MERLIN GERIN STR 58U 5000A Electronic Trip Unit
The Merlin Gerin STR 58U is a microprocessor-based electronic trip unit designed for high-current molded case circuit breakers with a frame rating of 5000A. It is part of the Masterpact series (now Schneider Electric) and provides precise protection for low-voltage distribution systems in industrial plants, data centers, and large commercial buildings. The trip unit features adjustable overload (L), short‑time (S), and instantaneous (I) protection functions.
Protection Settings
Long time (L): Adjustable current setting Ir = 2000–5000A (0.4–1.0 x In). Time delay tr = 0.5–24s (I²t off/on).
Short time (S): Pickup Isd = 1.5–10 x Ir. Delay tsd = 0.1–0.4s (I²t on/off).
Instantaneous (I): Fixed threshold Ii = 50000A (10 x In) or adjustable up to 12 x In on some variants.
The unit also includes ground fault protection (G) on some models (STR58U has G optional). All settings are made via rotary switches on the front panel, sealed to prevent tampering.
Breaking Capacity
The STR 58U is matched to a Merlin Gerin Masterpact M50 or M63 frame with a rated ultimate breaking capacity (Icu) of 100 kA at 600V AC (50/60Hz). Making capacity (Icm) is 220 kA. The unit is rated for insulation voltage (Ui) 1000V AC.
Microprocessor and Diagnostics
The trip unit contains an 8-bit microprocessor that continuously monitors phase currents via built-in current transformers. It features an LED bar graph for real-time load indication (20–100% of Ir). A red LED indicates trip events (overload, short circuit, ground fault). The unit includes a maintenance test port for connecting a portable test kit (Merlin Gerin Ekip Tester) to verify trip times without primary injection.
Power Supply and Auxiliaries
The STR 58U is self-powered from the line current (no external control voltage needed). It provides a dry contact output for remote trip indication (changeover contact, 250V AC/1A). An optional external 24V DC supply can be added for enhanced diagnostics and communication (Modbus via Ekip Com module).
Environmental and Mechanical
Operating temperature: -25°C to +70°C (derating above 55°C). Storage: -40°C to +85°C. Humidity: 95% non-condensing. The unit is sealed against dust and moisture (IP40 front, IP20 rear). Dimensions: 180 mm x 130 mm x 80 mm. Weight: 1.2 kg. The trip unit is interchangeable without removing the breaker (hot‑swappable with breaker open).
Compliance and Certifications
The STR 58U meets IEC 60947-2, UL 489 (under Schneider Electric label), and CCC. It is CE marked and RoHS compliant.
Typical Applications
Main feeders for large transformers (2500–5000 kVA), busbar protection, and generator circuit breakers. Also used in shipboard power systems.
Ordering and Documentation
Merlin Gerin part number: STR 58U (In=5000A). Manual: Masterpact NT/NW User Guide (LV492001). The unit is supplied with a sealing kit and label sheet.
