Westinghouse 774B472A12 Type AR Relay – Electromechanical Protection for DC Systems

The Westinghouse 774B472A12 Type AR Relay is a high-reliability electromechanical protective relay designed for 125 VDC control circuits. It is typically employed in substation and switchgear applications where dependable, simple, and robust protection is required. The relay is documented under Instruction Leaflet 41-759.3, which outlines its installation, setting, and maintenance procedures.

Design and Construction

This relay features a hinged-armature magnetic structure with a permanent-magnet damping mechanism to ensure consistent operation over a wide temperature range. The moving contact system is designed for high-speed action, minimizing arcing and contact wear. All internal components are housed in a rugged steel case with a transparent dust cover for visual inspection.

Electrical Specifications

• Coil Voltage: 125 VDC nominal (operating range 100–140 VDC)

• Pickup Adjustment: Continuously adjustable from 50% to 200% of rated current (or voltage, depending on application)

• Contact Configuration: 2 Form C (DPDT) with silver-tungsten contacts for DC breaking

• Operating Time: < 30 ms at 2x pickup

• Insulation Test: 1500 VAC between coil and contacts

Functional Applications

The Type AR relay can be used as an overcurrent relay, a differential relay, a voltage relay, or an auxiliary trip relay. Its versatility comes from interchangeable coils and resistor modules that allow different pickup characteristics. It is commonly found in feeder protection, transformer differential, and busbar protection schemes.

Setting and Calibration

Settings are made via a calibrated dial that adjusts the spring tension, changing the pickup level. An external resistor may be added to alter the time-current curve. The relay includes a target indicator that flags when the relay has operated, aiding in fault diagnosis.

Installation

Mounting is via standard 19-inch rack panel cutouts or individual surface mounting. Terminals are clearly numbered per ANSI standard, and the relay is polarity-sensitive for DC operation. The instruction leaflet provides detailed wiring diagrams for common protection schemes.

Testing and Maintenance

Periodic testing involves injecting DC current (or voltage) and verifying pickup and drop-out values. The contacts should be checked for pitting and cleaned as needed. The permanent magnet assembly does not require recalibration.

Reliability and Longevity

With few moving parts and robust materials, the 774B472A12 offers decades of service in properly maintained installations. Its electromechanical nature ensures immunity to electromagnetic interference and digital faults.

Conclusion

The Westinghouse Type AR Relay remains a trusted component in legacy and modern DC control systems, offering simplicity, dependability, and precise protection.