ABB 250A 4-pole TMG XT3N Circuit Breaker

ABB Tmax XT3N 250A 4P Circuit Breaker with TMG Relay

The XT3N is a molded-case circuit breaker (MCCB) from the Tmax XT from ABB, with the thermal-magnetic relay TMG, designed specifically to protect generators, unlike standard circuit breakers intended for networks with a stable utility power supply.

The key difference lies in the magnetic protection: in a generator, the short-circuit current decays rapidly because the power source is limited, not infinite like the utility grid. A circuit breaker with a standard magnetic threshold may not detect this decaying short circuit in time. The TMG relay sets this threshold to a low value (3 × In) precisely to respond to this current drop characteristic of alternators.

Electrical characteristics

Insulation voltage (Ui): up to 800 V AC
Surge voltage (Uimp): up to 8 kV; resistant to surges caused by switching operations or lightning strikes
Service breaking capacity (Ics): 100% of ultimate breaking capacity (Icu), 36 kA at 415 V. The equipment can resume safe operation after clearing a critical short circuit
Service life: up to 25,000 mechanical operations and 8,000 electrical cycles (at 415 V)

Thermal and Magnetic Protection of the TMG Relay

Adjustable thermal protection: Front-mounted rotary selector switch adjustable between 175 A and 250 A, to adjust the overload threshold to the exact rated current of the generator or cable.

Fixed magnetic protection: short-circuit threshold set at 700 A (3 × In, low impedance). This low-impedance configuration is what allows the system to detect the decreasing short-circuit current typical of generators, where a standard magnetic sensor would not respond in time.

Protected neutral (4P, InN=100%): The neutral conductor has exactly the same thermal and magnetic protection as the phase conductors. Suitable for systems with unbalanced loads or harmonics, which are common in installations with generator sets.

Mechanical and Physical Properties

Installation: Compact design, optimized for space savings in electrical cabinets
Mounting: Fixed front terminals (FF) as standard, for copper cables or busbars
Handle: clear indication of the three states (Open, Closed, Tripped)
Dimensions: 140 mm high × 150 mm wide × 70 mm deep
Weight: 2 kg (the original product listing also lists 2.1 kg in the description; this is a rounding difference with no practical significance)
Country of manufacture: Italy

References

Sorilux Part Number: G077787
ABB Part Number: 1SDA068264R1

What intensity do I need?

This circuit breaker is also available in lower- and higher-current versions (160A XT2N and 630A XT5N) within the same Tmax XT series with a TMG relay. If you have questions about which size is appropriate for your generator's power rating, contact us at.

Available documentation

The complete technical data sheet for the SACE Tmax XT series is available for download in the "Manuals" section of this page.