Air Circuit Breaker Trip Unit Types and Protection Functions Guide

What is an air circuit breaker trip unit? An air circuit breaker trip unit is the protection and measurement module inside an ACB — available in electronic or electromechanical variants — that monitors current continuously and triggers interruption based on configurable thresholds across long-time, short-time, instantaneous, and ground-fault (LSIG) protection functions defined under IEC 60947-2. Specifying an LI-only unit where LSIG is required leaves downstream equipment exposed to sustained ground faults, while oversizing protection bands tightens coordination margins and can cascade into upstream breaker operation. This guide covers what trip units do internally, decoding the LSIG protection functions, LI-versus-LSIG field selection criteria, time-current curve decision logic, IEC-versus-IEEE/NEMA nomenclature differences, and a practical trip unit sizing workflow.

What Exactly Does a Trip Unit Do Inside an ACB?

Strip away the marketing and a trip unit performs three jobs: it senses current (via Rogowski coils or current transformers in the pole assembly), it compares those readings against time-current curves stored in firmware or shaped by bimetal physics, and it fires a tripping coil that releases the mechanism's stored-energy latch. That's it.

The complexity is in the curves. A modern ACB trip unit must hold a 3000 A motor inrush for 8 seconds without flinching, then trip in 80 milliseconds on a 35 kA bolted fault three buses downstream. In our experience commissioning data center switchgear in Frankfurt and São Paulo, the difference between a project that passes its first selectivity audit and one that gets sent back for re-coordination almost always comes down to whether the engineer understood what the trip unit was actually measuring — true RMS, peak, or average — and over what window.

The Two Big Families: Thermal-Magnetic vs Electronic

Thermal-magnetic trip units use a bimetal strip for overload (long-time) and a solenoid for short-circuit (instantaneous). They're cheap, robust, and immune to firmware bugs. They are also imprecise — typical tolerance is ±20% on the long-time pickup, drift with ambient temperature, and offer no communication. You will still see them in older Masterpact M and Emax E1 frames in legacy plants.

Electronic trip units — ABB Ekip, Schneider Micrologic, Siemens ETU — sample current digitally (typically 8–32 samples per cycle), compute true RMS, and execute the curve in firmware. Tolerances tighten to ±10% or better, and you gain ground-fault protection, neutral protection, metering, and Modbus/Profibus/IEC 61850 communication. Every ACB Stoklink ships today — including the ABB 1SDA070701R1 E1.2B 630A with Ekip Dip LI — uses electronic trip technology.

Decoding LSIG: The Four Protection Functions Every Engineer Must Know

Trip unit catalog codes look like alphabet soup until you crack the convention. ABB calls them LI, LSI, LSIG, LSIRc; Schneider uses Micrologic 2.0, 5.0, 6.0, 7.0; Siemens uses ETU15B, ETU45B, ETU76B. They all map to the same four IEC functions:

L — Long-Time Protection (Overload)

The L function protects cables and busbars from thermal damage caused by sustained overcurrent. It implements an I²t inverse time curve that mimics conductor heating. You set two parameters: the pickup current Ir (typically 0.4–1.0 × In) and the time delay tr at 6 × Ir (typically 0.5–24 seconds, per IEC 60947-2 Annex E).

A common mistake is setting Ir at the cable ampacity. The trip unit doesn't trip at Ir — it trips at roughly 1.13–1.20 × Ir per the conventional non-tripping/tripping current rule of IEC 60947-2 §8.3.3.1. Set Ir at 0.95 × cable ampacity if you want margin, or use the step-by-step ACB sizing methodology.

S — Short-Time Protection (Selective Short Circuit)

The S function lets you delay the trip on a downstream short circuit so a feeder breaker downstream clears the fault first. You pick a pickup Isd (typically 1.5–10 × Ir) and a time delay tsd (typically 0.05–0.5 seconds). Modern trip units offer two modes: I²t-on (curve slopes) for coordination with fuses, and I²t-off (definite time) for coordination with other ACBs.

I — Instantaneous Protection

The I function is the no-delay solenoid-style protection that fires within 30–50 ms regardless of any time setting. It's there to protect the breaker itself from currents that exceed its withstand rating. On a 65 kA ACB you typically set Ii at 12–15 × In so high-magnitude faults clear immediately.

G — Ground Fault Protection

The G function detects residual (zero-sequence) current — the vector sum of the three phases plus neutral. In a healthy system this sum is zero; ground faults of even a few hundred amps show up clearly. NEC 230.95 mandates ground-fault protection on services ≥1000 A at 480Y/277V; IEC systems use it more selectively, often only on TN-S earthing per IEC 60364-4-41.

Field Reality: When LI Is Enough vs When You Need LSIG

Here's where engineers often overlook the economics. An LI trip unit (long-time + instantaneous only) like the one in the ABB 1SDA070741R1 E1.2B 800A costs roughly 30–40% less than the equivalent LSIG. Do you actually need S and G?

The honest answer: it depends on where the breaker sits in the hierarchy. For an incomer at the main switchboard feeding multiple downstream MCCBs, you absolutely need S — without time-current discrimination the main trips on every downstream feeder fault and you blackout the plant. For a feeder ACB serving a single dedicated load (a 600 kW chiller, a 1 MW PDU), LI is often sufficient because there's nothing to coordinate with downstream.

In practice, what we typically see in the field on European industrial projects:

Incomers and tie breakers — always LSI or LSIG. Subdistribution feeders 800–1600 A — LSI if budget-constrained, LSIG if the earthing system requires it. Single-load feeders below 1000 A — LI is fine. The ABB 1SDA070702R1 with LSI covers the subdistribution case at 630 A.

The Time-Current Curve: How Trip Units Actually Decide

The time-current curve is the contract between the trip unit and the system. Get it wrong and you either nuisance-trip on inrush or fail to clear faults before insulation breaks down.

Worked example: an ACB rated In = 1600 A with Ir set at 0.8 (so 1280 A) and tr set at 12 seconds at 6×Ir. A sustained 3200 A overload (2.5 × Ir) trips in t = 12 × (6 × 1280 / 3200)² = 12 × 5.76 = 69 seconds. That's the math behind every coordination study.

Why Engineers Get Burned on Inrush

A 1500 kW induction motor draws perhaps 7 × FLC (full load current) for 4–8 seconds during start. If your ACB Ir is set tight to FLC and tr is set short for fast cable protection, you'll trip on every start. The fix isn't a bigger breaker — it's the right curve. Dedicated motor protection profiles in modern Ekip and Micrologic units shape the curve to ride through inrush while still protecting the cable.

IEC vs IEEE/NEMA: Same Functions, Different Names

If you're working a project that mixes European and North American spec sheets, the terminology will trip you up before the breaker does. IEC 60947-2 calls overload protection "long-time" and uses Ir/tr; IEEE C37.13 (the Low-Voltage AC Power Circuit Breaker standard) calls it "long-time pickup/long-time delay" and uses LTPU/LTD. The math is the same; the labels differ.

NEMA AB 4 governs molded-case breakers more than ACBs, but you'll see NEMA-style enclosure ratings (1, 3R, 4, 4X, 12) on switchgear specifications. NEMA test points for ground fault are different from IEC — UL 1066 requires GF testing at 1200 A minimum sensor pickup, while IEC 60947-2 allows pickup as low as 0.2 × In on electronic units.

Sizing the Trip Unit: A Practical Workflow

Sizing is not picking a number off a nameplate. Here's the workflow that's served us across hundreds of switchgear projects:

Step one: establish the continuous current. Apply a 1.25 service factor to the calculated load per IEC 60364-5-52 or NEC 215.2(A)(1). For a 1100 A continuous load you need a frame ≥1375 A — pick a 1600 A frame like the ABB 1SDA070861R1 E1.2B 1600A and set Ir to 0.86 (1376 A).

Step two: verify the breaking capacity Icu against the prospective short-circuit current at the installation point. An E1.2B has Icu = 42 kA at 415V, Ics = 42 kA. If your transformer impedance gives 38 kA prospective, you're fine. If it gives 50 kA, step up to E2.2 frame (Icu = 65 kA) — something like the ABB 1SDA070981R1 E2.2B 1600A.

Step three: coordinate. Run an arc flash and selectivity study in ETAP or SKM. Adjust S settings on upstream breakers until each downstream fault clears at the lowest level. This is where the brand comparison matters — ABB and Schneider publish detailed selectivity tables; Siemens does too but you'll need SIMARIS Design.

Communication, Metering and the Smart Trip Unit

The version of "trip unit" includes a lot more than protection. ABB Ekip Touch, Schneider Micrologic 6.0 X, and Siemens ETU 8 series include:

True RMS metering of current, voltage, power, energy (Class 1 per IEC 61557-12), waveform capture for fault analysis, event logging with timestamps, Modbus RTU/TCP and IEC 61850 communication, integrated arc flash reduction maintenance switching (ARMS / ZSI). On a recent retrofit at a German automotive plant, the customer's payback case for upgrading to communicating trip units came entirely from energy submetering — they no longer needed separate power meters on each feeder.

Zone Selective Interlocking (ZSI)

ZSI is one of those features that sounds esoteric until you see it work. When a fault occurs, the downstream breaker that sees it sends a restraint signal up the chain. Upstream breakers that see the fault but receive a restraint signal hold their S delay. Upstream breakers that see the fault and receive no restraint trip immediately, bypassing tsd. The result: the closest breaker to the fault clears it fast, but if it fails, the next one up takes over without the full coordination delay. Critical for arc flash reduction.

Common Failure Modes and How to Diagnose Them

In our experience the top three trip unit issues on commissioned plants are: nuisance tripping on inrush (almost always a too-tight tr or I setting — see our nuisance tripping diagnosis guide), failure to trip on legitimate fault (usually wrong CT polarity on retrofit installations, or G function disabled in firmware), and communication dropouts (almost always Modbus baud rate mismatch or termination resistor missing).

Some engineers argue that you should test the trip unit with primary injection every year. In my experience, secondary injection through the test port covers 95% of failures and is roughly 10× faster. Reserve primary injection for commissioning and after any CT replacement. IEEE Std 3007.2-2010 supports this risk-based approach explicitly.

Procurement Notes: What to Specify When You Order

A trip unit specification should call out, at minimum: protection functions required (LI / LSI / LSIG / LSIRc), measurement type (true RMS mandatory above 400 A), communication protocol if any (Modbus RTU is the safe default), neutral protection level (50% / 100% — required at 100% for unbalanced single-phase loads on a three-phase ACB), local indication (LCD vs LED bargraph), and test port type.

For procurement, the SKU encodes most of this. Take ABB 1SDA070781R1: E1.2B = frame size and breaking class B (42 kA), 1000 = current rating, Ekip Dip = trip unit family (basic dip-switch configurable), LI = functions, 3p = three pole, F F = fixed/fixed mounting and connection. Decode that and you know exactly what's in the box.

For higher current applications, the E2.2 frame in the ABB 1SDA071021R1 E2.2B 2000A handles 2000 A continuous with 65 kA breaking. Browse the fullair circuit breakers range at Stoklink when you need to compare frames side by side. For lower-current branch protection downstream of an ACB, pair with selections from the miniature circuit breaker collection, and where personnel protection is required, residual current devices complement (not replace) the ACB G function.

Trip Unit Settings Walk-Through: A Real 1250 A Feeder

Let's make this concrete. Consider an ABB 1SDA070821R1 E1.2B 1250A with Ekip Dip LI feeding a 750 kVA dry-type transformer that powers a manufacturing line at 400V. Calculated load: 1080 A. Cable: 2 × 240 mm² Cu XLPE per phase, ampacity ~1140 A.

Long-time pickup Ir: set to 0.88 × 1250 = 1100 A. This protects the cable (1100 < 1140) and covers the load (1100 > 1080). Long-time delay tr: set to 12 s at 6×Ir. This rides through transformer inrush (typically 8 × FLC for 0.1 s, well below the 6×Ir reference point on the curve).

Instantaneous Ii: set to 10 × In = 12.5 kA. Below the 42 kA breaker rating, above any expected through-fault from the transformer. Done. Three settings, ten minutes of work, documented in the commissioning record.

If this same feeder were upstream of two 600 A subdistribution boards, you'd swap to LSI and add Isd = 6 × Ir (6600 A) with tsd = 0.2 s, allowing the 600 A downstream MCCBs to clear their faults first. That's selectivity.

Special Applications: Generators, UPS Outputs, and Solar

Standard trip unit curves assume a stiff utility source. They misbehave on weak sources. A 2 MVA standby diesel generator has a sustained short-circuit current of perhaps 3 × FLC — roughly 8.7 kA on a 400V genset — because the AVR and exciter can't sustain higher current without de-exciting. If your I setting is at 10 × In, the breaker never trips on a downstream short circuit. Use generator-specific protection: lower I pickup (3–5 × In), and consider voltage-restrained overcurrent if available.

UPS outputs are similar but worse — static bypass current is limited to 1.5–2 × nominal for a few cycles before the UPS folds back. Coordinate the trip unit with the UPS overload curve, not with utility short-circuit values. For data center applications specifically, see our ACBs in Data Centers selection guide.

Solar PV inverters present yet another case: they're current-limited at 1.1–1.2 × rated output and will trip themselves on any fault rather than feed it. The ACB protecting an inverter feeder rarely sees fault current at all — the inverter shuts down first. Set L tight, set I high, and rely on the inverter's internal protection.

Trip Unit Auxiliaries: Don't Forget These

The trip unit itself is one piece. The functioning protection system needs supporting elements that procurement teams often forget on the BOM:

Auxiliary power supply — most electronic trip units self-power above ~20% of In, but you'll want 24 VDC auxiliary supply for full functionality at low currents and during testing. Without it, your communication dies whenever the breaker is open. Trip coil (shunt release) — for remote tripping from external protection relays or fire alarm systems. Closing coil and motor operator — for remote ON commands. These are separate ABB part numbers and not included in the base breaker SKU. Undervoltage release — required by some codes (NFPA 70E for emergency disconnects). And finally CT inputs for external residual current detection if you're doing G protection via an external core balance CT instead of the internal Rogowski sum.

Don't underestimate the wiring. A typical communicating ACB needs ~24 wires landed in the secondary terminals for full functionality. Budget the labor.

Related Reading

• IEC 60947-2 for Air Circuit Breakers: Full Standard Breakdown
• How to Size an Air Circuit Breaker: Step-by-Step Selection Calculator
• ABB vs Schneider vs Siemens ACB: Brand Comparison for Engineers
• Air Circuit Breaker Nuisance Tripping: Causes, Diagnosis and Fixes

Conclusion

Trip units are not a commodity. The difference between LI, LSI, and LSIG, between thermal-magnetic and electronic, between basic and communicating, will determine whether your switchgear meets its selectivity targets, survives the next short circuit, and provides the metering data your operations team will demand five years from now. Specify deliberately. Set the curves with the load characteristics in mind, not just the cable rating. Test annually with secondary injection. Document everything in the commissioning record.

For the full selection methodology — frame sizing, breaking capacity verification, accessory selection and lifecycle costing — see our master Air Circuit Breaker Guide: How It Works, Selection, Sizing and Maintenance. For protection coordination across an entire LV distribution system, you'll also want to factor in protective relays upstream of the ACB and downstream MCCBs, MCBs, and RCDs. The trip unit is one link in that chain — get it right and the rest of the system has a chance to work as designed.