Beckwith M‑0170A AC Current Relay

The Beckwith M‑0170A is a solid‑state AC current relay designed for precise overcurrent detection in three‑phase or single‑phase power systems. It continuously monitors the line current via a current transformer (CT) and operates when the measured current exceeds a user‑adjustable pickup level for a specified time delay. The relay provides both instantaneous (ANSI 50) and time‑overcurrent (ANSI 51) functions, making it suitable for feeder protection, motor protection, and generator backup protection. Its robust design and accurate measurement ensure reliable operation in harsh industrial and utility environments.

Design and Operating Principle

The M‑0170A uses an analog solid‑state circuit with a precision rectifier and comparator to detect overcurrent conditions. The pickup level is adjustable over a wide range (typically 0.5 to 20 A secondary) via a front‑panel potentiometer, while the time delay is adjustable from 0.1 to 10 seconds using a separate dial. The relay features an inverse‑time characteristic option that mimics the thermal behavior of conductors and motors, providing better coordination with downstream devices. For instantaneous applications, the time delay can be set to its minimum position, giving an operating time of less than 20 ms.

The relay includes a built‑in test circuit that allows functional verification without applying primary current. A target indicator with manual reset provides clear trip indication, and an electrical reset input enables remote reset from a control panel. The output contact is rated 5 A at 250 VAC, capable of directly tripping a breaker or initiating an alarm.

Key Features and Adjustability

Independent adjustment of pickup current and time delay. Dropout ratio of approximately 95% prevents contact chattering near the threshold. Wide power supply range (24‑250 VDC or 120‑240 VAC) simplifies integration into various panel control voltages. LED indicator shows relay status (normal, trip, or fault). The relay is housed in a rugged metal enclosure with clear terminal markings, suitable for panel or surface mounting. It complies with IEEE C37.90 for surge withstand and has excellent noise immunity.

Technical Specifications

Input current: 1 A or 5 A secondary (user‑selectable via internal jumpers). Frequency: 50/60 Hz. Pickup range: 0.5‑20 A (adjustable). Time delay: 0.1‑10 s (adjustable). Accuracy: pickup ±2% of setting, timing ±5% or 50 ms (whichever greater). Output contact: 1 Form C (SPDT), rated 5 A at 250 VAC resistive, 0.5 A at 125 VDC. Power supply: 24‑250 VDC or 120‑240 VAC, less than 5 VA burden. Operating temperature: –20°C to +65°C. Enclosure: IP40, with mounting holes for panel installation. Weight: approximately 1.5 kg.

Applications

The M‑0170A is widely used for overcurrent protection of distribution feeders, branch circuits, large motors, and generators in industrial plants, commercial buildings, and utility substations. It serves as a cost‑effective backup protection for transformer and busbar faults. In motor applications, it can be set to protect against locked‑rotor and sustained overload conditions. The relay is also applied in renewable energy systems to protect inverters and cables from overcurrent.

Installation and Commissioning

Mount the relay securely on a panel using the provided screws. Connect the CT secondary leads to the appropriate terminals, ensuring correct polarity and CT grounding. Connect the power supply and output circuits. During commissioning, adjust the pickup level to a value above the maximum normal load current but below the minimum fault current. Set the time delay to coordinate with upstream and downstream devices. Perform secondary injection tests using a test set to verify pickup and timing accuracy. The relay includes a test jack for injection without opening CT circuits.

Reliability and Maintenance

The solid‑state design of the M‑0170A eliminates moving parts, providing high reliability and long service life. Regular calibration is recommended every two years to maintain accuracy, though drift is minimal due to temperature‑compensated components. Periodic inspection of connections and terminal tightness is advised. The relay's self‑monitoring circuitry detects internal power supply failures and provides a warning output.

Conclusion

The Beckwith M‑0170A AC current relay offers a simple yet effective solution for overcurrent protection in diverse power systems. Its adjustable settings, robust construction, and ease of installation make it a trusted choice for engineers seeking reliable and cost‑effective protection.