Engineering Guide · PCB
PCB Creepage and Clearance Design: Insulation Coordination, CTI, Pollution Degree, and Altitude
Design PCB creepage and clearance using insulation coordination, working and impulse voltage, pollution degree, CTI, altitude, and manufacturing constraints.
Updated October 11, 2026 · Intermediate · 18 min read
Insulation Coordination Is a Design Process
PCB safety spacing begins with the insulation barrier and the electrical stress across it, not with a remembered millimeter value. Creepage and clearance protect against different failure mechanisms, use different inputs, and can follow different physical paths through the same assembly.
This guide turns those inputs into a review workflow. It explains how to use preliminary calculators without mistaking them for standards tables, and how to carry the result through PCB layout, manufacturing tolerance, assembly review, testing, and certification evidence.
Clearance and Creepage Are Separate Checks
| Design question | Clearance | Creepage |
|---|---|---|
| Physical path | Shortest path through air | Shortest path along an insulating surface |
| Primary electrical stress | Impulse withstand and electric-field stress | Continuous or recurring working voltage |
| Important environment inputs | Altitude, field shape, transients, installation category | Pollution degree, condensation, contamination, surface tracking |
| Material influence | Solid-insulation CTI normally does not set the air distance | CTI-derived material group can affect the required surface path |
| Typical layout control | Direct air gap between conductive parts | Surface path around or across laminate, slots, grooves, and barriers |
| Common error | Using nominal DC voltage without checking impulse requirements | Using clearance distance as creepage without tracing the surface path |
PCB Insulation-Coordination Workflow
Complete the decisions in order and preserve their evidence. A downstream spacing value is only as reliable as the voltage, environment, material, and insulation classifications that produced it.
| Step | Decision | Engineering action |
|---|---|---|
| 1 | Define the barrier | Identify the two conductive nodes, accessible parts, protective earth, and whether the insulation is functional, basic, supplementary, double, or reinforced. |
| 2 | Identify every voltage stress | Record steady working voltage, repetitive peaks, mains system, overvoltage category, expected transients, and relevant single-fault conditions. |
| 3 | Choose the governing requirements | Start with the product standard and market requirements. Use a general insulation-coordination standard only where the product standard permits it. |
| 4 | Classify the microenvironment | Determine pollution degree at the actual insulation surface, including condensation, conductive dust, process residue, enclosure protection, and local heating. |
| 5 | Verify material evidence | Obtain CTI or material-group evidence for the released laminate and any credited coating or potting system. Generic FR-4 naming is insufficient. |
| 6 | Determine clearance | Use the required impulse withstand, field geometry, altitude, and applicable table or method to establish the shortest permitted air path. |
| 7 | Determine creepage | Use working voltage, pollution degree, material group, and insulation class to establish the permitted path along the insulating surface. |
| 8 | Add production constraints | Include copper registration, routing tolerance, board-edge tolerance, component lead position, solder, contamination, and assembly variation. |
| 9 | Review the complete assembly | Measure three-dimensional paths around slots, barriers, heatsinks, fasteners, connectors, and enclosure parts, not only copper-to-copper CAD spacing. |
| 10 | Verify and document | Record assumptions, standard edition, table references, measured paths, dielectric or impulse tests, inspection criteria, and certification evidence. |
Using the Calculators Correctly
Creepage Screening
Use working voltage, pollution degree, material group, and insulation type to explore a preliminary surface-path target. Confirm the exact voltage band, CTI evidence, coating treatment, and standard table before release.
Clearance Screening
Use operating context, overvoltage category, pollution degree, altitude, and margin to screen a through-air gap. The final requirement may be controlled by impulse voltage, field geometry, product clauses, or tests not represented by the simplified model.
Layout and Manufacturing Decisions
| Feature | Engineering purpose | Boundary |
|---|---|---|
| Slots and grooves | Can lengthen a qualifying surface path | May leave the shortest through-air path unchanged; minimum groove dimensions and measurement rules are standard-specific. |
| Conformal coating | Can control contamination when the complete coating system is qualified | A drawing note or generic coating name does not automatically permit reduced spacing. |
| Solder mask | May influence the local surface environment | Do not count it as certified solid insulation unless the governing requirements and process qualification explicitly allow it. |
| Board edge | Affects paths to chassis, enclosure, connectors, and accessible metal | Routing and depanelization tolerances can shorten the released path. |
| Component leads | Often determine the true assembly spacing | Bent leads, solder fillets, heatsinks, terminals, and mounting hardware must be included. |
| High altitude | Reduces air dielectric strength | Apply the prescribed clearance correction above the standard reference altitude; do not apply the same factor to creepage without justification. |
| Manufacturing allowance | Protects the nominal design against process variation | It is an engineering margin, not a substitute for the minimum required distance. |
Practical Design Examples
Example 1: Indoor Mains-Referenced Control Board
- A control board contains a mains-referenced primary circuit and a SELV user interface. The nominal mains value alone is not enough to release the spacing.
- First identify the insulation function between primary and SELV, the mains system, overvoltage category, working voltage across the barrier, required impulse withstand, installation altitude, and the applicable product standard.
- Next classify the PCB microenvironment. A clean indoor enclosure may support a PD2 assumption, but expected condensation, conductive process residue, vents, or service contamination can change that decision.
- Use the PCB Clearance Distance Calculator only as an early air-gap screening check and the PCB Creepage Distance Calculator only as an early surface-path check. Resolve final values from the governing standard and document manufacturing tolerances before release.
Example 2: Slot Used to Extend a Surface Path
- Two copper regions are separated by a routed slot intended to increase creepage. Trace the shortest permitted surface path around the slot using the governing measurement rules.
- Then measure the shortest three-dimensional air path across or through the slot. The slot may increase creepage while providing little or no increase in clearance.
- Check router tolerance, slot width, copper registration, board-edge quality, contamination, and whether the slot dimensions qualify under the selected standard.
- Record both measurements independently. A single CAD ruler measurement cannot demonstrate both creepage and clearance compliance.
Example 3: Product Used Above 2,000 m
- A board that meets a preliminary air-gap target at sea level may require greater clearance at high altitude because lower air density reduces dielectric strength.
- Apply the altitude method prescribed by the governing standard to the clearance requirement and include the intended maximum installation elevation in the product specification.
- Do not automatically multiply creepage by the same altitude factor. Reassess creepage from working voltage, pollution degree, material group, and insulation class.
- If the calculator and the applicable standard use different reference elevations or correction bands, the standard controls.
Verification Before Release
Review the current product standard and applicable regional deviations.
Verify CTI, coating, potting, laminate, and insulation-system evidence.
Measure the shortest air and surface paths on the complete 3D assembly.
Include PCB fabrication, component, solder, and mechanical tolerances.
Define inspection criteria for slots, coating coverage, contamination, and conductor position.
Perform dielectric-strength, impulse, or other verification tests where required.
Record unresolved assumptions and prevent them from becoming undocumented layout rules.
Repeat the review when voltage, environment, material, enclosure, or manufacturing process changes.
Common Design Mistakes
- Treating creepage and clearance as two names for the same copper spacing.
- Using nominal supply voltage without checking working-voltage and impulse definitions.
- Selecting pollution degree from the room description instead of the PCB microenvironment.
- Assuming all FR-4 belongs to one CTI material group.
- Applying a clearance altitude factor to creepage without a standards basis.
- Crediting solder mask or conformal coating without qualification and process controls.
- Measuring only in two-dimensional PCB artwork and missing shorter assembly paths.
- Ignoring routing, copper registration, component, solder, and board-edge tolerances.
- Copying a spacing value without its standard edition, voltage context, insulation class, and environmental assumptions.
- Treating a calculator result or a passed prototype test as product certification.
Related Calculators
Related Engineering Resources
Engineering Reference
PCB Creepage, Clearance and Insulation Coordination Reference
Look up terminology, pollution-degree context, CTI boundaries, and standards-review factors.
Engineering Guide
PCB Controlled Impedance Design
Keep high-speed signal spacing distinct from electrical-safety creepage and clearance.
Support reference
FAQ
What is the difference between PCB creepage and clearance?
Clearance is the shortest distance through air between conductive parts. Creepage is the shortest path along an insulating surface. They address different breakdown mechanisms and must be determined independently.
Which voltage should I use for creepage?
Use the working-voltage definition required by the governing standard, including applicable RMS, DC, repetitive peak, circuit, and fault conditions. Do not assume the nominal supply value is sufficient.
Which voltage should I use for clearance?
Clearance commonly depends on required impulse withstand as well as operating voltage. Establish it from the mains system, overvoltage category, installation, transients, and product standard.
Does CTI affect PCB clearance?
CTI primarily supports material grouping for creepage assessment. It normally does not determine a through-air clearance, although both checks belong to the same insulation-coordination review.
Why does altitude affect clearance?
Air dielectric strength decreases as elevation increases. Applicable standards therefore prescribe correction above a reference altitude, commonly 2,000 m, but the exact method and range must come from the governing requirement.
Does altitude also increase creepage?
Not automatically. Altitude primarily affects the air path. Determine creepage from its own working-voltage, pollution, material, and insulation criteria unless the governing standard specifies another treatment.
Can a PCB slot increase creepage?
A compliant slot or groove can lengthen the measured surface path. It may not increase the shortest air clearance, and its width, geometry, and measurement treatment must satisfy the applicable standard.
Does solder mask count as insulation?
Do not assume it does. Crediting solder mask requires explicit support from the applicable insulation system, material qualification, manufacturing controls, and product standard.
Can conformal coating reduce required spacing?
Only when the coating material, application process, coverage, environment, inspection, and governing standard support that treatment. A generic coating callout is not enough.
What pollution degree applies inside an enclosure?
Use the expected microenvironment at the insulation surface. Enclosure protection, condensation, conductive dust, local heating, service access, process residue, and coating qualification can all affect the classification.
Are the ECParts creepage and clearance calculators compliance tools?
No. They provide preliminary screening estimates. Final dimensions require the current applicable standards, product-specific clauses, material evidence, production tolerances, testing, and certification review.
How should I document an insulation-coordination review?
Record the barrier and insulation class, voltage stresses, environment, altitude, material evidence, governing standards and editions, selected tables or methods, measured paths, production allowances, verification tests, and unresolved assumptions.
Safety and Standards Disclaimer
This guide and the related calculators provide engineering education and preliminary screening only. They do not reproduce complete standards tables, establish certification values, or replace review by qualified safety engineers, test laboratories, certification bodies, and the authorities responsible for the finished product.