Creepage and Clearance

High-voltage circuits can arc or flash across a gap. Whether that discharge travels through the air or tracks along a surface depends on geometry, voltage, and environmental conditions. Two distinct minimum distances are therefore controlled in HV design.

Creepage — the shortest path between two conductors measured along the surface of an insulating material.

Clearance — the shortest path between two conductors measured through the air.

Both are defined in IEC 60664-1, the primary insulation coordination standard for systems up to 1,000 V AC / 1,500 V DC. For automotive battery systems, ISO 21498-1 and the VDA LV 123 standard also reference these principles. The purpose is to prevent two distinct failure modes: electrical breakdown across an air gap (clearance violation) and surface tracking along insulation (creepage violation).

Why it matters for battery packs

Modern EV packs operate at 400 V or 800 V DC nominal, with transient overvoltages that can exceed 1,200 V peak in 800 V systems. Unlike AC mains equipment, battery packs face a specific contamination risk: electrolyte leakage. A small amount of liquid electrolyte bridging an insufficiently short creepage path can cause immediate tracking current, progressive insulation degradation, and in the worst case, a ground fault that initiates thermal runaway.

Electrolyte contamination places the battery pack interior at Pollution Degree 3 under IEC 60664 — the standard for conductive or moisture-activated contamination. This classification has a significant effect on the required minimum distances compared to a clean electronics environment (Pollution Degree 2).

As system voltages increase towards 800 V and beyond, both clearance and creepage requirements scale upward. Physical packaging becomes more constrained. The interaction between voltage, material choice, contamination classification, and insulation type must be resolved at the design stage, it cannot be corrected at validation.

Governing factors

The minimum required creepage and clearance distances are determined by six factors. All must be established before any distance can be calculated.

Working voltage — the rated operating voltage under normal conditions. For DC systems such as a battery pack, this is the peak sustained DC voltage including any regulation tolerance. An 800 V nominal pack may see 850–900 V at maximum charge.

Pollution degree — a classification of the contamination environment (see table below).

Overvoltage category — defines the level of transient overvoltage a device is exposed to based on its position in the supply system (see table below).

Material group / CTI — the Comparative Tracking Index quantifies an insulating material’s resistance to surface tracking under electrical stress and contamination. Higher CTI allows shorter creepage distances.

Insulation type — whether the insulation provides functional, basic, supplementary, double, or reinforced protection. Reinforced insulation requires 2× the distances of basic.

Altitude — air density decreases with altitude, reducing the dielectric strength of air gaps. IEC 60664-1 requires clearance to be multiplied by a correction factor above 2,000 m. This is relevant for commercial vehicles and stationary storage deployed at altitude.

Pollution degree

IEC 60664-1 defines four pollution degrees. The selection drives creepage requirements more strongly than almost any other factor: moving from PD2 to PD3 can increase required creepage distance by 60% or more at high voltages.

For most battery pack designs, PD3 should be assumed for the general pack interior unless a qualified barrier separates the electrolyte environment from the component under assessment. Sealed sub-assemblies with qualified conformal coating may be assessed at PD1 under IEC 60664-3, but the coating must qualify as solid insulation under the standard, most liquid coatings do not.

One special case for Pollution Degree is cell venting. In soft and hard venting lots of materials can be ejected from the cell(s). Impacting the clearance to the extent that arcing occurs across what would have been large distances by normal standards. Also, a soft vent that goes undetected might leave a residue that significantly impacts creepage. → Thermal Runaway in High-Voltage Systems

Overvoltage category (OVC)

The overvoltage category defines the transient impulse voltage a device must withstand, based on where it sits in the power supply architecture. Higher OVC means larger required clearance.

HV battery pack main circuits are typically assessed at OVC III. For an 800 V system at OVC III, the rated impulse withstand voltage from IEC 60664-1 Table F.1 is in the range of 8 kV, which drives clearance requirements of 8 mm or more for basic insulation before altitude correction.

Comparative Tracking Index and material groups

The Comparative Tracking Index (CTI) is a standardised measurement of an insulating material’s resistance to surface tracking under contamination and electrical stress. Tracking is the progressive carbonisation of a material’s surface by leakage current, eventually forming a permanent conductive path. CTI is measured per IEC 60112.

IEC 60664-1 groups materials into four classes based on CTI range. The material group directly determines how long the creepage path must be for a given voltage and pollution degree.

Selecting Group I materials (CTI >= 600 V) over Group IIIa materials (CTI 175–399 V) can reduce required creepage distance by 30–40% at 400 V and PD3. Where packaging is tight, material selection is one of the most effective levers available to the designer.

Insulation types

Required distances depend on what level of protection the insulation must provide. IEC 60664-1 defines five types:

In battery pack design, reinforced insulation is typically required wherever a single fault would create a shock hazard — for example, across the galvanic isolation boundary in an on-board charger. The 2× multiplier on distances has a significant impact on component sizing and PCB layout at high voltages.

Altitude correction

Air breakdown voltage is proportional to air density. Above 2,000 m, clearance distances must be multiplied by an altitude correction factor from IEC 60664-1 Table F.4. Representative values:

• 2,000 m: factor 1.00 (no correction)
• 3,000 m: factor ~1.14
• 4,000 m: factor ~1.29
• 5,000 m: factor ~1.48

Creepage is not corrected for altitude, as it is governed by surface properties rather than air density. This means that at high altitude, clearance may become the binding constraint even for voltages where creepage would normally dominate.

Conformal coating

Conformal coating does not reduce clearance. Under IEC 60664, clearance is always measured through air and any coating is ignored. Creepage is different: if a coating qualifies as solid insulation and the pollution degree is legitimately reduced as a result, shorter creepage distances may be justified under IEC 60664-3. However, most liquid conformal coatings do not qualify. Parylene can qualify when a specific grade is tested and certified as part of a defined insulation system.

The design rule is: design the spacing to comply without relying on coating. Treat coating as environmental protection, not as a means of reducing distances.

Design rules of thumb

• At voltages above approximately 200 V DC, creepage will typically exceed clearance because surface tracking is the dominant failure mode.
• For a 400 V DC pack (PD3, Material Group IIIa, basic insulation), minimum creepage is on the order of 8–10 mm.
• For an 800 V pack under the same conditions, required clearance approaches 4–6 mm and creepage 16–20 mm depending on material group and insulation class.
• Manufacturing tolerances of ±0.5 mm on component placement can take an assembly from compliant to non-compliant. Tolerance stack-up analysis is required during design validation.
• Grooves and ribs in insulating parts add effective surface path length without increasing physical footprint, a useful technique for constrained packaging.
• Always verify distances under worst-case deformation:
  • connector float
  • busbar deflection under vibration
  • swelling of cells adjacent to a PCB all reduce effective gaps.

Creepage and Clearance Calculator – provides indicative minimum distances based on IEC 60664-1:2020 and ISO 21498:2021 table lookups with linear interpolation. It does not replace formal insulation coordination analysis by a qualified engineer.

References

1. IEC 60664-1:2020 — Insulation coordination for equipment within low-voltage supply systems. The primary standard for creepage and clearance calculations, defining pollution degrees, overvoltage categories, material groups, and minimum distances.
2. IEC 60664-3:2003 — Use of coating, potting or encapsulation to reduce the effects of pollution. Covers conditions under which solid insulation can be used to justify reduced pollution degree classification.
3. IEC 62368-1:2023 — Audio/video, information and communication technology equipment. Includes creepage and clearance requirements and altitude correction tables widely referenced for power electronics.
4. ISO 21498-1:2021 — Electrical systems for electrically propelled road vehicles. Part 1 specifies insulation resistance, creepage, and clearance requirements for HV components in automotive applications.
5. LV 123 (VDA) — Requirements for high-voltage components in road vehicles. German OEM-consortium standard referencing IEC 60664 with automotive-specific pollution degree and OVC assignments.
6. IEC 60112:2020 — Method for the determination of the proof and comparative tracking indices of solid insulating materials. Defines the CTI test method used for material group classification.
7. IPC-2221B — Generic standard on printed board design. Includes conductor spacing requirements for PCB layouts, complementary to IEC 60664 for PCB-level creepage and clearance.
8. IPC-9592B — Requirements for power conversion devices above 100 V. More specific than IPC-2221 for high-voltage power supply PCB design.
9. Comparative Tracking Index (CTI) is a standardized measurement that quantifies an insulating material’s resistance to surface “tracking”. Tracking is the progressive degradation of a material’s surface, where moisture, dust, and continuous voltage stress cause localized leakage currents that slowly carbonize the material, eventually forming a permanent conductive path.

See also: Electrical Breakdown, Photo Etching