ISO7240CF, ISO7240C, ISO7240M

ISO7241C, ISO7241M, ISO7242C, ISO7242M

SLLS868R – SEPTEMBER 2007 – REVISED SEPTEMBER 2015

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Device Support (continued)

Comparative Tracking Index (CTI) — CTI is an index used for electrical insulating materials that is defined as

the numerical value of the voltage which causes failure by tracking during standard testing. Tracking is the

process that produces a partially conducting path of localized deterioration on or through the surface of an

insulating material as a result of the action of electric discharges on or close to an insulation surface -- the higher

the CTI value of the insulating material, the smaller the minimum creepage distance required.

Generally, insulation breakdown occurs either through the material, over its surface, or both. Surface failure may

arise from flashover or from the progressive degradation of the insulation surface by small localized sparks. Such

sparks are the result of the breaking of a surface film of conducting contaminant on the insulation. The resulting

break in the leakage current produces an overvoltage at the site of the discontinuity, and an electric spark is

generated. These sparks often cause carbonization on insulation material and lead to a carbon track between

points of different potential. This process is known as tracking.

Material Groups — Materials are classified into four groups according to their CTI values. These values are

determined in accordance with IEC 60112. The groups are as follows:

•

Material group I: 600V

≤ CTI

•

Material group II: 400V

≤ CTI < 600

•

Material group II: 175V

≤ CTI < 400

•

Material group II: 100V

≤ CTI < 175

13.1.1.1 Insulation:

Functional insulation — Insulation needed for the correct operation of the equipment.

Basic insulation — Insulation that provides basic protection against electric shock.

Supplementary insulation — Independent insulation applied in addition to basic insulation in order to ensure

protection against electric shock in the event of a failure of the basic insulation.

Double insulation — Insulation comprising both basic and supplementary insulation.

Reinforced insulation — A single insulation system which provides a degree of protection against electric shock

equivalent to double insulation.

13.1.1.2 Pollution Degree:

Pollution is any addition of foreign matter, solid, liquid, or gaseous that can result in a reduction of electric

strength or surface resistivity of the insulation. There are four categories of pollution:

Pollution Degree 1 — No pollution or only dry, nonconductive pollution occurs. The pollution has no influence.

Pollution Degree 2 — Only nonconductive pollution occurs. However, a temporary conductivity caused by

condensation is to be expected.

Pollution Degree 3 — Conductive pollution occurs or dry non-conductive pollution occurs which becomes

conductive due to condensation which is to be expected.

Pollution Degree 4 – Continuous conductivity occurs due to conductive dust, rain, or other wet conditions.

13.1.1.3 Overvoltage Categories and Installation Classification:

Overvoltage Categories define transient overvoltage conditions. There are four different levels as indicated in IEC

60664.

I: Signal level — Special protected equipment or parts of equipment, e.g., circuit board inside a DVD player.

II: Local level — Portable equipment that is supplied from the wall outlet, e.g., a DVD player

III: Distribution level — Equipment in fixed installation such as HVAC system, Washers / Dryers, etc.

IV: Primary supply level — Equipment for use at the origin of the installations such as overhead lines, cable

systems, etc.

Lower level category is subject to smaller transients than the category above.

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