Comparative Tracking Index

The 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.

Creepage and Clearance

High voltage circuits can discharge across a gap, the gap can be through the air or across a surface.

• Creepage: distance between two conductors on the surface.
• Clearance: the distance between two conductors through the air.

Let’s look deeper into how CTI works, how it is categorized, and its impact on design:

1. How CTI is Tested

According to the IEC 60112 standard, the CTI value represents the maximum voltage (in volts) that a bulk material can withstand without forming a conductive track.

• The Test: A physical sample of the insulator is subjected to electrical stress while 50 drops of a 0.1% ammonium chloride solution (a conductive electrolyte) are applied to its surface.
• The Pass Criteria: To achieve its rating, the material must survive the test without tracking failure and with less than 0.5 amps of leakage current flowing.

Note on Limitations: CTI testing is strictly designed for solid, bulk materials that are at least 3 mm thick and 20 mm x 20 mm in size. Therefore, it cannot be used to evaluate very thin protective layers, like conformal coatings or the thin protective overcoats on film resistors.

2. Material Group Classifications

To simplify creepage calculations, safety standards like IEC 60664-1 categorize insulating materials into four distinct groups based on their CTI performance. The higher the CTI, the better the material resists tracking:

• Material Group I (CTI ≥ 600 V): These materials offer excellent tracking resistance. Examples include
  • specialized ceramics
  • glass
  • polytetrafluoroethylene (PTFE/Teflon)
• Material Group II (400 V ≤ CTI < 600 V): These offer good tracking resistance. Examples include:
  • reinforced polybutylene terephthalate (PBT)
  • certain engineering plastics
• Material Group IIIa (175 V ≤ CTI < 400 V): This group has fair tracking resistance and serves as the baseline for most general printed circuit board (PCB) designs. Standard FR-4 glass-epoxy laminates usually fall into this category.
• Material Group IIIb (100 V ≤ CTI < 175 V): These materials exhibit poor tracking resistance. Examples include:
  • lower-grade phenolics
  • some liquid crystal polymers (LCP)

3. Practical Impact on Design and Layout

The CTI value directly dictates how much creepage distance is legally required for your circuit. There is an inverse relationship between CTI and creepage: selecting a material with a higher CTI allows you to safely use a significantly shorter creepage distance.

For example, if you are designing a 400V DC system in an ordinary office environment (Pollution Degree 2):

• Using standard FR-4 (Group IIIa) requires a basic creepage distance of 4.0 mm.
• Upgrading to a high-performance Group I material shrinks that requirement down to just 2.0 mm.

Design Trade-offs

Choosing high-CTI materials is one of the most effective strategies for shrinking the physical size of dense designs like power supplies, medical equipment, or motor drives. However, the sources note that these premium high-CTI laminates often cost more and may have different thermal or mechanical characteristics compared to standard FR-4, requiring careful engineering trade-offs. If the material group of a board is unknown, safety standards dictate that you must conservatively assume it is the worst-performing group (Group IIIb), which forces you to use the largest possible spacing.

How CTI applies to Battery Pack Design

In the design of a high-voltage battery pack, particularly for electric vehicles (EVs) operating at 400V, 800V, or even 1000V, the Comparative Tracking Index (CTI) is a critical parameter that directly impacts safety, physical size, and material selection.

Here is how CTI applies specifically to battery pack design:

1. Preventing Electrolyte-Induced Thermal Runaway

Battery packs have a unique micro-environment because lithium-ion cells contain liquid electrolytes made of highly volatile organic solvents and conductive salts (like lithium hexafluorophosphate, LiPF6​).

• If a cell malfunctions and undergoes “soft venting,” it releases these vaporized solvents into the sealed battery enclosure.
• When this vapor condenses on cooler insulating plastics (like cell holders, busbar mounts, or BMS boards), it leaves a highly conductive, moisture-absorbing salt residue.
• This instantly changes the local environment from a clean state (Pollution Degree 2) to a highly contaminated state (Pollution Degree 3 or 4). If the battery pack was designed using low-CTI materials (like standard FR4), the existing creepage distances will be insufficient to halt the current. The conductive liquid film will initiate surface leakage and dry-band arcing, rapidly carbonizing the plastic. This forms a massive high-voltage short circuit that can melt components, ignite adjacent cells, and trigger a systemic thermal runaway.

2. Enabling Volumetric Compaction (Miniaturization)

Space and weight are heavily constrained in battery packs. The higher a material’s CTI, the shorter the legally required creepage distance between high-voltage components.

• Upgrading from a standard Material Group IIIa substrate (CTI ≈ 175 V) to a premium Material Group I substrate (CTI ≥ 600 V) cuts the required surface creepage distance by 38% to 50%.
• This massive reduction allows engineers to pack battery cells, busbars, and power modules much closer together, reducing the overall volume and weight of the battery pack, which also lowers the cost of structural housing materials.

3. Driving Polymer and Material Selection

Because of the severe consequences of tracking failures, CTI dictates exactly which plastics and films can be used for structural and electrical components inside the pack:

• Polybutylene Terephthalate (PBT) & Fluoropolymers: Specialized flame-retardant PBT and fluoropolymers (like PTFE) offer exceptional CTI ratings (≥ 600 V, Material Group I) and do not absorb moisture. They are heavily favored for critical high-voltage connectors, busbar wraps, and insulation sleeves.
• Polyamides (Nylon): While strong, nylons absorb moisture. This absorbed water acts as a plasticizer that dramatically increases ionic conductivity, severely degrading the material’s CTI. Consequently, standard nylons are generally unsuitable for high-voltage barrier insulation in 800V+ battery platforms.
• Polyimide Films (Kapton): Though polyimide films have incredible dielectric strength and heat resistance (often used for cell wrapping), they actually have a relatively low tracking resistance (CTI ≈ 150 V, Material Group IIIb). Designers must compensate for this by maintaining larger surface creepage distances when using Kapton in unsealed areas.
• Polycarbonate (PC): Commonly used for large structural covers, unfilled PC typically has a lower CTI (175 V to 250 V, Material Group IIIa), meaning it requires wider spacing between conductive nodes.

4. Strategic Engineering Guidelines

To meet automotive safety standards like ISO 6469-3 and IEC 60664-1, battery engineering teams utilize specific CTI-driven guidelines:

• Enforce Material Group I: Specify only Material Group I (CTI ≥ 600 V) substrates for all critical high-voltage interfaces operating at ≥ 400V DC. While these premium polymers cost 20% to 30% more, they offer a 10x reduction in surface tracking field failures under humid and contaminated conditions, providing a positive return on investment by reducing warranty claims.
• Design for Localized Pollution Degree 3: Because of the constant risk of moisture condensation and electrolyte venting, the interior insulation of the battery module should always be dimensioned assuming Pollution Degree 3 conditions to protect against rapid surface tracking.

References

1. Creepage Distance: Complete Guide to Measurement, Standards & Safety Requirements, viox
2. IEC 60112:2020 Method for the determination of the proof and the comparative tracking indices of solid insulating materials, IEC
3. Insulation Coordination in Automotive Power Module (IEC60664-1:2020), Infineon