ABB Emax 2 Arc Flash Mitigation and Zone Selective Interlocking Guide

What is ABB Emax 2 arc flash mitigation? ABB Emax 2 arc flash mitigation is a coordinated protection strategy applied to the Emax 2 open air circuit breaker series — rated 400–6300 A under IEC 60947-2 with breaking capacities to 150 kA — that reduces incident energy at the point of work by minimizing arcing fault duration through trip unit settings, zone selective interlocking, and maintenance mode switching. Misconfigured Ekip trip unit parameters or incorrect ZSI wiring defeat the fault-clearing speed advantage that IEEE 1584-2018 calculations assume, leaving workers exposed to incident energy levels that invalidate the selected PPE category. This guide covers the IEEE 1584-2018 calculation framework applied to a real panel, ZSI wiring implementation on Emax 2, Ekip settings that directly affect arcing duration, arc flash mitigation strategy comparisons, and a pharmaceutical clean utility MDB case study.

Published: 2026-04-27 | Last updated: 2026-04-27 | By Stoklink Engineering Team

Why arc flash mitigation drives Emax 2 specification, not the other way around

Most procurement specifications still list the ABB Emax 2 frame size first and the trip unit version last. That order is backwards. In our experience auditing switchgear builds across European and Middle Eastern industrial sites, the incident energy target — usually 8 cal/cm² or less so technicians can wear standard arc-rated coveralls — is what should drive the entire selection chain. Frame, trip unit, ZSI scheme, and even the busbar bracing all flow from that single number.

Here is why. Incident energy scales roughly linearly with arcing time. Cut the clearing time from 200 ms to 50 ms and you cut incident energy by about 75%. The Emax 2 platform, when paired with the right Ekip Dip or Ekip Touch trip unit, can hit total clearing times under 70 ms on the instantaneous function. But only if the engineer configures the I-pickup, the ZSI input, and the optional Ekip Arc Guard correctly.

The first overview of the ABB SACE Emax 2 platform covers the frame structure; what we are doing here is going one layer deeper into the protection logic.

What "fast" actually means on an Emax 2

ABB publishes an opening time of 30 ms and a total break time around 50–70 ms for the E1.2 through E6.2 frames on instantaneous trip. That figure assumes the Ekip unit issues the trip command immediately, with no intentional delay. Engineers often overlook that the S-function (short-time delay) and ZSI delay settings can stretch this to 200 ms or more if poorly configured. A common mistake is leaving the factory default S-delay at 100 ms across an entire selectivity chain — which inflates incident energy at every level above the feeder.

For complete protection function ranges, ZSI wiring details, and Ekip trip unit configuration data applicable to the ABB Emax 2, refer to the manufacturer's SACE Emax 2 Technical Catalogue.

The IEEE 1584-2018 calculation framework, applied to a real panel

Let us work through a concrete example sized around an ABB Emax 2 application. A 1600 A main feeder on a 400 V TN-S system, prospective short-circuit current 42 kA at the busbar, working distance 610 mm (24 inches), 32 mm electrode gap in a VCB enclosure (vertical conductors in a metal box). This is a textbook industrial main switchboard.

Run the numbers with a 200 ms clearing time and you get roughly 18 cal/cm² — Category 4 PPE territory, which means a 40 cal flash suit, hood, and a maintenance procedure most plants will not tolerate. Drop the clearing time to 60 ms (Emax 2 instantaneous, no ZSI delay) and the same panel calculates at about 5.5 cal/cm². That is the entire engineering case for ZSI in two sentences.

Zone Selective Interlocking on Emax 2: how the wiring actually works

ZSI on the ABB Emax 2 is conceptually simple. Each downstream breaker that detects a fault sends a "restraint" signal up the chain. The upstream breaker, on receiving the restraint, holds its short-time delay (S-function) and lets the downstream device clear the fault first. If no restraint arrives within a few milliseconds of the upstream device seeing the fault current, the upstream trips instantaneously — because that means the fault is between the two breakers, in the zone the upstream device is supposed to protect.

On Emax 2 with Ekip Dip or Ekip Touch trip units, ZSI uses two pairs of terminals: one for the S-function (short-time) and one for the G-function (ground fault), available on the LSIG variants. The signal is a simple 24 V DC pulse, current-loop driven, with a maximum cable length of around 300 m using shielded twisted pair. We typically see Belden 9841 or equivalent in our field installations.

Wiring topology — radial, not daisy-chain

A common installation error: wiring ZSI as a daisy chain between feeders. Don't. The correct topology is radial — every downstream breaker's ZSI-OUT goes to the same ZSI-IN terminal pair on the upstream device, wired in parallel. ABB's Ekip inputs are designed as wired-OR; any single feeder asserting restraint blocks the upstream short-time trip.

For a typical main-tie-main configuration with an ABB 1SDA070861R1 E1.2B 1600 A main breaker upstream of three ABB 1SDA070701R1 E1.2B 630 A feeder breakers, you wire all three feeder ZSI-OUT pairs in parallel to the single ZSI-IN of the 1600 A unit. Time delays drop from 200 ms to about 50 ms on the upstream device, because the restraint logic — not the time grading — is what provides selectivity.

Ekip trip unit settings that actually matter for arc flash

The Ekip Dip LI (long-time + instantaneous) variants — fitted on the 630 A, 800 A, 1000 A, 1250 A, and 1600 A ABB Emax 2 E1.2B SKUs — do not support short-time selectivity by themselves, so they are not ZSI participants on the S-function. They are still excellent at the bottom of a chain, where you want fast clearing without coordination concerns. For middle and top tiers in a hierarchy, you need LSI or LSIG trip units, such as the ABB 1SDA070782R1 E1.2B 1000 A Ekip Dip LSI.

I-function (instantaneous) pickup

Set the I-pickup as low as the inrush characteristic of the downstream load allows. For a transformer secondary feeder, this is typically 8–10× In. For a motor feeder fed through an MCC, you may need 12× In to ride through DOL starts. Setting I too high — say 15× — means the breaker uses the slower S-function on faults that should trigger instantaneous tripping, and your incident energy doubles.

S-function (short-time) settings under ZSI

With ZSI active, set the S-pickup at 4–6× In and the S-delay at the minimum (typically 0.05 s for I²t OFF, 0.1 s for I²t ON). The ZSI logic overrides the delay when restraint is received from downstream — but only if the trip unit firmware version supports it. Older Ekip Dip firmware (pre-2017) had a known issue where ZSI restraint did not properly bypass the I²t curve. We have seen this in retrofits and the fix is a firmware update via the Ekip T&P test unit.

G-function (ground fault) and ZSI

Ground fault ZSI is separately wired and separately configured. In TN-S systems with a 1600 A E2.2B such as the ABB 1SDA070981R1 E2.2B 1600 A Ekip Dip, set G-pickup at 0.2–0.4× In and G-delay at the minimum compatible with downstream RCDs and ground-fault relays. Coordination here is its own topic — see the discussion in our Emax 2 technical specifications guide.

Comparing arc flash mitigation strategies on Emax 2

ZSI is one of three primary arc flash mitigation techniques available on the ABB Emax 2. The others are arc flash relays (optical detection, e.g., Ekip Arc Guard or third-party units like Littelfuse PGR-8800) and maintenance switches (the "ARMS" or "MERS" function — a second I-pickup curve activated by a key switch during energized work). Each has tradeoffs.

In practice, the ZSI + maintenance switch combination handles 80% of industrial requirements. We reserve optical arc flash relays for 4000 A+ main buses where ZSI alone leaves you above 8 cal/cm². The Ekip Arc Guard module integrates directly with the Emax 2 trip unit, which avoids the wiring nightmare of a separate optical relay panel.

Real-world example: pharmaceutical clean utility MDB

A project we reviewed last year: 2500 A main distribution board for a sterile API plant in Ireland, 400/230 V, prospective fault current 38 kA at the bus, built around an ABB Emax 2 E3.2 frame. The owner specified 8 cal/cm² maximum at the working distance for any energized work in the panel. Initial design used time-graded selectivity with a 200 ms upstream S-delay. Calculated incident energy at the main bus: 14.2 cal/cm². Not acceptable.

The fix took three steps. First, swap the main from a generic 2500 A frame to an ABB 1SDA071021R1 E2.2B 2000 A Ekip Dip with a downsized but adequate rating (the load was 1650 A continuous, so 2000 A frame was fine). Second, implement ZSI between the main and the four 800 A feeders, each an ABB 1SDA070741R1 E1.2B 800 A unit. Third, add a maintenance mode switch tied to the Ekip Touch reduced-energy curve, key-operated, with a beacon on the panel front.

Result: normal-operation incident energy 6.1 cal/cm², maintenance-mode 3.4 cal/cm². The owner accepted, the technicians work in standard 8 cal arc-rated PPE, and the project closed. Total ZSI wiring cost: about €900 in cable and labor.

Common ZSI commissioning mistakes — what we see in the field

ZSI on the ABB Emax 2 fails silently. That is the dangerous part. A miswired ZSI loop will not cause any operational symptom under normal load — the breakers carry current happily. The fault only shows up during the 0.001% of the time when a fault actually occurs, and by then it is too late. So commissioning verification is non-negotiable.

Polarity errors

The Ekip ZSI-OUT and ZSI-IN terminals are polarized. Reverse the leads and the restraint signal never registers, so the upstream breaker trips on its full S-delay every time. We have seen this on perhaps one in five sites we audit. Always verify with a simulated fault injection using the Ekip T&P unit during commissioning.

Mixed firmware versions

If the upstream breaker has Ekip Touch firmware 2.5 and the downstream units have firmware 2.1, the ZSI handshake timing can drift. Symptoms: occasional spurious upstream trips on legitimate downstream faults, or delayed clearing. The fix is firmware alignment across all participating breakers in a ZSI zone.

Ground fault ZSI on three-wire systems

Engineers occasionally enable G-function ZSI on a three-wire delta system. The G-function on Emax 2 uses residual current summation across the three phases. On a three-wire ungrounded delta, there is no defined neutral path, and the G-function will not respond meaningfully to ground faults. Use a separate ground fault relay (e.g., a Bender IRDH575 insulation monitor) instead. Our note on Emax 2 nuisance tripping causes covers related earth-fault traps.

Quick incident energy estimator

Where Emax 2 fits in the wider mitigation hierarchy

ZSI is one tool. The full hierarchy of arc flash control, in rough order of effectiveness, runs: eliminate the hazard (de-energized work, IEC 60364-6 §6.4 verification), reduce probability (remote racking, infrared windows so technicians stay outside the arc boundary), and reduce magnitude (fast clearing — which is where the ABB Emax 2 lives).

For data center applications where uptime drives every decision, the calculus shifts slightly. See our specific notes on Emax 2 in data center MDB design for redundancy patterns that interact with ZSI. For a comparative view against the main competitor, the Emax 2 vs Schneider MasterPact MTZ comparison covers the equivalent ERMS and ZSI features on the Schneider side.

Procurement checklist for arc-flash-conscious Emax 2 specification

When writing an ABB Emax 2 specification, include these line items explicitly:

Trip unit version: minimum Ekip Dip LSI for any breaker that participates in ZSI as the upstream device. Ekip Touch LSIG for ground fault ZSI. Confirm firmware revision is current at delivery.

ZSI wiring: shielded twisted pair, 0.75 mm² minimum, single shield grounded at the upstream end only. Length budget under 300 m per zone.

Maintenance mode: specify the Ekip ARMS (Arc Reduction Maintenance System) function, including front-of-panel key switch and beacon. Wire the input to a dedicated auxiliary contact, not to a relay output that could fail open.

Commissioning: require a written ZSI verification procedure as part of factory acceptance testing, including secondary injection at each downstream breaker with the upstream breaker monitored to confirm restraint. ABB's recommended procedure is in document 1SDC200023D0204.

For sizing, the broader methodology is in how to size ABB Emax 2 step-by-step. For component-level browsing, the full Air Circuit Breakers collection at Stoklink includes all current Emax 2 frame sizes, with the ABB 1SDA070821R1 E1.2B 1250 A and ABB 1SDA070781R1 E1.2B 1000 A being the two most-specified mid-range frames in our order book.

Standards alignment: IEC, IEEE, and NEMA in the same panel

Global procurement creates an awkward overlap. A panel built in Italy and shipped to a US site must satisfy IEC 60947-2 for the breaker itself, IEEE 1584-2018 for the arc flash calculation, and NFPA 70E for the labeling and work practices. NEMA AB-4 covers the testing of low-voltage power circuit breakers in North American practice and references UL 1066 for the Emax 2's UL-listed equivalents.

The Emax 2 is dual-rated: IEC 60947-2 in its native form, and UL 1066 in the "E2.2 PR122" North American variants. The trip unit functions are the same. The ZSI wiring is the same. What changes is the labeling format on the panel and the working distance assumed in the calculation — IEEE 1584 uses 610 mm by default, while some European utilities specify 450 mm for medium-voltage adjacent equipment.

One subtle point. IEC 60947-2 §8.3.4 defines the rated short-time withstand current Icw, which is the current the breaker can carry for one second without tripping. This sets the upper bound of the S-function delay you can use safely. For an E2.2B with Icw = 42 kA for 1 s, you cannot specify an S-delay of, say, 800 ms at a fault current near 42 kA — the breaker would be at its thermal limit. ZSI sidesteps this by keeping the actual operating delay short, even though the configured delay setting may be longer.

Maintenance and lifecycle considerations for ZSI installations

ZSI wiring is the part of a switchboard that gets disturbed every time a breaker is racked out for maintenance. After ten years and forty rack-out cycles, terminals loosen, shields get nicked, and ground loops appear. We recommend a ZSI integrity test as part of every five-year maintenance cycle. This is not in the manufacturer's standard maintenance schedule, but it should be.

The test takes about thirty minutes per zone. Inject a secondary current at each downstream breaker using an Ekip T&P or equivalent, verify the upstream breaker shows the restraint signal on its diagnostic display (Ekip Touch shows it directly; Ekip Dip requires the Ekip Connect software via Bluetooth or USB), and verify that with the restraint removed, the upstream device trips within its instantaneous time. Document the results.

For coordination with downstream MCBs, residual current devices, and protection relays, the ZSI logic only operates between Emax 2 and Tmax XT/T-series ABB breakers. Mixing brands at the ZSI level is not supported. If you have a Schneider Compact NSX feeder downstream of an Emax 2 main, you cannot ZSI between them — you fall back to time-graded selectivity for that path.

Related Reading

• ABB Emax 2 Full Technical Specifications: Current Ratings, Breaking Capacity and Dimensions
• How to Size ABB Emax 2: Step-by-Step Calculator for LV Distribution Panels
• ABB Emax 2 in Data Centers: MDB Design, Redundancy and Uptime Considerations
• ABB Emax 2 Nuisance Tripping: Root Causes, Diagnostic Steps and Fixes

Conclusion

Arc flash mitigation on the Emax 2 is not a single feature you bolt on at the end of a project. It is a thread that runs through the trip unit selection, the ZSI wiring topology, the maintenance procedures, and the labeling format. Get the trip unit right (LSI minimum for upstream ZSI participation), wire the ZSI as a parallel restraint loop rather than a daisy chain, and verify the function with secondary injection at commissioning and every five years thereafter. The result is a switchboard that protects the people who maintain it, not just the cables and loads downstream of it.

The Emax 2 platform gives you the tools, but the engineering judgment is yours. There is no universal ZSI configuration that works for every site — the right delays, pickups, and zone boundaries depend on your downstream loads, your maintenance philosophy, and your local regulatory framework. For the full selection methodology and how arc flash mitigation fits into the wider engineering workflow, see our complete ABB SACE Emax 2 selection, application and maintenance guide, and contact our technical team at Stoklink for project-specific configuration support on any of the Emax 2 SKUs referenced in this article.