9.3 Post-Fault Breaker Maintenance & LV Trip-Unit Settings
Key Takeaways
- A breaker that has cleared a fault at or near its rating must be inspected and tested per NETA MTS before return to service — contact erosion, arc-chute damage, and trip-unit drift are all possible.
- Post-fault maintenance: visual inspection, clean/replace arc chutes, Ductor contact resistance, IR re-test, and trip-unit secondary-injection verification.
- Electronic LV trip units provide LSI (long-time, short-time, instantaneous) or LSIG (+ ground-fault) functions, each with pickup and time-band settings.
- Settings are determined by a coordination study that overlays time-current curves; field settings must match the study and be verified by secondary injection, not a "typical" value.
- Long-time pickup is a multiplier on the rating plug/sensor current (e.g., 0.8 x 1600 A = 1280 A); verify each dial against the coordination study and the nameplate.
Post-Fault Breaker Maintenance
When a breaker clears a fault — especially one near its interrupting rating — the internal damage is not always visible. The arc leaves contact erosion, deposits on the arc chute, and can shift an electronic trip unit's calibration. ANSI/NETA MTS requires that any breaker that has interrupted a fault at or near its rating be inspected and tested before return to service:
- Visual inspection — contacts (where visible), arc chutes (arc tracks, charring, cracked splitter plates), mechanism, and any puffer/bellows on air-magnetic units.
- Clean or replace arc chutes — replace splitter plates or the chute assembly if eroded past the OEM limit.
- Contact resistance (Ductor) — re-measure each pole; a rising or unbalanced reading after a fault points to contact erosion.
- Insulation resistance re-test — phase-to-ground and across open contacts; a drop indicates tracking or contamination.
- Trip-unit verification — secondary injection to confirm the trip unit still operates at its set points.
- Operational test — trip/close cycle, timing, and 52a/52b auxiliary contact continuity.
If any result is beyond OEM spec, the breaker is not returned to service — contacts or the trip unit are replaced, or the breaker is swapped out.
LV Electronic Trip Units — LSI and LSIG
Modern low-voltage power circuit breakers (ACBs and larger MCCBs) use electronic trip units that provide a stack of protection functions:
| Function | Protects against | Setting |
|---|---|---|
| L — Long-time | Overload | Pickup (multiplier of In, typically 0.4–1.0) + time band |
| S — Short-time | Short-circuit (coordinated delay) | Pickup (multiplier of In, ~2–9x) + delay band (flat or I-squared-t) |
| I — Instantaneous | Short-circuit (no delay) | Pickup (multiplier of In) — often a fixed hardware override on ACBs |
| G — Ground-fault (LSIG) | Ground faults | Pickup (multiplier of Ig) + delay band |
Each function has a pickup (the current at which it starts timing) and a time band (how long it waits before tripping). The long-time function follows an inverse-time curve; the short-time function can be flat or I-squared-t so it coordinates with downstream devices; the instantaneous function clears with no intentional delay.
Pickup Math
The pickup is a multiplier on the breaker's sensor (rating plug) current, not on the frame. Worked example:
A 1600 A ACB with L = 0.8 picks up at 0.8 x 1600 = 1280 A continuous. Above 1280 A the long-time inverse-time curve starts timing; actual clearing time depends on the time band and how far above pickup the current is.
The same pattern applies to short-time, instantaneous, and ground-fault — always a multiplier on the correct base (In for L/I/G, Ir for S on many OEMs).
Verifying Settings Against the Coordination Study
Trip-unit dials are not set to a "typical" or "reasonable" value in the field. They are set to the values from the coordination study — the engineering document that overlays the time-current curves of every series-connected protective device so the downstream breaker trips first, with a 0.2–0.3 s margin between curves.
On maintenance, verify each dial against the study and the nameplate:
- Is the rating plug (In) the size called for? A 1600 A plug on a 2000 A frame changes every multiplier.
- Is Ir (long-time pickup) at the study value?
- Is Isd (short-time pickup) and its delay band correct?
- Is Ii (instantaneous) enabled/disabled and at the right pickup? On main breakers with zone-selective interlocking, disabling or mis-setting Ii is a common defect.
- Is Ig (ground-fault) pickup and delay correct, and is ZSI wired correctly between the main and feeders?
A secondary-injection test set (the OEM tester) injects programmed currents and confirms the trip unit trips at the study's pickup and time — that is the verification record NETA wants.
Primary vs. Secondary Injection
Secondary injection feeds test current into the trip unit's CT secondaries through the OEM test plug — it verifies the electronics and the settings but does not exercise the main contacts or the CT primary. Primary injection pushes high current through the breaker from line to load (often thousands of amps from a dedicated primary-injection set) and exercises the CTs, the trip unit, and the main contacts end-to-end. NETA MTS prefers primary injection for commissioning and after major work because it validates the entire protection chain; secondary injection is the routine maintenance check and is far quicker. A common Level 2 trap is treating a passing secondary-injection test as proof the breaker will clear a real fault — it only proves the trip unit, not the CT wiring or the primary path.
Common Mis-Settings
- Rating plug swapped during a maintenance swap — every function shifts.
- Instantaneous enabled on a main that should coordinate with feeders — defeats selectivity.
- Ground-fault set too low — nuisance trips on inrush or unbalanced load.
- ZSI not wired / defeated — the main clears on a feeder fault instead of the feeder.
- Long-time band too fast — trips on normal motor inrush.
These are all caught by a dial-vs-study check plus a secondary-injection test — not by a "looks right" walkthrough.
Worked Example
A 2000 A feeder ACB has L = 0.9, S = 6x / 300 ms, I = OFF, G = 0.3x / 120 ms per the coordination study. On inspection, the rating plug is 1600 A (wrong — should be 2000 A), so the long-time pickup is actually 0.9 x 1600 = 1440 A instead of 0.9 x 2000 = 1800 A. The breaker has been nuisance-tripping on a 1700 A load. The plug is replaced, all dials are re-verified against the study, and a secondary-injection test confirms operation before re-energization.
A 1600 A LV air circuit breaker has its Long-Time pickup set at 0.8. It will start timing toward a trip at approximately:
Trip-unit settings on an LV breaker in the field should be:
A low-voltage breaker has just interrupted a fault near its rating. Per NETA MTS, the next step is to: