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Main Components

Discrimination

Back-up Protection

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Discrimination

Ideally only C1should operate for all overloads

up to its rated overload breaking capacity

Supply Load

C2 C1

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Back-up Protection

Ideally C2should operate if C1 fails to operate

within operating time or if short circuit current

exceeds rated overload breaking capacity of C1

Supply Load

C2 C1

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Some Important Parameters

Icu, rated ultimate short circuit breaking

capacity

Ics

, rated service short circuit breaking capacity

SCPD, short circuit protection device

Is, selectivity limit current

IB, take over current

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Design Method

Use the characteristic curves of the breakers

to decide operating conditions

Use practical tests to verify or decide the set

points of breakers

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Some practical considerations

ISshould not be too lowthis can cause loss

of discrimination

ICU

of C1

should not be smaller that IB

of the

association

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Some Examples

A

Pre arching characteristic of fuse

BOperating characteristic of fuse

COperating characteristic of the non

current limiting circuit breaker

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Since the instantaneous release cannon be stopped once started

a practical test must be done to determine selectivity in the

shaded area

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C1Non current limiting circuit breaker

C2Current limiting circuit breaker

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C1,C2Non current limiting circuit breakers