Model Series: S74D Series (Motor Protection Relays)

Technical Specifications:

• Rated Current (In): 1 A to 10 A (selectable via DIP switches)
• Voltage Range: 100V AC to 690V AC (3-phase)
• Phase Monitoring (PB0): Phase failure, phase reversal, phase unbalance (>15%)
• Overvoltage Setting (F0V0): 400V to 500V (adjustable, default 440V)
• Undervoltage Setting: Disabled (PB0 configuration)
• Frequency Range: 45 Hz to 65 Hz
• Output Contacts: 2 × Changeover Relay (8A / 250V AC) + 1 × Alarm Contact (NO, 5A)
• Accuracy: ±1% of setting value
• Response Time: <30 ms (phase failure) / <100 ms (overvoltage)
• Operating Temperature: -25°C to +60°C
• Protection Rating: IP20 (front panel: IP40 with cover)
• Mounting: DIN rail 35 mm (EN 60715)
• Power Supply: 100–240V AC/DC (auxiliary supply)
• Dimensions: 90 mm × 70 mm × 62 mm
• Weight: 280 g
• Standards Compliance: IEC 60255-1, IEC 60947-6-2, UL 508, CSA C22.2

Functional Features:

• Phase failure detection — trips within 30 ms if any phase is lost
• Phase reversal detection — trips if L1/L2/L3 sequence is incorrect
• Phase unbalance protection — trips if voltage unbalance exceeds 15%
• Overvoltage protection (F0V0) — trips if voltage exceeds 440V (adjustable)
• Start-up delay (adjustable 0–30 s) to allow inrush current decay
• Curtain timer — adjustable delay before re-energization after fault (0–600 s)
• LED status indicators: Power (green), Run (green), Fault (red), Alarm (yellow)
• Test button — simulates fault condition for relay verification

Structural Characteristics:

• Modular DIN rail housing (90 mm width)
• Front cover with hinged transparent window
• Screw terminal blocks for all power, voltage sense, and output connections
• DIP switch array (8 switches) on front for full parameter configuration
• LCD display option (on higher variants) — not present on PB0 base model

Working Principle:
Three-phase voltages are sampled via resistive voltage dividers at 4 kHz sampling rate. The 16-bit MCU performs FFT analysis to detect phase sequence, calculate RMS values, and measure phase-to-phase unbalance. The phase failure algorithm uses a voltage threshold + time-delay logic (30 ms). The overvoltage function (F0V0) compares RMS voltage against the setpoint with a 100 ms time delay to avoid nuisance trips from transient spikes. Upon fault detection, the output relay de-energizes, opening the motor contactor circuit.

Advantages & Highlights:

• Phase reversal protection — prevents motor damage from incorrect wiring (critical for new installations)
• 30 ms phase failure response — faster than most electromechanical relays
• No external CTs required — direct voltage sensing eliminates current transformer costs
• DIP-switch configuration — fully operational without PC software
• Test button enables commissioning verification without creating actual fault

Applicable Industries:

• Critical process motors (pumps, compressors) where phase loss causes catastrophic damage
• Generator feed protection
• UPS output monitoring
• Marine and offshore motor protection
• Data center cooling systems

Installation Requirements:

• Mount on DIN rail TS35 with minimum 30 mm spacing between adjacent devices
• Connect L1, L2, L3 to voltage sensing terminals (do not connect neutral)
• Wire output changeover contacts in series with motor contactor coil (A1–A2)
• Provide dedicated auxiliary supply (100–240V AC/DC) — do not derive from motor supply
• Use shielded cable for voltage sense leads in high-EMI environments

Usage Precautions:

• Verify phase sequence before energizing — PB0 will trip on reversal
• Set overvoltage threshold 5% below motor nameplate voltage rating
• Set phase unbalance threshold at 15% (do not go below 10% — causes nuisance trips)
• Perform relay test via test button monthly as per maintenance schedule
• After fault trip, use curtain timer delay before restart — do not manually bypass
• Do not modify DIP switches while relay is energized — power down first