ABB Emax 2 as Bus Tie Breaker in Double Busbar Configuration Guide

What is an ABB Emax 2 bus tie breaker in a double busbar configuration? An ABB Emax 2 bus tie breaker is an IEC 60947-2-compliant air circuit breaker, rated 400–6300 A with breaking capacities up to 200 kA at 690 V, positioned between two independent busbars to couple or isolate them during normal operation, load transfer, or fault events. Incorrect specification — undersized Icu ratings, missing zone-selective interlocking, or absent mechanical/electrical interlock schemes — exposes the system to simultaneous source paralleling faults and loss of selectivity across three protection directions. This guide covers bus tie sizing methodology, Ekip Dip LSI trip unit requirements, mechanical and electrical interlocking logic, three-directional selectivity coordination, and open versus closed transition automatic transfer schemes.

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

Why a Double Busbar Needs a Properly Specified Bus Tie

A double busbar arrangement exists because single-source switchgear cannot meet the availability requirements of refineries, data centers, hospitals, or cement plants. You have two transformers, two incomers, and two bus sections. The bus tie — often built around a frame like the ABB Emax 2 — sits between them. Its job is simple to describe and surprisingly easy to get wrong: keep the two sections separated under normal operation, and close the gap when one source fails — but never let two out-of-phase sources connect through it accidentally.

In our experience, the most common mistake on greenfield projects is treating the bus tie like a feeder breaker. It isn't. A feeder protects a downstream load. A bus tie protects the bus itself, transfers load between sources, and participates in an automatic transfer scheme (ATS) that — if mis-coordinated — will black out the entire facility in under 100 ms.

Where the Emax 2 Fits

The Emax 2 family covers 630 A to 6300 A across E1.2, E2.2, E4.2, and E6.2 frames. For bus tie duty in typical industrial LV distribution (400/690 V), the practical range sits between 1000 A and 4000 A. The ABB 1SDA070781R1 E1.2B 1000A handles small process panels; the ABB 1SDA071021R1 E2.2B 2000A covers most data center and pharma plants; for large MV/LV substations feeding pumping stations, you step up to E4.2 4000 A. For background on how these ratings map to frames and breaking capacity, the full technical specifications guide is the right starting point.

For complete electrical and mechanical specifications of the ABB Emax 2 air circuit breaker family, refer to the manufacturer's ABB SACE Emax 2 product documentation.

How Do You Size an Emax 2 as a Bus Tie?

Engineers often overlook that during a transfer, the bus tie carries the load of both sections. If each incomer normally carries 1500 A, after one transformer trips the surviving incomer plus the ABB Emax 2 bus tie must carry up to 3000 A — assuming load shedding is in place. Without load shedding, the surviving feeders draw whatever they need until thermal protection bites.

The rated continuous current of the bus tie should equal the larger of the two incomer ratings, not their sum. Why? Because in a properly designed scheme, only one incomer is closed onto the full bus at a time after a transfer. The bus tie sees, at most, what one transformer can deliver.

For breaking capacity (Icu), the bus tie must withstand the worst-case parallel fault — both transformers contributing to a fault on either bus section. Even if your operating philosophy forbids paralleling, you size for the credible fault scenario. With two 2 MVA, 6% impedance transformers in parallel at 400 V, expect roughly 96 kA prospective short-circuit current. That pushes you toward an E2.2N (66 kA) or E2.2H (85 kA), not the standard B variant.

For deeper sizing methodology including let-through energy and cable coordination, the step-by-step Emax 2 sizing guide walks through worked examples from real industrial projects.

Trip Unit Configuration: Why Ekip Dip LSI Is the Minimum for Bus Ties

A bus tie running with only L (long-time) and I (instantaneous) protection is a problem waiting to happen. You need S (short-time delay) to achieve time-grading selectivity with both upstream and downstream breakers. On the ABB Emax 2, the ABB 1SDA070782R1 with Ekip Dip LSI is the entry point for proper bus tie protection. For larger frames, Ekip Touch or Ekip Hi-Touch with measurement, communications, and zone selective interlocking (ZSI) is what we typically specify on critical installations.

Setting the Time-Current Curve

A common mistake is setting the bus tie L-pickup at the same value as the incomer. Don't. The bus tie should pick up at roughly 1.05–1.10 × bus tie rated current, with a slightly faster I²t curve than the incomer so it sees a feeder-bus fault first. Typical settings on a 2000 A bus tie feeding two 1500 A bus sections:

• L: I1 = 0.9 × In, t1 = 12 s at 6×I1
• S: I2 = 4 × In, t2 = 0.2 s (I²t = ON)
• I: I3 = 10 × In (or OFF if ZSI is used and selectivity demands it)

The S-band is what makes selectivity work. Without it, an instantaneous trip on the bus tie races the incomer's instantaneous trip, and you cannot guarantee which one opens first. With S delayed 100–200 ms below the incomer's S setting, the bus tie clears bus-section faults while the incomer rides through.

Mechanical and Electrical Interlocking: Avoiding the Parallel-Source Fault

Three breakers, two of which can be closed simultaneously, but never all three. That is the interlock requirement of a double busbar with an ABB Emax 2 bus tie. The standard rule is: any two of (Incomer 1, Bus Tie, Incomer 2) may be closed, but not all three — unless the operator has explicitly enabled synchronizing mode and a check-sync relay confirms phase angle, frequency, and voltage match within tolerance.

Key Interlock Methods

ABB offers three coordinated approaches for Emax 2 interlocking:

In practice, on critical projects we use both mechanical and electrical interlocks. The mechanical cable interlock is the last line of defense — even if firmware crashes, no one closes three breakers at once. The electrical interlock handles the fast logic and supports controlled paralleling for closed-transition transfers, which is increasingly demanded in data center MDB designs where any voltage dip causes IT load events.

Selectivity in Three Directions: The Hardest Part of Bus Tie Design

A feeder breaker only worries about coordination with one upstream device. The ABB Emax 2 bus tie has to coordinate upward with both incomers and downward with every feeder on both bus sections. That is selectivity in three directions, and it requires careful curve plotting.

Time-Current Coordination Example

Consider a typical layout: 2× 2500 kVA transformers, 2× 4000 A incomers, 1× 4000 A bus tie, downstream feeders ranging from 250 A MCCBs to 1600 A Emax frames such as the ABB 1SDA070861R1 E1.2B 1600A. The coordination chain works like this:

1. Largest feeder (1600 A): S-band at 0.1 s, I-band ON at 8×In
2. Bus tie (4000 A): S-band at 0.25 s, I-band OFF (ZSI active)
3. Incomer (4000 A): S-band at 0.4 s, I-band OFF, with reverse-power protection

Without ZSI, you cannot turn off the bus tie's instantaneous protection and still meet selectivity — fault currents above 30 kA would race past the time-graded bands. With ZSI, the bus tie knows whether the fault is upstream or downstream of itself based on a fast wired signal from the feeder breakers, and it delays only when appropriate. This is one of the genuine advantages of the Ekip platform over older trip units.

For sites where you're comparing platforms, the Emax 2 vs MasterPact MTZ comparison covers how ZSI implementations differ between manufacturers.

Automatic Transfer Schemes: Open vs Closed Transition

The behavior of the ABB Emax 2 bus tie during a source loss defines whether your facility rides through or blacks out. There are three transfer modes in common use:

Open Transition (Break-Before-Make)

The failed incomer opens, then the bus tie closes after a defined dead time (typically 100–500 ms). Simple, safe, and unforgiving — every contactor with a hold-in voltage above ~70% drops out. Used on sites where some downtime is acceptable and motor-restart sequencing is in place.

Fast Transition

The bus tie closes within 4–6 cycles of the incomer opening, before motor residual voltage decays past the safe re-closure threshold (typically 25% phase angle drift). Requires careful coordination with motor inertia and fast-acting breakers. The Emax 2 closing time of 80 ms makes this feasible but tight.

Closed Transition

Two sources are paralleled briefly (under 100 ms typically) while the load transfers, then one source disconnects. This requires synchrocheck, voltage matching, and explicit operator authorization. The bus tie must withstand the parallel-source fault current during the overlap window. This is where the proper Icu rating earns its keep.

What we typically see in the field: industrial process plants choose fast transition; data centers demand closed transition; older facilities live with open transition and hope. The choice affects bus tie sizing because closed transition exposes the breaker to higher fault currents.

Installation, Cabling, and Common Field Mistakes

The ABB Emax 2 in fixed execution (F) versus withdrawable (W) is a meaningful choice for bus ties. Fixed is cheaper and slightly more reliable mechanically. Withdrawable lets you isolate the bus tie for maintenance without de-energizing both bus sections — a major operational advantage. For bus tie service in any installation that runs 24/7, we strongly recommend withdrawable. The ABB 1SDA070981R1 E2.2B 1600A in F-HR (Fixed, Horizontal Rear) configuration is fine for greenfield panels designed around it; for retrofits into existing switchgear, withdrawable saves you from outage planning headaches.

Cable and Bus Connection

The bus tie is typically connected via copper bars on both sides, not cables. Bar sizing follows IEC 61439-1 §10.10 thermal verification. Common mistake: undersizing the bar between the bus tie and the busbar based on bus tie In, when in fact the bar must carry full transferred load including downstream short-circuit duty. Verify bar Ipk against the breaker's let-through Ipk peak — ABB publishes these in the Emax 2 technical catalog tables.

Anti-Pumping and Operating Voltage

Bus tie closing coils must be on a reliable control voltage source — typically a 110 V DC station battery, not the AC bus they're switching. Engineers occasionally tie bus tie control to the bus it sits on. That works fine until the bus is dead and you need the bus tie to close. We've seen this on a paper mill in Poland; the post-incident report blamed "unexpected control loss." The control wiring was the cause.

Maintenance, Testing, and Nuisance Trip Avoidance

A bus tie that has not operated in three years is a bus tie you cannot trust. Mechanical exercising every 6–12 months is required by most utility-grade maintenance specifications and recommended by ABB in the Emax 2 service manual. The ABB Emax 2 Ekip trip unit's self-test function should be exercised quarterly; we've seen Ekip Dip units sit fine for years and then fail their first real trip because a contact had oxidized. Periodic primary injection testing every 3–5 years catches that.

Nuisance tripping on a bus tie is particularly destructive because it can cascade into a full plant trip if the surviving incomer overloads. The most common causes — harmonic distortion, high inrush from large motor restarts, and incorrect S-band settings — are covered in detail in our Emax 2 nuisance tripping diagnostic guide.

Spare Parts Strategy

Procurement managers should hold a complete spare bus tie breaker on site, or at minimum a complete trip unit and closing coil. Lead times on Emax 2 frames have historically been 8–16 weeks; on critical sites, that is unacceptable. Alternatively, browse the ABB air circuit breakers collection for in-stock alternatives that match your frame and trip unit specification.

Specification Checklist for Procurement

When releasing the ABB Emax 2 bus tie purchase order, the data sheet should specify:

• Frame and rated current (e.g., E2.2B 2000 A) — verified against transferred load
• Icu / Ics at operating voltage — 100% Ics highly recommended for bus tie service
• Trip unit: minimum Ekip Dip LSI; Ekip Touch + ZSI for critical installations
• Execution: F (fixed) vs W (withdrawable) — withdrawable for 24/7 service
• Auxiliary contacts: minimum 4 NO + 4 NC for interlock and status
• Closing coil voltage: 110 V DC (or per site standard)
• Synchrocheck module if closed transition is required
• Communications: Modbus RTU or Profibus for SCADA integration
• Bell alarm contact and electrical reset

For the foundational understanding of how Emax 2 differs from its predecessors, the introduction to ABB SACE Emax 2 features and models remains the right reference for procurement teams new to the platform.

Verifying Compliance Documents

Every Emax 2 ships with a routine test certificate per IEC 60947-2 §8.3.3.1 covering mechanical operation, dielectric withstand, and trip unit calibration. For bus tie service, additionally request the type test certificate for Ics at the relevant voltage (per IEC 60947-2 §8.3.5) and the seismic certificate if the installation is in a qualified zone (IEEE 693 or IEC 60068-3-3). On a recent project for a Turkish refinery, the procurement team accepted the routine certificate alone; during commissioning, the customer's third-party inspector demanded type test documentation and the breaker shipment sat in the warehouse for three weeks. Specify it upfront.

Communications and Integration with Plant Control Systems

Modern industrial facilities expect every breaker above 800 A to report status, current, voltage, and trip cause to the plant SCADA or DCS. The ABB Emax 2 with Ekip Com modules supports Modbus RTU, Modbus TCP, Profibus DP, Profinet, and EtherNet/IP. For bus tie applications, the relevant data points exposed should include:

• Breaker status (open/closed/tripped/racked-out)
• Real-time RMS current per phase, three-phase voltage, active and reactive power
• Trip cause (L, S, I, G, or external)
• Wear indicators (contact wear %, operations counter)
• Interlock status from upstream and downstream breakers

The Emax 2 platform supports IEC 61850 through the Ekip Com IEC 61850 module — relevant if your bus tie sits in a substation automation system rather than a process DCS. In our experience, IEC 61850 is overkill for standard industrial MDB applications and worth the cost only when the LV switchgear is integrated into a utility-class substation with GOOSE messaging. For a typical chemical plant, Modbus TCP over a redundant ring topology delivers everything operations needs at one-third the engineering cost.

Cybersecurity Considerations

Engineers often overlook that a networked bus tie breaker is a potential attack surface. The Ekip Com modules support role-based access and signed firmware updates, but only if you configure them. Default passwords on commissioned breakers are a finding we encounter on roughly 60% of the audits we participate in. Change them. Segment the breaker network from the corporate IT network. Disable unused protocols on the trip unit. None of this is exotic — it is just routine ICS hygiene per IEC 62443-3-3.

Real-World Case: 25 MW Petrochemical Plant Bus Tie Retrofit

A petrochemical site in the Middle East operated two 4000 A incomers fed from independent 11/0.4 kV transformers, with a 4000 A bus tie originally installed in the 1990s. The legacy bus tie used an electromechanical trip unit without any S-band, and selectivity below 5 kA was nonexistent — every feeder fault tripped the bus tie and one incomer simultaneously, blacking out half the plant.

The retrofit replaced the legacy unit with an E4.2 4000 A frame, Ekip Hi-Touch trip unit with ZSI, withdrawable execution, and synchrocheck module to support closed-transition transfers. Mechanical and electrical interlocks were rewired with the bus tie control fed from a dedicated 110 V DC battery rather than the LV bus.

Commissioning revealed two issues that didn't show up in the design review:

1. The existing busbar bracing was rated for 80 kA peak; the new breaker's let-through under closed-transition fault was 95 kA peak. Bracing had to be reinforced before energization.
2. The legacy SCADA polled the bus tie at 5-second intervals; the operator wanted sub-cycle event capture for transfer studies. An Ekip Signalling module with internal event log at 1 ms resolution solved this without a SCADA replacement.

Eighteen months post-commissioning, the site recorded two incomer losses. Both were handled by closed-transition transfer with measured voltage dip below 3% — well inside ITIC ride-through tolerance for sensitive instrumentation. Pre-retrofit, the same events would have caused 200–400 ms outages and downstream process trips costing roughly $80,000 each.

Related Reading

• ABB Emax 2 in Data Centers: MDB Design, Redundancy and Uptime Considerations
• How to Size ABB Emax 2: Step-by-Step Calculator for LV Distribution Panels
• ABB Emax 2 Nuisance Tripping: Root Causes, Diagnostic Steps and Fixes
• ABB Emax 2 vs Schneider MasterPact MTZ: Technical Specs, Features and Price Compared

Conclusion

Using the ABB Emax 2 as a bus tie in a double busbar configuration is straightforward in concept but unforgiving in execution. The breaker is excellent — Ekip trip units, ZSI, modular communications, and the full range from E1.2 to E6.2 cover virtually every industrial scenario. What separates a robust installation from a fragile one is not the breaker selection but the surrounding engineering: proper sizing for transferred load, correct trip unit configuration with S-band and ZSI, layered mechanical and electrical interlocking, control supplies fed from independent DC sources, and procurement specifications that include type test documentation and adequate spares.

The bus tie carries quiet responsibility. It sits idle most of the time, then on the day a transformer fails it has 80–500 ms to either save the plant or contribute to its blackout. Treat it as a critical asset from specification through commissioning through periodic exercise, and it will earn its place. For the complete selection methodology, coordination guidance, and lifecycle planning across the Emax 2 portfolio, the ABB SACE Emax 2 selection, application and maintenance guide consolidates the engineering practice that informs every recommendation in this article. To browse in-stock frames matched to your bus tie sizing, the air circuit breakers collection at Stoklink covers E1.2 through E2.2 ratings with lead times suitable for retrofit projects, while the relays collection and miniature circuit breakers cover the auxiliary protection and control devices that complete a properly engineered double busbar lineup.