How to Set MCCB Trip Settings (Ir, Isd, Ii)

How do you set the trip parameters on an MCCB electronic trip unit? An electronic trip unit protects a circuit through three independently adjustable stages — long-time (Ir), short-time (Isd with delay tsd), and instantaneous (Ii) — each guarding against a different fault condition and each expressed as a multiple of the breaker's frame rating In. Set one stage wrong and the breaker either nuisance-trips a healthy motor start or lets a bolted fault burn through a busbar before it clears. This article covers how to derive Ir from actual load current, how Isd and tsd interact to preserve selectivity between upstream and downstream breakers, what Ii protects against and why it's usually left near maximum, and the adjustable ranges found on Ekip, Micrologic, and ETU trip units.

Why Thermal-Magnetic Breakers Skip This Step Entirely

TM and TMD trip units are calibrated once, at the factory. The bimetal strip is sized to In, and the magnetic trip fires at a fixed multiple — typically 5-10x In depending on the curve. There's no dial to turn. If the load doesn't match the breaker rating, you change the breaker, not the setting.

Electronic trip units replace the bimetal-plus-magnet mechanism with a current transformer feeding a microprocessor. That microprocessor lets a single physical frame cover a current band instead of one fixed value. An ABB Ekip Dip on a Tmax XT4 250, for example, covers roughly 100-250 A on one frame by moving the Ir dial. Fewer breaker sizes to stock, field-adjustable protection, and — critically — a setting that has to match the actual circuit, not just the frame it's bolted into.

Ir — Long-Time (Thermal) Protection

Ir is the long-time pickup: the current above which the breaker starts an inverse-time countdown toward a trip, mimicking the thermal behavior of a cable heating up under sustained overload. It's an I²t function, not instantaneous — a small overload takes minutes to trip, a large one takes seconds. This is the setting that protects the conductor, not the load.

Derivation is straightforward: read the design load current Ib from the load schedule or a clamp meter under normal running conditions, confirm the frame's In is equal to or above Ib, then set Ir so the pickup sits between Ib and the cable's continuous current rating Iz. Set Ir too close to Ib and a legitimate load swing trips the breaker; set it near Iz and you've deleted the cable's overload protection.

What we see in the field: Ir gets set once at commissioning to whatever the panel builder assumed the load would be, then never revisited after the actual load profile settles in. A motor feeder sized for a future expansion that never happened sits with Ir well above Ib for years — the cable is protected on paper but the setting is doing nothing useful until someone re-measures.

Isd and tsd — Short-Time Pickup and Selectivity

Isd is the short-time pickup, expressed as a multiple of Ir (roughly 1.5-10x Ir on most electronic trip units). It distinguishes a genuine short circuit from a heavy but survivable overload — motor locked-rotor current, transformer inrush — that would otherwise sit above Ir and trip needlessly on the long-time curve. Above Isd, the breaker starts a separate, much faster clock: the short-time delay, tsd.

tsd is what makes selectivity between two breakers in series possible. On a fault downstream, both the upstream and downstream breaker sense the same fault current briefly. If the downstream breaker's total clearing time is shorter than the upstream breaker's tsd, the downstream breaker clears first and the upstream one never opens — only the affected circuit loses power. Get the stagger wrong and a fault on one feeder trips the main, taking the whole board down.

Most trip units offer tsd in fixed steps — 0, 0.1, 0.2, 0.3, 0.4 s is a common set on Micrologic — plus an I²t on/off choice. With I²t on, the delay shortens as fault current rises (a ramping curve); with I²t off, the delay is flat regardless of how far above Isd the fault current sits. I²t off gives more predictable selectivity margins; I²t on limits let-through energy better on severe faults. Coordinating two breakers in series generally means giving the upstream unit a tsd step at least one increment above the downstream unit's total clearing time, plus a margin: commissioning engineers typically build in on the order of 100 ms beyond the downstream breaker's own operating time to cover tolerance.

Ii — Instantaneous Protection

Ii is the instantaneous pickup: no intentional delay, tripping as fast as the mechanism allows once current crosses the threshold. It exists to protect the breaker and downstream busbar from a bolted, high-magnitude short circuit that would otherwise ride out even a short tsd delay long enough to do damage. On many electronic trip units Ii is adjustable across a wide band — commonly on the order of 2-15x In — though some models ship it fixed near the top of that range or allow it to be switched off entirely.

Not always on, though. Switching Ii off is a deliberate selectivity technique — usually paired with zone-selective interlocking on air circuit breakers — that lets tsd do all the short-circuit clearing so the upstream unit never races the downstream one on instantaneous trip. On MCCBs this is less common than on ACBs, mainly because MCCBs generally have limited or no short-time withstand rating (Icw): leaving Ii off for an extended tsd period risks damaging the breaker itself before it opens, not just losing selectivity.

Typical Ranges on Ekip, Micrologic, and ETU

ABB's Ekip Dip (dip-switch) and Ekip Touch (LCD) units on the Tmax XT range set Ir continuously across roughly 0.4-1.0x In, with Isd adjustable as a multiple of Ir and Ii either fixed or adjustable depending on the trip unit tier — Ekip Touch/Hi-Touch add full LSIG metering and communications on top of the same setting logic. Schneider's Micrologic units follow the same 0.4-1.0x In pattern for Ir; Micrologic 5 and 6 (LSI and LSIG) add the Isd/tsd short-time stage with the I²t on/off choice described above, and Ii typically sits fixed near 12-15x In unless the model explicitly allows adjustment. Siemens ETU units on the Sentron 3VA2 range span from the basic ETU320 up to the graphic-display ETU850 with comms and metering — Ir and Isd adjustability is standard across the tier, while how much Ii range is exposed depends on which ETU model is fitted.

Exact dial values differ by frame size within each brand, so the multiplier ranges above should be read as class figures, not a substitute for the specific trip unit's datasheet.

Setting Order and Field Verification

A practical sequence: measure or calculate Ib for the circuit, confirm the frame's In covers it with margin for growth, set Ir between Ib and Iz, then check Isd and tsd against the breakers immediately upstream and downstream for selectivity before touching anything else. Leave Ii at its default unless a selectivity study specifically requires disabling it. Document every setting on the panel schedule — dial positions drift in institutional memory much faster than they drift physically.

None of this is done by eye. Commissioning verification means primary or secondary current injection through a test set to confirm the breaker actually trips at the dialed thresholds and within the expected time bands, not just that the dial is pointed at the right number. A trip unit with a stuck relay or a miscalibrated CT can look correctly set and still fail to protect anything.

Common Setting Mistakes We See in the Field

The most frequent error is leaving every breaker in a panel at its factory-default Ir, Isd, and tsd — effectively un-commissioned protection dressed up as a finished installation. A close second: setting Isd on an upstream breaker equal to Isd on the downstream one, on the assumption that "higher current threshold" alone gives selectivity. It doesn't. Without a tsd stagger, both breakers can still race each other on the same fault. A third, more subtle one — motor feeders with Ir set to nameplate full-load current instead of locked-rotor-adjusted current, which produces nuisance trips on every start under load.

Getting these three stages right is one part of correctly specifying a breaker in the first place — pair trip-setting calculation with the frame and rating checks covered in our MCCB selection checklist and the sizing approach in calculating MCCB rating for a feeder circuit. Motor-specific Ir and Isd derivation, including locked-rotor multiples, is covered separately in sizing an MCCB for motor load. For the standard clauses referenced throughout this article, see our summary of IEC 60947-2 standards for molded case circuit breakers. Broader background on frame families and trip unit tiers sits in the MCCB engineering guide, and current stock across Ekip, Micrologic, and ETU-equipped frames is in our molded case circuit breakers collection.

Conclusion

Three settings, three jobs: Ir protects the cable against sustained overload and should track measured load current between Ib and Iz. Isd and its tsd delay separate genuine short circuits from survivable overcurrent spikes and are what actually deliver selectivity between breakers in series — the delay stagger matters more than the pickup value alone. Ii clears bolted faults with no intentional delay and should generally stay near default on MCCBs, given their limited short-time withstand rating. None of the three should be left at factory default on a commissioned panel, and none should be trusted without an injection test confirming the dial matches the actual trip behavior.