ISOLATION GLOSSARY

ISO721M-EP

Creepage Distance — The shortest path between two conductive input-to-output leads measured along the

surface of the insulation. The shortest distance path is found around the end of the package body.

Clearance — The shortest distance between two conductive input-to-output leads measured through air (line of

sight).

Input-to-Output Barrier Capacitance -- The total capacitance between all input terminals connected together,

and all output terminals connected together.

Input-to-Output Barrier Resistance -- The total resistance between all input terminals connected together, and

all output terminals connected together.

Primary Circuit -- An internal circuit directly connected to an external supply mains or other equivalent source

that supplies the primary circuit electric power.

Secondary Circuit -- A circuit with no direct connection to primary power, and derives its power from a separate

isolated source.

Comparative Tracking Index (CTI) -- CTI is an index used for electrical insulating materials. It is defined as the

numerical value of the voltage that 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 CTI value of the

insulating material, the smaller the minimum creepage distance.

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.

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