ACB frame and LSIG worksheet equations
The worksheet derives pickup starting points but leaves time delay and coordination to the exact trip-unit study.
Ir = min(Ib × Fmargin, Iz, In)Isd = kS × IrIi = kI × IrIg = kG × InIcu >= Ikwhere:
- Ir
- Long-time pickup current[A]
- Isd
- Short-time pickup current[A]
- Ii
- Instantaneous pickup current[A]
- Ig
- Ground-fault pickup current[A]
- In
- ACB frame or rated-current reference[A]
Final LSIG pickup and delay settings require manufacturer curves, conductor limits, source data, and a protection-coordination study.
How to build an ACB LSIG worksheet
Enter feeder design current, corrected conductor ampacity, prospective fault current, candidate breaking capacity, and the desired pickup multiples. The result produces frame and pickup starting points for a later manufacturer-curve study.
- Choose a frame that can carry the design current.
- Keep long-time pickup between load current and protected-conductor ampacity.
- Enter short-time and instantaneous pickup multiples.
- Enter a ground-fault pickup starting point only when applicable.
- Complete time-delay and selectivity studies using exact curves.
Meaning of L, S, I, and G
L is long-time overload protection, S is delayed short-time protection, I is instantaneous short-circuit protection, and G is ground-fault protection. Pickup current and operating delay are separate settings, and not every trip unit provides the same ranges or functions.
| Function | Pickup | Main coordination purpose |
|---|---|---|
| Long time | Ir | Load and conductor overload protection |
| Short time | Isd | Delayed fault clearing and downstream selectivity |
| Instantaneous | Ii | Fast high-current fault clearing |
| Ground fault | Ig | Ground-fault pickup and delayed clearing |
Why final LSIG settings need a study
Generic multiples cannot establish coordination. Final settings require utility and transformer source data, minimum and maximum fault current, downstream protective-device curves, conductor damage limits, arc-flash implications, system earthing, and the exact ACB trip-unit manual.
Calculate short-circuit currentScreen breaker selectivityCheck cable short-circuit withstand
ACB LSIG worksheet example
For 800 A design current, 1000 A conductor ampacity, and 10% long-time margin, the worksheet proposes an 880 A Ir starting point within a 1000 A frame. Entered 6× and 10× multiples produce Isd and Ii references, but their delays and coordination remain unresolved until exact curves are studied.
Assumptions
- Entered current and ampacity are already corrected design values
- LSIG numbers are pickup starting points only
- Ground-fault protection is applicable to the selected system
Important Warnings
- Preliminary engineering estimate only. Verify the final design against the current applicable standard, local rules, manufacturer data, and project conditions.
- Final Ir, tr, Isd, tsd, I2t, Ii, Ig, and tg settings require the exact trip-unit ranges, manufacturer curves, conductor damage curves, source data, and selectivity study.
- Do not energize equipment using generic LSIG settings from this worksheet.