Importance of the tracking resistance of insulating materials for developers

The CTI value of a material indicates how strong (or weak) the tendency of an insulation material is to form conductive paths on the surface. The higher the voltage, the better the material’s CTI value must be.

An insulating material is defined by a number of parameters such as temperature resistance, flammability and breakdown voltage (see also IEC 60664, Insulation coordination). High-voltage engineers have long known that voltages above approximately 400 V result in an additional load: corona and sliding discharges can occur on the surface of insulating materials because air is considerably less voltage-resistant than the insulator itself. Today, non-sinusoidal voltage characteristics and frequencies >>30 kHz expose the insulated section to an additional load due to the air. Air has only 80% of the dielectric strength at approximately 2.5 MHz that it has at 50 Hz.

Large distribution transformers or high-voltage switchgear systems deal with this problem by using either appropriately large distances or insulation gas or oil. They have a significantly higher dielectric strength than ambient air.

However, these methods are at odds with the ever smaller manufactured sizes in the (automotive) industry. Achieving greater power output in smaller volumes requires the use of higher operating voltages (power is the product of current strength and voltage level). That is why many insulations today are exposed to a greater load than the classic mains voltage of 230/400 VAC at a sine frequency of 50 Hz.

From the insulator into the ambient air

Standards almost always measure the dielectric strength of an insulating material by using a comparatively homogeneous electric field perpendicular to the material. There are also other types of load in practice: two parallel conductor tracks on a printed circuit board lie in one plane. The electric field that forms in between runs along rather than through the surface of the carrier material (e.g. FR4 material).

The insulating air between the conductor tracks has a dielectric strength of only ~3kV/mm @DC or ~0.35 kVeff/mm @AC compared to the several tens of kV/mm that the usual insulating materials have.

The “comparative tracking index” or “tracking resistance” is an indication of how well an insulating material withstands certain loads on its surface. In even more general terms, tracking resistance is an indication of how resistant a material is to environmental influences.

Other loads occur in practice alongside partial discharges, which slowly destroy an insulating material. The insulation surface can be contaminated as early as during assembly, and dust and abrasion are added during operation. This, in conjunction with moisture, considerably reduces the “surface resistance” of insulating materials. A conductive path forms along these particles at sufficiently high voltage and in the presence of air humidity (condensation). The resulting creepage current gradually carbonises the covering material and the insulation material (polymer). The creepage path keeps growing (creeping).

The remaining “air gap” along the surface of the material becomes ever shorter until the remaining clearance is eventually breached.

The CTI value (tracking resistance) indicates a material’s tendency to form a creepage path when exposed to contamination and high humidity. The better a material can resist destruction thanks to high field strengths and creepage currents, the more likely it is to be used in the high-voltage range. Measurements are made according to a standardised method in order to characterise insulating materials. Two of the most important standards are IEC 60112 (Methods for determining the test number and the comparative tracking path index of solid insulating materials) and IEC 60587 (Electrical insulating materials used under severe conditions – test methods for determining resistance to tracking path formation and erosion).

 

Excerpt on common materials

insulating group  (EN 50124)	CTI(IEC 60112)	PTI  (UL746)	typical materials
I	>600	0	PTFE; PP; PE; PA; PFA; FEP
II	400....599V	1	Polyester (PET), PEN
IIIa	250….399 V	2	Polycarbonat
IIIa	175….249 V	3	PPS
IIIb	100….174 V	4	Polyimid; PEI; PSU; PEEK
	< 100V	5

Like many UL standards, insulation coordination according to IEC 60664 makes use of the CTI value, which is also the most common. Relevant standards include, for example, IEC 60587 and IEC 60112 as well as UL standard 749.

Classification of insulating material groups according to IEC 60601:

CTI value	insulating group
600 ≤ CTI	I
400 ≤ CTI < 600	II
175 ≤ CTI < 400	IIIa
100 ≤ CTI < 175	IIIb

There is a draft application guide (design examples and insulation tests) for part 2 (DIN IEC 60664-2-1) of the basic standard DIN IEC 60664 (insulation coordination).

Usefulness of the CTI value specification in designing electrical devices

The tracking index helps to identify a suitable material based on the intended use as well as standard specifications. Given the expected degree of contamination, this makes it possible to select a material that is sufficiently resistant to the formation of conductive surface coatings. Alternatively, you will be provided with an overview of the materials that can be used at the beginning of the development, allowing you to adapt the design accordingly.

The possibility of reducing the required creepage distances represents another important benefit. A material with a cti=0 (400 - 600 V) requires a shorter creepage distance than a material with a cti=4 – and this is independent of the degree of contamination (except for full encapsulation). This, in turn, significantly reduces the required installation space.

In particular the commonly used high-performance materials such as PI, PEEK or PSU are relatively poor when it comes to tracking resistance. This is attributable, among other things, to the manufacturing process, which involves polycondensation. Polycondensation can be reversed by adding (heat) energy and water, which results in the gradual destruction of the polymer. The ability to absorb water also has a significant influence on the polymer’s tracking resistance properties.

A “good” CTI value also facilitates the reduction of the required creepage distances resulting from standard specifications. The distance reduction when using a cti=1 material compared to a cti=4 material is considerable, particularly in light of ever smaller installation spaces.

Alternatives to materials with a good CTI value:

A suitable product cannot be found for all combinations of cti, HWI, RTI, HAI and flame retardancy, as this combination of required values results not from the list of available insulating materials, but from specifications including operating voltage, rated surge voltage, the overvoltage category, degree of contamination and other general conditions.

If, for example, the determination of the required material properties shows that a material with cti=0 (>600V) and an RTI=160°C is needed, then it will be difficult to find a suitable material.

However, adjusting the degree of contamination can sometimes help in such cases. Changing the degree of contamination from 3 to 1 in turn reduces the requirements for the CTI value. This change in the degree of contamination can be achieved by encapsulation, protective coating or the use of hermetically sealed housings (e.g. IP X7) and cleaning.