ABB XT3N 250 TMD 250-2500 3p F F Moulded Case Circuit Breaker

The XT3N 250 TMD 250-2500 3p F F, order code 1SDA068059R1, is a fixed three-pole SACE Tmax XT3 moulded-case circuit breaker in the N breaking-capacity class. It has front terminals and a TMD thermomagnetic release rated 250 A with a fixed 2500 A instantaneous threshold. The XT3 is the 250 A frame of the Tmax XT range for power distribution at up to 690 V AC. More protection devices are listed under circuit breakers, and the wider SACE Tmax XT family is described on its own page.

Technical specifications

Thermomagnetic protection of the XT3 TMD

The TMD trip unit protects against overload with an adjustable thermal threshold from 0.7 to 1 times In and against short circuit with a fixed instantaneous threshold of 10 times In. For the 250 A unit that means the thermal setting runs from 175 A to 250 A and the magnetic trip is fixed at 2500 A. ABB’s trip curve example for an XT3N 250 TMD shows that a 3 x I1 overload trips between 107.9 s and 31.5 s from cold and between 31.5 s and 7.0 s from a hot state, and that fault currents above 2500 A trip magnetically. The thermal setting is made with a rotary cursor on the front.

Selecting the XT3N class and terminals

The N class of the XT3 gives 36 kA at 380 to 415 V and 25 kA at 440 V, so check the prospective fault current at the busbar before choosing it over the XT3S, which the catalogue lists at 50 kA at 415 V. The fixed front-terminal version is wired directly; extended, spread or multicable terminals and a plug-in version are separate items in the accessory chapter. A four-pole version and other trip units, such as TMG for generators, have their own ordering codes.

Ordering code and what to confirm

Read the designation as follows: XT3 is the frame, N the breaking class, 250 the size in amperes, TMD the thermomagnetic trip unit, 250-2500 the In and the magnetic threshold, 3p the poles, and F F fixed with front terminals. Please confirm the voltage, fault level, terminals, release options and accessories before ordering. The catalogue linked here is dated 2010; ask Seven Star LLC to check the current ABB documentation for revisions.

Sizing the XT3N 250 TMD for a feeder

This breaker is chosen by three numbers: the load current, the prospective short-circuit current at the point of installation and the system voltage. The adjustable thermal setting from 175 A to 250 A lets one breaker cover a range of feeder loads, so set I1 to suit the cable and the downstream load rather than leaving it at 250 A by default. Then compare the fault level calculated at the panel with the Icu figure for the actual voltage: 36 kA at 380 to 415 V and 25 kA at 440 V for this N-class unit. If the calculated fault current is higher, move to the XT3S or consider back-up protection arrangements documented by ABB.

The fixed 2500 A magnetic threshold also needs a check. On long cable runs the minimum fault current at the far end of the circuit can fall below 2500 A, in which case the breaker would only clear the fault on its thermal characteristic. A designer should confirm cable length and earth-fault loop impedance against the instantaneous threshold, especially on motor and sub-distribution feeders.

Panel installation and commissioning

• Mount the fixed breaker on DIN EN 50022 rail or on the panel plate, allowing for its 105 x 70 x 150 mm envelope plus cable bending space at the front terminals.
• Use cable lugs or bars that suit the F front terminals; if larger conductors or several cables per pole are needed, order the extended, spread or multicable terminals from the accessory chapter instead of forcing oversize lugs.
• Tighten terminals to the torque given in ABB’s instructions and record the values for the commissioning file.
• Set the thermal rotary cursor to the design I1 value and mark the setting on the panel drawing.
• If a shunt or undervoltage release is fitted, test it electrically; the total opening time with these releases is listed as 15 ms.
• Carry out insulation resistance testing with appropriate precautions before energising the outgoing circuit.

Maintenance and troubleshooting

With a mechanical life of 25,000 operations and an electrical life of 8,000 operations at 415 V AC, the XT3 is rarely limited by switching duty in normal distribution panels. Periodic maintenance therefore focuses on terminal tightness, signs of overheating or discolouration around the connections, and operating the breaker manually to confirm free movement. Repeated thermal trips usually mean the I1 setting is below the real load or the panel is running hot; compare measured current with the setting before raising it, and never raise it above the cable rating. A magnetic trip indicates a fault current above 2500 A and should be investigated before reclosing.

Gulf climate considerations

Thermomagnetic releases respond to temperature, so the inside of the enclosure matters. Panels in outdoor kiosks or plant rooms in Oman can run well above the ambient conditions assumed in catalogue tables during summer, which may lead to nuisance thermal tripping near full load. Ventilated or air-conditioned enclosures, shading and sensible spare capacity in the I1 setting help. Dust ingress in desert locations calls for enclosures with suitable sealing and regular cleaning of the breaker front and terminals. Near the coast, salt deposits and humidity accelerate corrosion of terminals and lugs, so include thermographic or visual inspection of the connections in the annual maintenance plan.

Documents and quotation

The linked ABB technical catalogue covers the whole SACE Tmax XT range with construction characteristics, ranges, accessories, curves, dimensions, wiring diagrams and ordering codes. Seven Star LLC is an independent supplier in Muscat and quotes on request. No price or stock is shown on this page.