Creepage vs. Clearance: A Worked Example on an Isolated Power Board
Clearance is the shortest distance through air between two conductive parts. Creepage is the shortest distance along the surface of an insulator between the same two parts. A safety standard gives you a minimum for each, and your board must meet both.
Flux, the AI hardware engineer, does this work for you. Tell Flux the safety standard, the inputs, and the distances. Flux saves them as project requirements and sets the distances as layout rules on the board. Then it runs the design rule check, fixes the reported errors, checks the air paths from the board to the enclosure, and prepares the safety analysis for your test lab.
This is a 24 W isolated AC/DC supply in a grounded aluminum enclosure, to IEC 62368-1. Keep all primary-side nets 6 mm from the secondary-side nets on every layer, and 3 mm from the mounting holes, the board edge, and the enclosure. Run the design rule check and fix the errors.The rest of this post shows where those numbers come from, with a worked example for one isolated AC/DC supply, and then how to do each step in Flux.
Creepage vs. clearance: the difference
Both distances stop the same failure: current that crosses an insulation barrier. They stop different mechanisms.
- Clearance stops arcs. A voltage peak or a mains surge can break down the air between two conductors. Clearance depends on the peak voltage and on the transients that the mains can deliver. Measure it as a straight line, in three dimensions, through air.
- Creepage stops tracking. Dust and moisture on a surface make a conductive film. Over time, leakage current burns a carbon track along that film. Creepage depends on the RMS working voltage, the pollution degree, and the insulating material. Measure it along the surface, around every edge and over every component body.
The two paths start and end at the same conductors, but they are not the same length. A slot in the board makes the surface path longer and does not change the air path. A tall part near a metal wall makes the air path shorter and does not touch the board surface. Check both paths at every point where the barrier is crossed.
The inputs you need before you look up a number
A creepage or clearance value without its inputs is not a requirement. Write these down first:
| Input | What it sets | Value in this example |
|---|---|---|
| Product safety standard | Which tables and rules apply | IEC 62368-1 (audio/video, IT and communication equipment) |
| Insulation grade | Functional, basic, supplementary, or reinforced | Reinforced from primary to the 12 V output; basic from primary to the earthed enclosure |
| Working voltage | RMS value sets creepage; peak value adds to clearance | 250 Vrms across the barrier |
| Mains transient voltage | Clearance for mains-connected circuits | 2500 V peak (overvoltage category II, 230 V mains) |
| Pollution degree | Creepage multiplier for dust and condensation | Pollution degree 2 (indoor, occasional condensation) |
| Material group | Creepage for the board and part bodies, from the CTI | Material group IIIa (standard FR-4, CTI 175) |
| Altitude | Clearance multiplier for thin air | Up to 2000 m |
Change one input and the numbers change. A higher-CTI laminate (material group I or II) gives shorter creepage. A product for 5000 m altitude needs more clearance. A medical product uses IEC 60601-1, which has its own tables and its own means-of-protection rules.
Worked example: a 24 W isolated AC/DC supply
The product is an industrial controller in a grounded aluminum enclosure. A 24 W flyback converter makes 12 V from 100 to 240 Vac mains. The 12 V output goes to a connector that users touch, so it must be safe to touch.
Step 1: Draw the barrier
Sort every net into a side of the barrier.
- Primary (hazardous) side: L, N, the bulk capacitor rail, the switch node, the auxiliary winding, and the primary ground.
- Secondary (touch-safe) side: +12 V, the secondary ground, and the feedback circuit.
- Earthed metal: the enclosure, the mounting screws, and the standoffs.
Only three parts cross the primary-to-secondary barrier: the transformer T1, the optocoupler U2 in the feedback loop, and the Y1 safety capacitor CY1. Every one of them is a place to check.
Step 2: Look up the minimums
Read the clearance tables in clause 5.4.2 and the creepage tables in clause 5.4.3 of IEC 62368-1:2018 with the inputs above. For reinforced insulation, the creepage is two times the basic value.
| Barrier | Insulation | Minimum clearance | Minimum creepage | Design target on the board |
|---|---|---|---|---|
| Primary to secondary | Reinforced | 3.0 mm | 5.0 mm | 6.0 mm |
| Primary to earthed metal | Basic | 1.5 mm | 2.5 mm | 3.0 mm |
The design target adds margin for etch tolerance, solder fillets, and parts that sit off-center in their pads. Use the target in the layout rules, and use the standard's minimum to judge each measured path.
Step 3: Find which value controls each path
On a flat board, the surface path and the air path between two pads are about the same length. Creepage (5.0 mm) is larger than clearance (3.0 mm), so creepage controls everywhere on the board surface. Clearance controls in three dimensions, where no surface connects the two conductors:
- The bulk capacitor can and the enclosure wall above it.
- The transformer core and the secondary-side heat sink next to it.
- The mounting screw and the nearest primary copper.
Step 4: Measure copper edge to copper edge
Measure from the edge of one pad to the edge of the other, not from pin center to pin center. Solder mask does not count as insulation in this calculation, so measure from the copper.
The Y1 capacitor shows why this matters. A disc capacitor with 7.5 mm lead pitch and 2.0 mm pads has 5.5 mm between its pad edges. That passes the 5.0 mm minimum, but not the 6.0 mm design target. There are two fixes:
- Use a part with 10 mm lead pitch. The pad gap becomes 8.0 mm.
- Mill a 1.5 mm slot between the pads, and extend it 3 mm past each pad. The surface path must now go around the end of the slot, which is more than 6.0 mm. The slot is more than 1 mm wide, so the path across it counts as an air path. That air path is still 5.5 mm, more than the 3.0 mm clearance.
For the optocoupler, select a package with wide lead spacing and a datasheet creepage of 8 mm or more. Then keep the pad gap on the board at the full 6.0 mm target. For the transformer, use a bobbin that puts the primary and secondary pins on opposite sides, and route no primary copper under the secondary pins.
Step 5: Turn the numbers into layout constraints
| Constraint | Value |
|---|---|
| Primary nets to secondary nets, all layers | 6.0 mm |
| Primary nets to mounting holes and earthed copper | 3.0 mm |
| Primary copper to board edge (the enclosure is earthed metal) | 3.0 mm |
| Copper pours inside the barrier zone | None |
| Slot under CY1 | 1.5 mm wide, 3 mm past each pad |
| Primary parts to the enclosure wall, through air | 3.0 mm |
Inner layers have no surface path, but they have a distance through the laminate. Apply the same 6.0 mm net spacing on inner layers to keep the stack-up simple. A silkscreen line along the barrier and a high-voltage symbol next to the mains connector tell the assembler and the service technician where the barrier is.
Do the worked example in Flux
Flux is an AI hardware engineer. It designs the board and the enclosure in one project, so the safety distances are checked in both.
Save the requirements in the project
Tell Flux the standard, the insulation grades, and the inputs. Flux saves them in the project Knowledge, and uses them for each later layout change.
Add to this project's knowledge: IEC 62368-1, reinforced insulation from primary to secondary, basic insulation from primary to the earthed enclosure, 250 Vrms working voltage, pollution degree 2, FR-4 with CTI 175. Keep primary-to-secondary spacing at 6.0 mm and primary-to-earth spacing at 3.0 mm.Set the rules on the board
Flux sets the distances as Keep Out rules. Other tools call this rule "spacing" or "clearance". A Keep Out rule can apply to one object against all other objects, or only against the objects that you specify.
Keep all primary-side nets 6 mm away from the secondary-side nets on every layer.Add a 3 mm keep out around the mounting holes for the primary-side nets, and keep primary copper 3 mm from the board edge.Check the layout
Flux runs the design rule check, fixes the reported layout errors, and runs the check again after each change.
Run the design rule check. Fix the clearance and overlapping-copper errors, then report the remaining violations.Then measure each point where the barrier is crossed: T1, U2, CY1, the mounting holes, and the board edge. Use Measure Distance (keyboard shortcut M), click the two copper edges, and compare the straight-line result with the clearance target. Where there is no slot, the straight line is also the surface path, so it must meet the creepage target too. At a slot, such as the one under CY1, measure the surface path in two parts: from each pad to the end of the slot. Add the two parts and compare the sum with the creepage target.
Check the air paths to the enclosure
The air path from a tall primary part to a metal wall is not on the board, so a board check alone does not see it. Flux has the enclosure in the same project and checks the clearance between the board, its parts, and the enclosure.
Check the clearance from the bulk capacitor, T1, and the primary heat sink to the aluminum enclosure. Each one needs at least 3 mm.Prepare the safety documents
Flux writes a safety analysis for mains-powered products and AC/DC equipment from the project data, with the board and its mechanical housing. Give it to your test lab with the design files.
Prepare a safety analysis for this AC/DC power supply and its aluminum enclosure.See how Flux designs the schematic, the board, and the enclosure in one project.
FAQs
Clearance is the shortest distance through air between two conductive parts. Creepage is the shortest distance along an insulating surface between the same parts. Clearance prevents arcs from voltage peaks. Creepage prevents tracking from surface contamination.
No. The surface path between two conductors is never shorter than the straight air path. When the creepage minimum from the table is smaller than the clearance minimum, use the clearance value for creepage too.
The RMS working voltage, the pollution degree, the material group of the insulator (from its CTI), and the insulation grade. Reinforced insulation needs two times the basic creepage.
Yes. A slot that is at least 1 mm wide (pollution degree 2) makes the surface path go around its ends. The path across the slot then counts as an air path, so it must meet the clearance value.
Ordinary solder mask does not count. Measure from the copper. A qualified coating system, tested to the rules of the standard, can reduce the pollution degree for the coated area.
Yes. Tell Flux the distances, and it sets them as Keep Out rules between the nets, holes, and board edge that you name. Flux then runs the design rule check, fixes the reported errors, and checks the clearances to the enclosure. Use Measure Distance to confirm the air path and the surface path at each barrier crossing.
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