The ST420F integrates a low-voltage microprocessor-based line protection relay, multifunction power meter, and residual current electrical fire monitoring detector into a single intelligent device. It provides comprehensive low-voltage circuit protection, measurement, management, and communication functions — while simultaneously monitoring residual current for electrical fire hazards and triggering alarms before fire incidents occur.

The ST420F has passed dual certification testing and is widely applicable in industrial, commercial, and civil power systems as well as substations.

Key Features

Electrical Fire Monitoring

• Monitors 1-channel residual current (20–1000mA)
• Audible and visual fire alarm with graphical color display
• Fire linkage: 2-channel relay output
• 2-channel digital input (DI)

Comprehensive Low-Voltage Line Protection

• Inverse-time overcurrent protection
• Definite-time overcurrent protection
• Zero-sequence overcurrent protection
• Inverse-time zero-sequence overcurrent protection
• Instantaneous overcurrent (fast trip) protection
• Residual current protection
• Phase-loss protection
• Undervoltage / overvoltage protection
• Current unbalance protection

Multifunction Metering (Accuracy Class 0.5)

• Three-phase current & voltage
• Active / reactive power
• Active / reactive energy (kWh)
• Power factor & frequency
• Positive / negative sequence current & voltage
• Three-phase current & voltage unbalance
• 1st–7th harmonic current & voltage
• Residual current
• Average current

Integrated Solution Advantages

• Eliminates redundant investment in separate fire monitoring detectors and residual current transformers
• Optimized design — simplified installation, reduced failure points
• Unified HMI for easy operation
• Optimized communication channels, reduced infrastructure cost

Communication & Networking

• Two independent RS485 interfaces (Modbus protocol)
• RS485 Port 1: connects to power monitoring master station or communication manager
• RS485 Port 2: connects to electrical fire monitoring system network
• Both systems operate independently — meets fire inspection requirements

Event Records

• 16 fault records
• 8 digital input change records
• 8 trip records
• 8 protection reset records

I/O Specifications

• Three-phase current input: 1A / 5A
• Three-phase voltage input: AC 380V
• Digital input (DI): 2 channels, built-in DC 24V power supply
• Relay output (DO): 2 channels — Channel 1: AC 220V, resistive 5A; Channel 2: AC 220V, resistive 8A
• Residual current input: 1 channel, 20–1000mA

Standards & Certifications

Dual certification tested. Suitable for industrial, commercial, and civil power distribution systems and substations.