(in accordance with EN 60664-1:2007 and VDE 0110-1)

For the purposes of the above mentioned standards, materials are classified into four groups according to their CTI values. These values are determined in accordance with IEC 60112 using  solution A. The groups are as follows:

Insulation materials group I                  600 ≤ CTI

Insulation materials group II                400 ≤ CTI < 600

Insulation materials group IIIa            175 ≤ CTI < 400

The proof tracking index (PTI) is used to verify the tracking characteristics of materials. A material may be included in one of these four groups on the basis that the PTI is not less than the lower value specified for the group.