LV Breaker Selectivity

Check total selectivity between an upstream main breaker and a branch breaker by the instantaneous-trip setting ratio and by the downstream let-through I²t vs the upstream breaking I²t capability.

Inputs

Upstream rated Iₙ (main) A

Upstream instantaneous Iᵢ A

Branch rated Iₙ A

Branch instantaneous Iᵢ A

Fault level I_k kA

Branch cut-off time ms

Upstream I²t withstand kA²s

Results

Note — Selectivity holds when the upstream instantaneous setting clears the branch let-through current with margin and the upstream I²t breaking capability exceeds the downstream let-through I²t. The 1.4× ratio is a typical design margin; confirm against the manufacturer energy-limiting (I²t) curves and a certified engineer. For partial selectivity use only the I²t criterion.