Electrical
LSIG Breaker Settings
Long-time, Short-time, Instantaneous and Ground-fault settings for an electronic-trip circuit breaker — snapped to the discrete steps real breakers actually offer, not idealised continuous values.
Standard method — stated scope limits apply
Enter breaker data and generate.
About This Calculation
LSIG (Long-time, Short-time, Instantaneous, Ground fault) is the standard four-zone protective curve used on molded-case and power circuit breakers with adjustable trip units — each zone targets a different fault magnitude and timescale, and getting the settings right (and properly coordinated with upstream/downstream breakers) is what actually determines which breaker opens for a given fault, not just whether a breaker CAN open for it. Long-time protects against sustained overload (like a thermal element); Short-time and Instantaneous protect against progressively larger fault currents at progressively faster speeds; Ground fault specifically targets line-to-ground faults, which can otherwise be too small to reliably trip the phase elements.
Frequently Asked Questions
Why does a breaker need four separate zones instead of one curve?
Because overload, moderate fault, and severe fault conditions genuinely need different response speeds and thresholds to coordinate correctly with other breakers in the system — a single curve can’t simultaneously ride through normal motor starting current, coordinate selectively with a downstream breaker on a moderate fault, and still clear a severe bolted fault fast enough to limit damage.
What is the purpose of the short-time delay specifically?
It deliberately delays tripping on a moderate-to-high fault long enough for a downstream breaker (closer to the actual fault) to clear it first — this is what "selective coordination" means: only the breaker nearest the fault opens, keeping the rest of the system energized.
Why is instantaneous protection sometimes turned off entirely?
On an upstream (main or intermediate) breaker in a fully-coordinated system, instantaneous tripping would defeat the selective coordination the short-time delay was set up to provide — a severe fault downstream would trip BOTH the downstream breaker and the upstream instantaneous element simultaneously, causing unnecessary loss of unaffected loads.
Why is ground fault protection a separate zone from phase overcurrent?
A line-to-ground fault, especially through some impedance (like a wet insulation failure), can draw current far below the phase overcurrent pickup but still be a real, dangerous fault — dedicated ground fault protection, often using a zero-sequence or residual current measurement, catches faults the phase elements alone would miss.