Circuit protection

ACB Frame & LSIG Trip-Setting Worksheet

Build preliminary air circuit breaker frame size and Long-time, Short-time, Instantaneous, and Ground-fault (LSIG) setting ranges from the feeder load current, cable capacity, fault level, and the largest downstream device. Starting ranges for a coordination study — final settings come from the manufacturer’s curves.

Feeder design current I_b (A)

Busbar / cable capacity I_z (A)

Prospective I_sc (kA)

Largest downstream device (A)

ⓘLong-time protects the cable (I_b ≤ I_r ≤ I_z). Short-time is set above the downstream device so the ACB holds during a downstream fault — the basis of selectivity.

ACB frame I_n

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L — Long-time I_r

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S — Short-time I_sd

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I — Instantaneous I_i

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G — Ground fault I_g

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What each LSIG band does

Band	Function	Typical setting range
L	Long-time overload — protects the conductor	I_r = 0.4–1.0 × I_n
S	Short-time — rides through downstream faults for selectivity	I_sd = 1.5–10 × I_r, delay 0.1–0.4 s
I	Instantaneous — clears close-up high faults	I_i = 2–15 × I_n, or OFF (ZSI)
G	Ground fault — earth-fault pickup with delay	I_g = 0.2–1.0 × I_n, delay 0.1–0.5 s

How the worksheet builds the settings

Frame I_n: the next standard ACB frame at or above the feeder design current.
Long-time I_r: set to carry the load yet protect the conductor, so I_b ≤ I_r ≤ I_z. The worksheet starts I_r just above I_b.
Short-time I_sd: raised above the downstream device’s clearing region with a short delay, so the ACB waits while the downstream breaker clears its own fault.
Instantaneous I_i: covers a close-up fault. It can be switched off where the ACB’s short-time withstand (I_cw) and zone-selective interlocking provide full selectivity.
Ground fault I_g: a lower earth-fault pickup with a delay, coordinated with downstream earth-fault protection.

⚠These are starting ranges, not a coordination study. Real selectivity, the short-time withstand rating I_cw, and the delay grading must be verified against the actual manufacturer time–current curves and the fault levels at each level of the installation.

Frequently asked questions

Why turn the instantaneous band off?

To achieve full selectivity with a downstream breaker, the ACB must hold during a downstream fault. If its short-time withstand (I_cw) covers the fault level, the instantaneous element can be disabled so only the downstream device trips.

What is I_cw and why does it matter?

I_cw is the current the ACB can carry for a short, rated time (for example 0.5 s or 1 s) without tripping or damage. It sets how long the short-time delay can safely be, and therefore how much selectivity is achievable.

ACB or MCCB for this feeder?

ACBs suit large frames (typically 630–6300 A), high short-time withstand, and full LSIG adjustability for main and tie duties. Below that, or where breaking-only performance is enough, an MCCB is usually more economical.

Select the ACB

Match the frame, I_cw, trip unit, and poles to a device, then confirm the settings with a coordination study.

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Standards referencedIEC 60947-2 (circuit-breakers, I_cu/I_cs/I_cw, trip units) · IEC 60364-4-43 / 5-53 (overload, fault, and coordination).

Preliminary ranges for guidance only. Final frame, ratings, and LSIG settings must be established by a qualified person through a protection-coordination study using the manufacturer’s curves and the installation’s fault levels.