Creepage and Clearance: The Two Distances That Decide Certification
Most transformer designs that fail a safety certification do not fail on performance. They fail on a distance: the gap along the surface of the bobbin or the air distance between the primary winding and the secondary winding is a fraction of a millimetre short of the requirement.
This guide explains the difference between creepage and clearance, how the numbers are derived from the standard, and the construction techniques that reliably meet reinforced insulation without inflating the component size.
Definitions
- Clearance — the shortest distance through air between two conductive parts. It protects against flashover and is dimensioned mainly by the transient overvoltage the insulation must withstand.
- Creepage — the shortest distance along the surface of an insulating material between two conductive parts. It protects against tracking across a contaminated, humid surface and is dimensioned by working voltage, pollution degree and material group.
Both must be satisfied at every point of the insulation barrier, including the winding ends, the pin-to-winding connections, the tape margins and the core itself if it is not earthed.
How the Required Values Are Derived
Under IEC 62368-1 (and its predecessors IEC 60950-1 / 60065), the required distances depend on four inputs:
- Working voltage — RMS and peak, plus the transient category for clearance.
- Pollution degree — PD1 (sealed, potted), PD2 (normal indoor, the usual default), PD3 (industrial, conductive dust or condensation).
- Material group — determined by the comparative tracking index (CTI) of the bobbin material: group I ≥ 600 V, group II 400–599 V, group IIIa 175–399 V.
- Insulation type — functional, basic, supplementary or reinforced. Reinforced is the sum of basic plus supplementary and is what separates mains from an accessible secondary.
Indicative values for a 250 V RMS mains input, PD2, material group IIIa (a typical PBT-GF30 bobbin), 50/60 Hz:
| Insulation | Clearance (mm) | Creepage (mm) |
|---|---|---|
| Functional | 0.5–1.0 | 1.0–1.5 |
| Basic | 1.5–2.0 | 2.5–3.2 |
| Supplementary | 1.5–2.0 | 2.5–3.2 |
| Reinforced | 3.0–4.0 | 5.0–6.4 |
These are typical values for orientation, not substitutes for the table in your applicable standard. Switching-supply designs must also consider the working voltage across the barrier, which for a 230 V mains flyback can exceed 400 V peak, and the relevant table entry rises accordingly.
Construction Techniques That Work
Bobbin design
- Use a dedicated mains-side and secondary-side winding bay separated by a moulded barrier of the full creepage height, rather than relying on tape alone.
- Choose a material with a known CTI: PBT-GF30 is typically group IIIa; phenolic and some PPS grades reach group I. Group I roughly halves the creepage requirement, which is often cheaper than a larger core.
- Add creepage slots at the pin exit: a 1 mm slot adds about 1 mm of surface path and breaks the tracking line.
Winding technique
- Margin tape at both ends of each winding, wide enough that the wire cannot slip past it. Margin must cover the full creepage distance, not just the clearance.
- Triple insulated wire (TIW) for the secondary: the wire itself provides reinforced insulation, so the winding can touch the primary-side margin without extra tape. This is the standard trick for compact flyback transformers.
- End tape and crossover insulation: the point where a winding returns to its start pin is the most common creepage failure. Insulate the crossover explicitly.
- Keep the winding ends inside the bobbin bay; hand-wound prototypes often creep outward, and the production unit then inherits the problem.
Core and assembly
- If the core is not earthed, treat it as a live part on the primary side and maintain the full distance to the secondary; if it is earthed, the secondary must still meet reinforced insulation to the core.
- Potting with a qualified compound can reduce clearance requirements (pollution degree 1 inside the potting), but only if the compound is void-free — vacuum potting, not pour potting.
- Wire and varnish must be rated for the temperature class; a class F design with class B tape fails the thermal ageing test even if the distances are correct.
Verification
Certification laboratories measure both distances on the actual sample with a pin gauge and a calliper, at the worst-case point, and then apply the tests: dielectric withstand, insulation resistance after humidity conditioning, and for reinforced insulation a partial-discharge check. Build the measurement into your own first-article inspection: a 5-minute check on the first unit of each batch catches bobbin tooling wear before it becomes a certificate problem.
Typical Failure Points
| Location | Failure mode | Fix |
|---|---|---|
| Winding start pin | Wire crosses the margin tape corner | Add end tape; route the start turn inside the bay |
| Bobbin pin exit | Creepage path is a straight line along the moulding | Creepage slot or a moulded rib |
| Core-to-winding | Core not earthed but treated as safe | Earth the core or add insulation tape rated for reinforced |
| Between two secondaries | Assumed functional only | Check the actual circuit potential and required insulation level |
| PCB interface | Distances OK inside the transformer, too small on the PCB | Include the PCB in the safety review; add slots under the part |
Why Core Material Also Matters
The insulation system sits on top of the core. Nanocrystalline and amorphous cores conduct and must be treated as a conductive part; Mn-Zn ferrite is insulating in bulk but its surface can become conductive when coated, and its mechanical tolerance affects how much margin space the winding has. Shaanxi SiRui Technology supplies coated and uncoated cores with documented coating thickness and breakdown voltage, so the barrier calculation can be closed with real numbers.
Related reading: Insulation class and temperature rise · Core coating comparison · Isolation transformer selection
About Shaanxi SiRui Technology Co., Ltd.
Custom transformers and inductors built to IEC 62368-1, 60601-1 and 61558 requirements, with creepage/clearance documentation supplied per part number.
Siri Yuan, Sales Manager · Email: sales@sirui-tech.com · Web: www.sirui-tech.com
No. 2555, Chanba Avenue, Chanba Ecological Zone, Xi’an, Shaanxi Province