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ISOLATION GLOSSARY

Insulation:

Pollution Degree:

Installation Category:

AMC1203

SBAS427 – FEBRUARY 2008

Creepage Distance: The shortest path between two

the leakage current produces an overvoltage at the

conductive input to output leads measured along the

site of the discontinuity, and an electric spark is

surface of the insulation. The shortest distance path

generated. These sparks often cause carbonization

is found around the end of the package body.

on insulation material and lead to a carbon track

between points of different potential. This process is

known as tracking.

Operational insulation—Insulation needed for the

correct operation of the equipment.

Basic

insulation—Insulation

provide

basic

protection against electric shock.

Clearance: The shortest distance between two

conductive input to output leads measured through air

Supplementary

insulation—Independent

insulation

(line of sight).

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.

Input-to Output Barrier Capacitance: The total

capacitance between all input terminals connected

Pollution

Degree

1—No

pollution,

only

dry,

together, and all output terminals connected together.

nonconductive pollution occurs. The pollution has no

Input-to Output Barrier Resistance: The total

influence on device performance.

resistance between all input terminals connected

Pollution Degree 2—Normally, only nonconductive

together, and all output terminals connected together.

pollution occurs. However, a temporary conductivity

Primary Circuit: An internal circuit directly connected

caused by condensation is to be expected.

to an external supply mains or other equivalent

Pollution Degree 3—Conductive pollution, or dry

source that supplies the primary circuit electric power.

nonconductive pollution that becomes conductive

Secondary

Circuit:

circuit

with

direct

because of condensation, occurs. Condensation is to

connection to primary power that derives its power

be expected.

from a separate isolated source.

Pollution Degree 4 – Continuous conductivity occurs

Comparative Tracking Index (CTI): CTI is an index

as a result of conductive dust, rain, or other wet

used for electrical insulating materials. It is defined as

conditions.

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

Overvoltage Category—This section is directed at

localized deterioration on or through the surface of an

insulation coordination by identifying the transient

insulating material as a result of the action of electric

overvoltages that may occur, and by assigning four

discharges on or close to an insulation surface. The

different levels as indicated in IEC 60664.

higher CTI value of the insulating material, the

I: Signal Level: Special equipment or parts of

smaller the minimum creepage distance.

equipment.

Generally,

insulation

breakdown

occurs

either

II: Local Level: Portable equipment, etc.

through the material, over its surface, or both.

III: Distribution Level: Fixed installation.

Surface failure may arise from flashover or from the

progressive degradation of the insulation surface by

IV: Primary Supply Level: Overhead lines, cable

small localized sparks. Such sparks are the result of

systems.

the

breaking

surface

film

conducting

Each category should be subject to smaller transients

contaminant on the insulation. The resulting break in

than the previous category.

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