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Relay protection closing self-holding

Fiber and communications infrastructure knowledge brief Published 22 Jun 2023

Use this reference to prepare product questions for CJC FIBRE. Check interfaces, capacity, protection and installation requirements against the current product documentation.

Self-holding in relay protection allows a relay or contactor to remain energized after the initiating signal is removed, maintaining the circuit until a stop or reset action occurs. Principle of Self-HoldingSelf-holding, also called self-locking, is a control technique where a relay or contactor maintains its energized state even after the start signal (like a push button) is released. This is achieved by wiring a normally open (NO) auxiliary contact of the relay in parallel with the start button.

When the start button is pressed, the relay coil energizes, and the NO contact closes, forming a holding circuit that keeps the coil powered independently of the button.

Typical Wiring and Operation

Start Button (NO): Connects the relay coil to the voltage source temporarily. Relay Coil: Energizes when the start button is pressed.

Auxiliary N

O Contact: Wired in parallel with the start button; closes when the relay energizes, maintaining current to the coil. Stop Button (NC): Breaks the holding circuit when pressed, de-energizing the relay. This setup ensures that once the relay is activated, it remains closed until a deliberate stop signal interrupts the circuit.

Applications in Protection Relays

In power system protection, self-holding circuits are used to: Keep circuit breakers or protective relays closed after a fault is detected until a reset or trip occurs. Enable sequential control in automation, where one relay's operation triggers the next step in a process. Implement interlocking and mutual locking, preventing unsafe simultaneous operations of multiple devices.

For example, in a medium-voltage network, a self-holding relay can maintain a breaker closed to isolate a faulted section while ensuring other breakers remain unaffected, enhancing selectivity and system stability.

Key Considerations

Response Time: Relay contacts must operate quickly to avoid delays in protection. Circuit Design: Parallel and series connections of auxiliary contacts determine whether the system stops immediately or completes a cycle before stopping. Safety: Interlocks and mutual locks are often combined with self-holding to prevent unsafe operations.

Self-holding circuits are fundamental in industrial automation, motor control, and electrical protection systems, providing reliable and predictable operation while simplifying operator control.

Relay protection closing self-holding - CJC FIBRE CONSULTANTS [PDF]

Relay protection closing self-holding

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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