Essential Protection Settings of Air Circuit Breakers (ACBs)

1. Introduction

The air circuit breaker (ACB) is the workhorse at the top of most low-voltage distribution systems — the incoming device on main switchboards, the tie between bus sections, and the feeder breaker for the largest downstream loads. Unlike a simple thermal-magnetic moulded-case breaker, a modern ACB is fitted with an electronic trip unit (also called a protection relay or release) whose behaviour is defined almost entirely by how it is set. Two identical breakers, same frame, same rating, can protect a system beautifully or trip a plant offline at every motor start — the difference is the settings.

This is why protection engineers treat ACB configuration as a discipline in its own right. The settings determine how much current the breaker allows before it acts, how long it waits, and how it coordinates with the devices above and below it so that a fault is cleared by the nearest breaker only, leaving the rest of the system energized. Get it right and you have selective, reliable protection that safeguards cables, busbars and equipment. Get it wrong and you have either nuisance tripping that erodes availability, or sluggish protection that lets fault energy damage the very assets it is meant to protect.



This article, prepared by the engineering team at Keentel Engineering, explains the essential ACB protection settings — the widely used LSIG model of Long-time, Short-time, Instantaneous and Ground-fault protection, plus neutral protection — what each parameter does, how the parameters interact, and how they are coordinated and commissioned in practice. It is written for protection, electrical and commissioning engineers. A detailed FAQ and three fully anonymized case studies follow the main text.

2. First, Get the Reference Right: In versus Ir

Every ACB setting is expressed as a multiple of a base current, so the first thing to be clear about is which base. In is the rated (sensor) current of the trip unit — the reference the manufacturer builds the device around. Ir (sometimes written I1) is the long-time pickup, the actual overload threshold the engineer sets, expressed as a fraction of In (for example Ir = 0.8 × In). Downstream settings such as short-time and instantaneous pickup are then usually expressed as multiples of Ir, while ground-fault pickup is generally a fraction of In. Confusing In with Ir is one of the most common — and most consequential — setting errors, because it shifts the entire protection curve.

3. The LSIG Protection Model at a Glance

Modern ACB trip units implement up to four current-based protection functions, remembered by the acronym LSIG. Each function is a “band” on the time–current curve, and each is defined by a pickup (how much current) and, except for instantaneous, a delay (how long). A separate neutral-protection setting extends overload/fault protection to the neutral conductor in three-phase four-wire systems.

4. Long-Time Protection (Ir + Tr): The Overload Band

Long-time protection is the ACB’s thermal-overload function. The pickup Ir defines the boundary between “normal” and “overload”; below Ir the breaker carries current indefinitely, above it the breaker will eventually trip. Ir is set to protect the downstream cable and equipment, so it is chosen at or below the continuous current-carrying capacity of the circuit — never above it — while sitting comfortably above the expected maximum load current.

The long-time delay Tr sets how long an overload is tolerated before tripping, and the characteristic is inverse: the larger the overload, the shorter the time to trip, following an I²t (constant thermal-energy) law that mimics the heating of a conductor. Tr is therefore usually quoted at a reference multiple such as 6 × Ir. A longer Tr lets brief, benign overloads — a cluster of motors starting, a transformer energizing — pass without tripping; a shorter Tr protects thermally sensitive cable more aggressively. Many trip units also provide thermal memory, so repeated or closely spaced overloads accumulate rather than resetting the timer each time, reflecting the real thermal state of the conductor.

5. Short-Time Protection (Isd + Tsd): Riding Through and Discriminating

Between ordinary overload and a bolted short circuit lies a band of higher overcurrents — motor and transformer inrush, feeder through-faults — where the engineer wants control over both threshold and timing. That is the short-time band. The pickup Isd (a multiple of Ir) sets where short-time protection begins; the delay Tsd introduces an intentional, deliberate wait.

Why wait deliberately during a fault? Because of selectivity. If a fault occurs on a downstream feeder, the downstream breaker should clear it while the upstream ACB holds in, so that only the faulted circuit is lost. A short-time delay on the upstream device gives the downstream device time to operate first — the essence of time-current discrimination. The trade-off is that any delay lets more energy through, so Tsd is kept as short as coordination allows, and it must remain within the breaker’s short-time withstand rating (Icw).

Most trip units offer an I²t ON / I²t OFF choice for the short-time region. With I²t OFF, the delay is a fixed (definite) time above Isd. With I²t ON, the delay follows an inverse ramp at lower currents before flattening — this shapes the curve to nest neatly above downstream fuses and thermal-magnetic breakers, improving discrimination with I²t-limited devices. Selecting the right mode is often the key to clean coordination.

6. Instantaneous Protection (Ii): No Deliberate Delay

The instantaneous function trips the breaker with no intentional time delay once current exceeds the pickup Ii, giving the fastest possible clearance for very high-magnitude faults close to the breaker. Because it has no delay, instantaneous protection cannot discriminate with downstream devices — so on an upstream ACB where full selectivity is required, Ii is often raised high or switched OFF (subject to the breaker’s withstand rating and to any short-circuit protection provided by short-time). On the lowest-level device, by contrast, a low instantaneous setting clears faults quickly and limits let-through energy. The correct choice depends entirely on where the breaker sits in the coordination scheme.

7. Ground-Fault Protection (Ig + Tg): Catching Earth Faults Early

Earth faults often start small — a degraded insulation path, a loose termination — and draw current that returns through the earth/ground rather than the phase or neutral conductors. Such currents can be far below the phase-overcurrent pickups, so a dedicated ground-fault function is needed to detect them before they escalate into fire or equipment damage. The trip unit computes the residual (vector sum) of the phase currents (and neutral, where measured); a non-zero residual indicates current leaking to earth.



Ground-fault pickup Ig is set as a fraction of In, low enough to catch developing faults but above normal standing leakage and expected imbalance. The delay Tg lets the function ride through brief transient imbalances (for example during switching) and, importantly, allows earth-fault selectivity so a downstream earth fault is cleared downstream. Ig and Tg together protect people and plant from the insidious, low-level faults that phase protection can miss.

8. Neutral Protection (N): Not an Afterthought

In three-phase four-wire systems the neutral can carry significant current — from single-phase load imbalance and, increasingly, from triplen (3rd, 9th…) harmonics produced by electronic loads, which add arithmetically in the neutral rather than cancelling. An undersized or unprotected neutral can overheat even when the phases look healthy. Neutral protection extends the ACB’s overload/short-time logic to the neutral pole, typically selectable as OFF, 50% or 100% of the phase setting.

Where the neutral is full-sized and harmonic content is high, 100% neutral protection is common; a half-rated setting suits reduced-neutral installations.Choosing this correctly prevents a hidden thermal risk from going unprotected.

9. Worked Interpretation of a Typical Trip-Unit Readout

It helps to read a real settings string the way the breaker does. Taking a representative electronic trip-unit display of the kind shown on ACB relay units, the values translate as follows (In being the trip-unit rated current):

Read together, this string describes a selective incomer: a realistic overload threshold, delays on short-time and ground-fault to let downstream devices act first, and a high instantaneous pickup so the ACB only trips instantly for the most severe faults. Every value has a reason, and every value must be justified against the cable data, the load profile and the coordination study.

10. Coordination and Selectivity: The Whole Point

Individual settings matter, but the objective is system behaviour: when a fault occurs, only the breaker nearest the fault should operate. This is selectivity (discrimination), achieved by separating the time–current curves of series devices so they never overlap in a way that trips the wrong breaker. Engineers use several, often combined, methods.

The upshot is that ACB settings can never be chosen device-by-device in isolation. They emerge from a protection coordination (discrimination) study that plots every series device on a single time–current chart, checks fault levels at each point, and verifies that the curves are properly nested from the load right up to the source.

11. Commissioning and Good-Practice Setting Rules

A setting is only as good as its verification. Sound practice, echoing the guidance summarized on the source infographic, comes down to a short discipline:



1. Coordinate every setting with upstream and downstream devices — never set a breaker in isolation; work from a discrimination study.
2. Stay within the manufacturer’s recommended ranges and the device’s ratings (In, Icw, Icu) — settings outside the intended envelope are unreliable or unsafe.
3. Match Ir to the protected cable and Ig/N to the earthing and neutral arrangement — protect the conductor, not just the load.
4. Test every protection function after adjustment — primary or secondary injection to confirm pickups and delays behave as set.
5. Document the as-left settings, and review and update them whenever loads, generation, or the network topology change.
   

12. Why It Matters — and Where Keentel Helps

Correctly configured ACB protection delivers four things at once: reliable protection through accurate, selective tripping; system stability by avoiding unnecessary trips of healthy circuits; easy, repeatable commissioning; and equipment safety that extends asset life. These are not competing goals — a good coordination study achieves them together. The reverse is equally true: a single mis-set delay or a confused In/Ir reference can either black out a facility on a routine motor start or let a downstream fault damage a main switchboard.



This is precisely the work Keentel Engineering supports — protection coordination and discrimination studies, trip-unit setting calculations, ZSI and selectivity design, and commissioning and injection-test support — so that every ACB in a distribution system is set with intent, verified by test, and documented for the life of the installation.

Confidential Case Studies

The following three case studies are fully anonymized. They contain no client names, project names, locations or other identifying details, and describe generalized scenarios composited from typical protection-engineering assignments to illustrate how ACB setting decisions are made in practice.

Case Study 1 — Eliminating Nuisance Tripping on Motor Inrush

Situation

A distribution board feeding several large induction motors experienced repeated, seemingly random trips of the section’s air circuit breaker, always during periods of high starting activity. Production was interrupted with no fault ever found on inspection — the classic signature of protection that is mis-set rather than a genuine fault.

Investigation

A review of the trip-unit settings and the motor data showed the short-time pickup (Isd) had been left low and the short-time delay (Tsd) short, so the combined inrush of simultaneous direct-on-line starts — several times full-load current for a few hundred milliseconds — was crossing into the short-time trip region. The instantaneous setting was also low enough to be grazed by inrush peaks. Nothing was faulty; the curve simply did not accommodate legitimate starting current.

Resolution

The engineering team recalculated the short-time band to sit above the aggregate starting inrush with margin, adjusted the short-time delay to ride through the starting period while remaining within the breaker’s short-time withstand, selected the appropriate I²t mode to preserve coordination with downstream motor protection, and raised the instantaneous pickup so it responded only to genuine short-circuit magnitudes. Long-time pickup was confirmed against the feeder cable rating so overload protection was retained.

Outcome

• Nuisance trips during motor starting were eliminated, restoring uninterrupted operation.
• Genuine overload and short-circuit protection remained fully intact and within device ratings.
• Downstream coordination was preserved, so a real motor-circuit fault would still be cleared locally.
• The corrected settings were documented and injection-tested to confirm behaviour.
  

Case Study 2 — Restoring Selectivity Between an Incomer and a Feeder

Situation

On a switchboard supplying a facility that could not tolerate loss of the whole board, a fault on one downstream feeder tripped not only that feeder’s breaker but also the main incoming ACB, de-energizing every circuit on the board. A local fault had become a total outage — a selectivity failure.

Investigation

Plotting the incomer and feeder time–current curves on a single chart revealed overlap in the short-time region: the incomer’s short-time delay was not long enough above the feeder’s clearing time to guarantee the feeder tripped first, and at the prevailing fault level the two curves crossed. In effect both devices “saw” the fault as instantaneous and raced, with the incomer sometimes winning.

Resolution

The team re-graded the short-time settings to open a proper coordination margin between feeder and incomer, applied I²t curve-shaping to maintain discrimination up to the board’s fault level, and — because simply lengthening the incomer’s delay would have raised let-through energy — implemented zone-selective interlocking so the feeder could signal the incomer to hold, while preserving fast incomer tripping for a fault on the bus itself. Instantaneous on the incomer was coordinated accordingly.

Outcome

A downstream feeder fault is now cleared by the feeder breaker alone; the rest of the board stays energized.

The incomer still trips quickly for a genuine busbar fault, so protection speed was not sacrificed.

Let-through energy at the incomer was kept low through ZSI rather than long fixed delays.

The coordination study and as-left settings were documented for future changes.

Case Study 3 — Catching a Hidden Earth Fault and Neutral Overheating

Situation

An installation with a high proportion of non-linear electronic loads showed signs of thermal distress — elevated temperatures at the switchboard neutral and intermittent tripping — while the phase currents appeared within limits. Phase-overcurrent protection alone was not telling the whole story.

Investigation

Two issues surfaced. First, triplen harmonics from the electronic loads were adding in the neutral conductor, driving neutral current well above expectations while phase readings looked normal, and the neutral protection had been left disabled. Second, a low-level earth fault was drawing residual current below the phase pickups, so the phase functions never reacted; the ground-fault settings were too insensitive and slow to catch it early.

Resolution

The team enabled and set neutral protection appropriate to the (full-sized) neutral and the harmonic loading so the neutral pole was protected against the elevated current, and recalculated the ground-fault pickup (Ig) to a sensitivity that would catch a developing earth fault while staying above normal standing leakage and imbalance, with a ground-fault delay (Tg) chosen to ride through switching transients and coordinate with downstream earth-fault clearing.

Outcome



• The developing earth fault was detected and cleared early, before it could escalate to equipment damage or fire.
• Neutral protection now guards against harmonic-driven neutral overheating that phase protection cannot see.
• Ground-fault and neutral settings were coordinated with downstream devices to avoid nuisance tripping.
• All changes were injection-tested and documented, with a recommendation to re-review as harmonic loads grow.
  

Work With Keentel Engineering

Whether the issue is nuisance tripping on starting loads, a selectivity failure that turns a local fault into an outage, or hidden earth-fault and neutral risks in a harmonic-rich installation, the answer usually lies in a rigorous protection coordination study and disciplined, tested trip-unit settings. Keentel Engineering supports facility owners, contractors and utilities with discrimination studies, ACB and trip-unit setting calculations, ZSI and selectivity design, and commissioning and injection-test support. Talk to our protection team about your switchboard.

Frequently Asked Questions

Detailed answers to the questions engineers most often ask about ACB protection settings.