Protection - Selectivity Check

Protection selectivity check - downstream breaker must trip before upstream for faults on its branch.

ProtectionSelectivityRelay

Downstream trip delaymsUpstream trip delayms

Time margin: 120ms

Status: Selective (≥ 100 ms margin)

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Formula, assumptions, and limits

\Delta t \geq 100\,ms

Symbol	Meaning	Unit
$Δt$	Time grading margin	ms

Source: IEC 60255 / ABB Selectivity

In short

Protection selectivity check - downstream breaker must trip before upstream for faults on its branch.

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