Overview of Spring-Based Operating Mechanisms

The operation of a vacuum circuit breaker (VCB) is governed by a precisely engineered spring energy storage mechanism, where the closing spring and opening spring perform distinct yet interdependent functions. This separation ensures controlled mechanical motion, reliable contact engagement, and rapid fault isolation during operation.

At the core of this mechanism lies the principle of mechanical energy accumulation and release. The springs act as energy reservoirs—storing potential energy during the charging phase and releasing it instantaneously to drive the moving components during switching operations.

Function of the Closing Spring

The closing spring serves as the primary actuator during the breaker’s closing operation.

• Energy Storage: During the energy accumulation stage, the closing spring is compressed through manual or motorized means. This stored potential energy remains locked within the mechanism until a closing command is issued.

• Energy Release: Upon activation, the spring releases its stored energy abruptly, propelling the transmission linkages and engaging the breaker contacts. The motion must be swift and decisive to overcome contact pressure and magnetic counterforces, ensuring a firm and arc-free closure.

The controlled release of the closing spring defines the closing speed profile, directly influencing contact pressure buildup and arc suppression performance during energization.

Function of the Opening Spring

The opening spring functions as the driving element during the tripping phase. Unlike the closing spring, it remains largely passive during closing but plays a critical role during circuit interruption.

• Energy Discharge: When a trip signal is issued, the opening spring releases its stored energy, separating the breaker contacts and initiating the arc-extinguishing process within the vacuum interrupter.

• Energy Restoration: After each opening operation, the mechanism resets, and the opening spring recharges during the subsequent closing action, preparing for the next trip sequence.

For systems requiring auto-reclosing operations (O–0.3s–CO–180s–CO), the rapid re-energization capability is essential. Since the closing spring requires around 10 seconds to recharge, the opening spring alone cannot sustain the 0.3-second reclosing cycle, necessitating independent energy storage drives for fast reclosing functionality.

Determining the Parameters of the Closing Spring

To ensure coordinated motion, the mechanical characteristics of the operating mechanism are represented by a load characteristic curve (equivalent resistance diagram).

In this diagram:

• Curve 1 depicts the equivalent resistance from the opening spring.

• Curve 2 represents the self-closing force of the vacuum interrupter.

• Curve 3 shows the contact spring force.

A noticeable resistance jump occurs before and after contact engagement, illustrating the sudden mechanical transition during closing.

Based on empirical data from a 1250A, 31.5kA vacuum circuit breaker, the forces acting on the operating mechanism are quantified as follows:

• The combined vacuum negative pressure and bellows tension per pole amount to 21.64 kgf, or 64.9 kgf across three poles, opposing the closing force.

• Each opening spring exerts 59.3 kgf of tension (totaling 177.9 kgf for three poles), acting as a closing resistance. The pre-tension before closing measures 26.7 kgf per pole (totaling 80.1 kgf).

• The contact spring, a composite of disc springs, exhibits negligible resistance between 0–10 mm compression. Between 9–12 mm, resistance rises nearly linearly, and from 12–13 mm, a sharp rebound force appears, stabilizing at 3800 N per pole, or 11400 N total, equivalent to 1163 kgf, representing the maximum closing resistance.

Matching the Output Force with Load Resistance

In an optimized spring operating mechanism, the output force curve of the spring (typically shown as curve 3 in the diagram) must align with the load resistance curve (curve 1). However, these curves inherently diverge due to nonlinear elasticity and mechanical losses.

For reliable closing, the driving force must always exceed the equivalent resistance throughout the motion. The ideal output force profile (curve 2) should closely parallel the load curve, ensuring stable acceleration, adequate contact pressure, and minimal mechanical stress.

To determine appropriate spring parameters, the designer must derive the spring output characteristic from these conditions.

Key Requirements for Closing Spring Characteristics

According to the operational demands of high-performance circuit breakers, the output force characteristic of the closing spring must satisfy:

1. Sufficient Initial Force: The initial spring output must exceed the system’s initial resistance, ensuring smooth motion initiation.

2. Adequate Total Work: The total mechanical work performed by the spring must surpass the total energy required for full contact engagement, guaranteeing complete closure.

3. Optimized Force Profile: The force distribution should yield an appropriate closing velocity curve, balancing mechanical impact and dynamic response.

These requirements ensure that the breaker closes with controlled speed, avoiding contact bounce or excessive impact, while maintaining sufficient force to overcome counteracting spring loads and magnetic drag.

Contact Pressure and Spring Design

In high-interrupting capacity circuit breakers, significant contact pressure is essential to withstand high fault currents and prevent pre-arcing. To achieve this within compact geometries, disc springs (Belleville springs) are employed.

These springs generate immense pressure through minimal deformation and are typically installed within insulated pull rods, forming a compact, mechanically robust assembly. The use of disc springs ensures high mechanical strength and stability under repetitive high-energy switching conditions.

For ring main unit (RMU) vacuum circuit breakers, where short-circuit breaking capacity typically reaches 20 kA, the contact pressure requirement is relatively lower. Consequently, cylindrical compression springs—either rectangular or circular in cross-section—are used.

Mechanism Architecture in RMU Circuit Breakers

In compact RMU designs, domestic vacuum circuit breakers often employ an insulated main shaft mechanism. The rotational motion of the shaft drives a cam structure, achieving both opening and closing of the vacuum interrupter.

While this cam-driven configuration simplifies design and reduces cost, it inherently lowers the opening velocity, making it unsuitable for high short-circuit breaking capacities. However, for moderate ratings typical of RMU applications, this limitation has negligible impact and provides an optimal balance between mechanical simplicity, safety, and performance.

Conclusion

The operating mechanism of a vacuum circuit breaker epitomizes the interplay of mechanical precision and electrodynamic control. Each spring — whether closing, opening, or contact — fulfills a distinct role in translating stored mechanical energy into swift, reliable switching actions.

From the heavy-duty disc springs of high-capacity breakers to the compact compression springs in RMU units, the spring-based architecture ensures dependable performance, rapid reclosing capability, and compact integration. The precise calibration of force curves, energy profiles, and timing defines not only the efficiency but also the longevity and safety of the vacuum circuit breaker — the silent guardian of modern power systems.