Merlin Gerin STR 58U In=3200A Masterpact Trip Control Unit

The STR 58U is an electronic trip unit designed for Merlin Gerin Masterpact NW (formerly M-Series) air circuit breakers. Rated for a sensor nominal current of 3200 A, it provides adjustable long-time, short-time, instantaneous, and ground-fault protection. The unit also offers metering of current, harmonic distortion, and event logging, making it suitable for low-voltage main switchboards and generator protection.

Protection Functions

• Long-Time (L): I_r = 0.4–1 x In (1280–3200 A), t_r = 0.5–24 s (I²t or fixed time)

• Short-Time (S): I_sd = 1.5–10 x I_r, t_sd = 0.1–0.4 s (I²t off/on)

• Instantaneous (I): I_i = 2–15 x In (6400–48 kA), disableable

• Ground-Fault (G): I_g = 0.2–1 x In, t_g = 0.1–0.8 s, with external toroid

Metering and Diagnostics

The STR 58U measures true RMS current per phase (accuracy ±2% from 0.1 to 1.5 x In), neutral current, and ground current. It calculates power, energy, and total harmonic distortion (THD) up to the 31st harmonic. The LCD display (4 lines, 20 characters) shows real-time values, pre-alarm warnings, and trip history (last 10 events with time stamp).

Communication and Integration

A built-in RS485 port supports Modbus RTU protocol, enabling remote monitoring and adjustment via a supervisory system. Optional communication modules (DigiTrip, ULP) allow integration with Masterpact M6/M8 controllers and power management software (PowerLogic). The trip unit also has a test port for portable tester (MER51000) to verify all functions offline.

Physical and Operational Data

The STR 58U is powered by current sensors (self-powered) – no external control voltage required for protection. For display and communication, a 24 VDC auxiliary supply is optional. Dimensions: 96 mm x 96 mm (panel-mount) or direct plug-in to breaker. Operating temperature: -25°C to +70°C; calibration accuracy unaffected by temperature. The unit meets IEC 60947-2 and UL 489.

Applications

Used in Masterpact NW20 (2000A), NW32 (3200A), and NW40 (4000A) breakers with appropriate sensors. Typical installations include main low-voltage switchgear, transformer secondary protection, large motor control centers, and standby generators.