AMC1200

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SBAS542A

– APRIL 2011 – REVISED AUGUST 2011

ISOLATION GLOSSARY

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 that 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.

Insulation:

Operational insulation

—Insulation needed for the correct operation of the equipment.

Basic insulation

—Insulation to provide 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 that provides a degree of protection against electric shock

equivalent to double insulation.

Copyright

© 2011, Texas Instruments Incorporated

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