enamelled wire with suitable barriers, FIW or TIW as permitted for the required insulation function. Define winding order, bobbin, tapes, sleeves, termination protection and winding-to-core separation in a controlled construction drawing.
Dimension and verify insulation paths
Use the applicable standard method to determine clearance, creepage and solid-insulation requirements. Record the governing conditions and required minima, then check the finished transformer and PCB interface with manufacturing tolerances and termination geometry included.
Perform the required verification tests
Establish dielectric withstand, partial-discharge applicability, thermal and mechanical tests for the selected construction. Define connections, voltages, timing and acceptance criteria, and distinguish qualification testing from production acceptance testing.
Complete approval documentation and change control
Compile construction drawings, material specifications, calculations, test records and relevant component certificates or reports. Confirm acceptance in the final equipment and establish traceability, supplier change notification and reassessment rules for substitutions.

References

The following standards summaries, presentation and manufacturer documents support the technical discussion. Construction decisions and compliance testing require the applicable complete standards and component documentation.