You're right to expect most SEL-751 functions to auto-reset after power is restored and the fault condition clears. However, there are several factors that can prevent this from happening, and troubleshooting why it's not automatically resetting is key. Here's a breakdown of common causes and things to check when a SEL-751 relay doesn't automatically reset after a fault or temporary power outage

1. Latch Bits and Manual Reset Settings:
Latching

The most common reason for needing to manually reset an SEL-751 is that certain fault elements or functions are configured to latch. This means that once they operate (trip or alarm), they remain active until manually reset, even after the fault is gone. This is often done to provide a record of the event and require operator investigation. Examples of functions often set to latch include:

Trip Outputs (e.g., Trip Coil Energized signal)

These outputs are designed to stay tripped even after the fault clears to prevent auto-reclosing into a potentially still-faulty system.

Alarm Contacts

Similar to trip outputs, alarms may be latched to ensure that personnel are aware of a previous event that needs investigation.

Overcurrent Protection (50/51)

Sometimes latched, depending on system criticality and operating philosophy.

Ground Fault Protection (50G/51G)

Similar to overcurrent.

Volts/Hertz (V/Hz) Protection (81)

Often latched as these can indicate more serious system issues.

Breaker Failure (50BF)

Definitely latched.

SEL Logic Equations

Examine the SEL logic equations within the relay's settings. A simple AND gate with a trip signal and a LATCH bit will ensure the trip output remains asserted until the latch bit is cleared.

2. Settings Group Dependency



Settings Group Selection

If the fault caused the relay to switch to a different settings group (via voltage, current, or digital input conditions), and that settings group is configured with latching, then you'll need to reset the relay. Even if the settings group reverts back after the fault is cleared, the latched condition from the triggered settings group might still persist.

3. Power-Up Issues



Insufficient Voltage/Inrush Current

The relay might not be fully powering up correctly after the outage. A weak battery or power supply issue could cause inconsistent behavior.

Startup Delays

Some relays have configurable startup delays that might affect how quickly they recover after power is restored. Check the settings related to power-up behavior.

Corrupted Settings

While rare, a power surge during the outage could potentially corrupt the relay's settings, leading to unexpected behavior.

4. Hardware Fault



Relay Malfunction

Although less common, the relay itself could have a hardware fault that prevents it from resetting properly. Internal components might have been damaged by the fault or the power outage.

5. Communication Issues



SCADA/HMI Interference

If the relay is controlled or monitored by a SCADA or HMI system, communication problems could prevent the reset command from being properly transmitted.

Troubleshooting Steps


Check the Event Report (SER – Sequential Events Recorder)

This is the first and most important step. The SER will provide a detailed timestamped record of what happened before, during, and after the fault. Look for: Which protection elements operated (e.g., 50, 51, 50G, etc.). Whether the relay tripped. Any alarms that were activated. The status of the latch bits. Any settings group changes.2.

Inspect Relay Status Indications

Look for any LEDs or display messages indicating a trip condition, alarm, or other error. Check the relay's faceplate or display for any specific error codes.3.

Review SEL-751 Settings

Carefully examine the relay's settings file using SEL AcSELerator QuickSet. Focus on: The latching configuration of each protection element and function. The settings group configuration and the logic that triggers settings group changes. Any logic equations that control trip outputs or alarms. Power-up and reset settings.4.

Attempt a Manual Reset



Via the Front Panel

Use the "Reset" button on the relay's front panel.

Via Communication Port (SEL AcSELerator QuickSet)

Connect to the relay using SEL AcSELerator QuickSet and issue a reset command through the software.

Via SCADA/HMI

If the relay is connected to a SCADA or HMI system, attempt to reset it through the system's interface.5.

Cycle Power

If a manual reset fails, try cycling the power to the relay (briefly disconnect power, then restore it). Be very careful when doing this and ensure it's safe to de-energize the relay. This can sometimes clear hung states or corrupted data.6.

Check Voltage Levels

Verify that the relay is receiving the correct voltage levels from its power supply. Measure the voltage at the relay's power input terminals.7.

Verify CT/PT Connections

Incorrect or loose CT or PT connections can cause incorrect relay behavior. Double-check the connections to the relay.8.

Consult the SEL-751 Instruction Manual

The SEL-751 instruction manual contains detailed information about all of the relay's functions, settings, and troubleshooting procedures.9.

Contact SEL Support

If you've exhausted all other troubleshooting steps and are still unable to resolve the issue, contact Schweitzer Engineering Laboratories (SEL) technical support. They have experts who can provide further assistance.

Example Scenario

Let's say a 50/51 (overcurrent) element is set to latch and trips. After the fault clears, the trip output will remain asserted. The SER will show the 50/51 element operating, the trip output asserting, and the latch bit being set. You would need to manually reset the relay to clear the trip output.

Important Considerations



Safety First

Always follow proper safety procedures when working with electrical equipment. Wear appropriate personal protective equipment (PPE) and de-energize circuits whenever possible.

Record Changes

Document any changes you make to the relay's settings.

Coordination

Ensure that any changes you make to the relay's settings are coordinated with other protection devices in the system.

Training: Proper training on the SEL-751 relay is essential for effective troubleshooting and maintenance.By carefully following these steps, you should be able to diagnose and resolve most issues that prevent an SEL-751 relay from automatically resetting after a fault or temporary power outage. Remember that understanding the relay's settings and using the SER data are critical to successful troubleshooting. Flag for review