The ST400S Series is a microprocessor-based line protection relay designed for low-voltage applications up to AC 660V at rated frequencies of 50/60Hz. It integrates protection, measurement, control, management, and communication functions into a single intelligent device — the ideal choice for smart power distribution systems.

The series features a modular design comprising the main unit and dedicated current transformers. Its compact, space-saving structure allows direct installation in 1/4-modular and larger withdrawable switchgear cabinets. Widely applied in power generation, petroleum, petrochemical, chemical, metallurgy, coal, paper, and rail transit industries.

Complies with GB/T 14598.2-2011 (Measuring Relays and Protection Equipment — Part 1: General Requirements) and GB/T 14598.26-2015 (Part 26: Electromagnetic Compatibility Requirements).

Product Variants

Communication Options

• ST400S1 — No communication
• ST400S3 — 1× Modbus-RTU (RS485)
• ST400S5 — 2× Modbus-RTU (RS485)
• EVTP — Modbus-TCP (optional)

CT Installation Types

• Integrated (L) — 1A / 5A CTs, mounted directly on the unit
• Split (F) — 1A, 5A, 25A, 100A, 160A, 400A CTs, externally mounted; cable lengths: 0.5m, 2m, or 3m

Rated Current Range (CT Selection)

• 1 (0.2–1A)
• 5 (0.5–5A)
• 25 (2.5–25A)
• 100 (10–100A)
• 160 (16–160A)
• 400 (40–400A)

Optional Add-on Functions

• V1 — Voltage protection, AC 100V
• V2 — Voltage protection, AC 400V
• V3 — Voltage protection, AC 660V
• M1 — 1× 4–20mA analog output
• M2 — 2× 4–20mA analog output (uses 5 relay outputs)
• L — Residual current protection (requires external ZCT)
• R — Fault waveform recording (3-phase current + 3-phase voltage, 6 channels)
• T2 — Temperature protection, 2-wire (PT100)

Expansion Modules (Optional)

• EDP — 1× Profibus-DP (DPV0/V1, DB9 interface)
• E2DP — 2× Profibus-DP (DPV0/V1, DB9 interface)
• EPN / EMTP — 1× RS485 (RJ45 interface)

Residual Current Transformers (for L option)

• LZT46-1A — Bore 46mm, range 0.1–1A
• LZT80-1A — Bore 80mm, range 0.1–1A
• LZT120-1A — Bore 120mm, range 0.1–1A

Display Module (Optional)

• ST523L — Multi-line color panel display with WiFi wireless data upload/download, historical data analysis, and fault management

Digital Input Power Supply Options

• Built-in DC 24V (internal supply for optically isolated DI)
• External AC 220V / DC 110V / DC 220V

Relay Outputs

• Standard: 6 relay outputs
• With M2 or T2 option: 5 relay outputs