The increased safety protection method is based on the improvement of the safety standards of some types of equipment. In this context it is of primary importance to avoid the formation of sparks or electric arcs, possible triggers of an explosion.

To understand the possible situations in which an electric arc is formed, it is necessary to study the so-called "tracking phenomena" [1].

The formation of an electric arc between two bare conductors can occur through a current path that develops on the surface of the insulating material. This path is referred to as a "tracking". Some insulating materials characterized by a low "surface resistance" are more prone than others to manifest this phenomenon. 

It is known that inorganic insulators such as aluminum oxide, ceramics, or mineral glasses [2] are not subject to this type of effect, while most materials of organic origin, based on hydrocarbons, suffer from this problem. 

The key to this phenomenon lies, in fact, in the carbonization of the surface of the material. 

In organic insulators the initial failure burns the surface with the consequent formation of carbon. Charred areas conduct electrical current more than pristine insulation, and the higher the current, the more heat is generated which leads to insulation degradation. 

At the end of the process the material becomes conductive. 

One question might arise: where does carbon come from? Polymers, or plastics, are also made up of carbon, one of the main constituents together with hydrogen, oxygen, and nitrogen [3] of organic and polymeric chemistry.

To classify insulating materials and their resistance to the tracking phenomenon, the method reported in the international standard IEC 60112 is used.This standard divides the different materials into four groups, with the tracking resistance index improving from group IIIb to group I.

Table 1: the classification of CTIs

Going back to what was previously explained, glass and ceramics, which do not undergo the phenomenon of carbonization, are conventionally classified as group I materials as shown in table 1.That is, they are among the insulating materials that are less affected by tracking.

Table 2: Typical materials vs material group

It is necessary to underline how plastic materials can improve their CTI thanks to particular additives, a common practice to give polymers better physical properties or to facilitate their production and molding.For this reason, the same plastic material, for example polyamide, can be found on the market with different levels of CTI, the designer's task is to select the right composition.

Figure 1 – TEST setup for determining the CTI of a material according to IEC 60112

We often hear that the components used in contexts at risk of forming an explosive atmosphere are made with particular materials. 

This is particularly true in the case of plastic materials, which are finding more and more space in the electrotechnical industry. In contexts designed to form insulation between conductors, the materials used are selected from among those that express better tracking resistance. 

Other parameters on which to intervene to inhibit the formation of electric arcs are the surface distance between the conductors, the control of over voltages and the protection against pollution.

 Condensation and dust, as it is easy to imagine, increase the risk of electric arc formation.

 
Notes, reference standards and bibliography 

[1] "tracking phenomena" is the English translation that we can find in technical regulations. See NOTE 2 CEI EN 60079-7:2008-11 at point 4.4.1

[2] CEI EN 60079-7 2016-07 4.4.1

[3] in the classic definition of organic chemistry we also find sulfur and chlorine, in modern organic chemistry there are also other elements