CTI - Insulation materials with good resistance to creep path formation

Insulation materials with good resistance to creepage. Why the CTI value is becoming increasingly important in the electrical future of renewable energies.

Electrical power is the product of voltage and flowing current. The greater the required power, the more current must flow. This leads to ever thicker cable cross-sections, because even the best copper can only transport a certain amount of electrons per cross-sectional area. More power therefore leads to heavier and heavier cables.

Weight is a valuable commodity when it comes to transporting goods or people. This is because more weight consumes more drive energy and reduces the payload. 

So to avoid having to transport more and more heavy copper, the other multiplier in the equation is changed: the voltage. While 48V can only generate a few KW of drive power at most, 350...400 V is already enough to power a car. In addition, electric motors are extremely efficient compared to combustion engines.

For even higher outputs, such as those required in trucks, rail transport, ships or, in the future, in aviation, the voltage continues to rise. A voltage level of 950 VDC is currently established, but developments are going much further. The IEC/TS 62993:2020 guideline already extends the insulation coordination of IEC 60664 to operating voltages from >1000 VAC to 2000 VAC (>1500 VDC to 3000 VDC).

Such high voltages require completely different measures to those required for low voltages. There are even considerable differences compared to the usual mains voltage of 230 VAC. At 950 VDC operating voltage, the required insulation distances across the surface of the insulation materials are up to 4 times greater in unfavorable cases.

Air and creepage distance as insulation

In addition to insulation through the air (air gap) and insulation through surrounding insulation materials (e.g. cable insulation), there are other constellations that separate potential-carrying conductors from each other for the purpose of functional insulation, basic insulation or reinforced insulation. 

This can be illustrated quite well using the example of a printed circuit board: in a double-sided printed circuit board, the top and bottom layers are insulated from each other by the insulating material (e.g. FR4 glass fabric epoxy resin). The conductor tracks themselves are separated from each other by the distance between them. This distance is made up of the air gap and the creepage distance. This creepage distance describes the shortest path that an electric current would take across the surface of the insulating material.

This distance is usually defined as the insulation distance. However, there are circumstances that can shorten this distance for the electric current. In the simplest case, it is metallic contamination, e.g. from the soldering process, which remains between the conductor tracks. However, grease (e.g. fingerprints) or later soiling caused by dust or abrasion can also shorten the insulation distance on the surface. This is particularly the case when high humidity and condensation create a conductive coating. The current that then begins to flow slowly but surely destroys the insulation surface. In most organic insulators, carbon is produced when the polymer chains break down. This carbon, together with the carbon from dust deposits, for example, creates an increasingly conductive connection between the different electrical potentials. The higher the voltages and the resulting field strength, the faster this development takes place. In extreme cases, a partial discharge occurs via the surface of the insulator. In the worst case, a short circuit occurs and the electronic component or the entire assembly fails.

The so-called comparative tracking index indicates how strongly a material tends to form a conductive tracking path under applied voltage and the influence of moisture and impurities on the surface. 

The Comparative Tracking Index (CTI) is measured to determine the tendency of a material to form a creep path. In this standardized test method, a standardized test solution is dripped onto a test plate between two electrodes (4 mm apart) in accordance with IEC 60112. The CTI value granted to the material corresponds to the maximum voltage at which no current flow above 0.5 A occurs for a maximum of 0.2 seconds after 50 dripping applications. The CTI value corresponds to the voltage up to which the failure criteria were not reached. 

General conditions for determining the right insulating material

The practical benefit of a high CTI value is primarily the reduction in the required insulation distances. For example, while a polyester adhesive tape (CTI value >600V) only requires a distance of 2.5 mm at an operating voltage of 500 V, an adhesive tape based on Kapton®HN already requires a distance of 5 mm (in accordance with IEC 60664).

However, the CTI value is only one of the material properties that an insulation material must fulfill. The following other values must also be taken into account:

• Thermal class into which the material is grouped

• Tendency to ignite when exposed to flame (flammability class)

• Resistance to electric arcs (HAI)

• Resistance to penetration by hot objects (HWI)

• Dielectric strength

• Possibly also the behavior against UV exposure and cold

The ambient conditions are also important and are determined by the following values:

• How "dirty" is the operating environment? (degree of soiling)

• What is the maximum height at which it can be used? (height correction factors)

• Which transient voltage peaks are possible? (near-grid, far-grid, equipment class)

• What is the maximum possible operating voltage?

Sometimes the combination of product standards and the basic standard for insulation coordination IEC 60664 results in requirements for the insulation materials used that are difficult to meet. This is particularly the case if the aim is to achieve minimum sizes. As the voltage level increases, this aspect becomes increasingly relevant. Foils made of PEEK, a very good high-performance material, already require a distance of 10 mm (pollution degree 2) at 1050 V (voltage level for truck drives). This is definitely a problem in the limited installation space of converters, high-voltage machines and busbars. This is because many electrical connections are designed as solid metal stamped or bent parts due to the high current load. There is no "standardized insulation" for this, as is the case with round cable insulation. In many cases, stamped parts made of insulation foil or adhesive tape are used.

The following selection of high-quality insulation foils with good CTI values may help to take the space problem of creepage distances into account when designing electrical systems:

(Source: https://iq.ulprospector.com; availability of materials varies greatly)

Many manufacturers of insulation films (and insulation materials in general) have not had the CTI value determined, as it has played a subordinate role for the usual applications to date. However, every supplier and user is free to have the CTI value determined. An accredited test laboratory such as that of the VDE (Offenbach) or UL (Krefeld) can measure the value so that you have reliable proof. Then nothing stands in the way of an application in modern high-voltage technology.

Some materials such as fluoropolymers (e.g. PTFE, FEP) and silicones can achieve cti values of >1000 V. These high values can result in even shorter insulation distances for the creepage distance. However, the IEC 60112 test standard has so far been limited to measurements up to 600 V. However, work is being done on this, just as in America on UL746.

Section of common materials

Like many UL standards, insulation coordination in accordance with IEC 60664 uses the CTI value, which is the most common. Relevant standards are, for example, IEC 60587 and IEC 60112 as well as UL standard 749.