Basler BE127 Under Voltage Relay

The Basler BE127 is a solid‑state under voltage relay designed to monitor single‑phase or three‑phase voltage levels and provide alarm or trip signals when the voltage falls below a preset threshold. It is widely used in generator, motor, and transformer protection applications to prevent damage from prolonged undervoltage conditions.

Relay Operation

The BE127 continuously measures the RMS voltage of the monitored circuit. When the voltage drops below the adjustable pickup setpoint for a specified time delay, the output relay activates. The relay returns to normal when the voltage rises above the dropout setpoint, which includes a hysteresis to prevent chattering.

Key Features

• Adjustable pickup voltage: 70–130 % of nominal (user‑selectable).

• Definite time delay: 0.1–10 seconds.

• Hysteresis setting: 2–10 % of pickup.

• True RMS sensing for accurate measurement.

• Wide auxiliary supply: 24–250 VDC or 120/240 VAC.

• Form C output contacts (SPDT) rated 5 A at 250 VAC.

• Front‑panel LED for relay status.

Technical Specifications

Input voltage range: 100–480 VAC (single‑phase) or 100–480 VAC line‑to‑line for 3‑phase. Frequency: 50/60 Hz ±10 %. Burden: < 5 VA. Accuracy: ±2 % of setting over the full temperature range. Operating temperature: -20 °C to +60 °C. The relay is housed in a robust case suitable for panel mounting.

Application Areas

The BE127 is commonly used in generator automatic voltage regulator (AVR) systems to initiate load shedding or generator tripping during sustained undervoltage. It is also applied in motor control centres to prevent low‑voltage starting, which can damage motors, and in transformer protection to avoid core saturation.

Installation Guidelines

Mount the relay on a panel using the provided brackets. Connect the voltage sensing leads to the appropriate phases, ensuring correct phase rotation for 3‑phase systems. The auxiliary power must be connected to the designated terminals; polarity must be observed for DC supplies. Use fuses or circuit breakers on the voltage input as per local codes.

Setting the Relay

Set the pickup voltage to the minimum permissible operating voltage, typically 80–85 % of nominal. Adjust the time delay to coordinate with upstream protections; for example, a longer delay may be used to ride through transient dips. The hysteresis should be set to avoid nuisance trips due to voltage fluctuations.

Testing and Commissioning

Apply the rated voltage and reduce it gradually using a variable autotransformer. Observe the pickup point and measure the timing with a stopwatch. Verify that the output contact changes state as expected. Test the dropout and hysteresis by slowly increasing the voltage.

Maintenance

Periodically inspect the relay for dirt and moisture. Check all connections for tightness. Re‑test the pickup and timing every two years or after any system change. The relay does not require routine calibration if settings remain unchanged.

Comparison with Other Undervoltage Relays

The BE127 offers a wider adjustment range and higher accuracy compared to electromechanical types. Its solid‑state design provides longer life and faster response, while the true RMS sensing ensures accurate measurements even with distorted waveforms.

Conclusion

The Basler BE127 Under Voltage Relay is a reliable and versatile protective device for voltage monitoring. Its adjustable parameters and robust construction make it suitable for a wide range of industrial and utility applications.