Advantages & Applications of Low-Voltage Vacuum Circuit Breakers

Low-Voltage Vacuum Circuit Breakers: Advantages, Application, and Technical Challenges

Due to their lower voltage rating, low-voltage vacuum circuit breakers have a smaller contact gap compared to medium-voltage types. Under such small gaps, transverse magnetic field (TMF) technology is superior to axial magnetic field (AMF) for interrupting high short-circuit currents. When interrupting large currents, the vacuum arc tends to concentrate into a constricted arc mode, where localized erosion zones can reach the boiling point of the contact material.

Without proper control, overheated areas on the contact surface emit excessive metal vapor, which may lead to dielectric breakdown of the contact gap under the transient recovery voltage (TRV) after current zero, resulting in interruption failure. Applying a transverse magnetic field—perpendicular to the arc column—within the vacuum interrupter drives the constricted arc to rapidly rotate across the contact surface. This significantly reduces localized erosion, prevents excessive temperature rise at current zero, and thereby greatly enhances the breaker’s interrupting capability.

Advantages of Vacuum Circuit Breakers:

• Contacts require no maintenance

• Long operational life, with electrical life nearly equal to mechanical life

• Vacuum interrupters can be mounted in any orientation

• Silent operation

• No risk of fire or explosion; the arc is fully contained within the sealed vacuum chamber, making them suitable for hazardous, explosion-proof environments such as coal mines

• Performance is unaffected by surrounding environmental conditions such as temperature, dust, humidity, salt fog, or altitude

• Capable of withstanding high voltages across very small vacuum gaps

• Current interruption typically completed at the first current zero crossing

• Environmentally friendly and easily recyclable

Low-voltage vacuum circuit breakers share the same comprehensive protection, extensive measurement capabilities, and rich diagnostic features as conventional Air Circuit Breakers (ACBs). However, they offer superior advantages, including higher electrical and mechanical endurance, greater number of rated short-circuit breaking operations, stronger arc-quenching capability, and true "zero arc flash" performance.

These characteristics make them especially suitable for harsh environments and high-voltage low-frequency systems such as AC690V and 1140V in TN, TT, and IT configurations—commonly found in photovoltaic and wind power applications. They enable high-voltage collector systems that reduce transmission losses. Beyond line protection, these breakers can also protect motors (meeting GB50055 requirements) and generators (meeting GB755 standards), providing users with a safer, more reliable, and comprehensive low-voltage power distribution protection solution.

Why Aren’t Vacuum Circuit Breakers More Widely Used in Low-Voltage Applications?

The primary reason lies in the significant energy demands of the operating mechanism:

Low-voltage circuit breakers typically employ lightweight operating mechanisms with compact components. In contrast, vacuum circuit breakers require substantially more operating energy—especially those designed for high-breaking-capacity applications. Due to their small contact gap, extinguishing the arc requires intense energy. To withstand electromagnetic forces during fault interruption, high contact pressure is essential. For example:

• A 31.5kA vacuum breaker requires approximately 3200N contact force.

• To maintain adequate pressure after contact wear, a contact travel of 4mm is needed.

• Consequently, the total energy required from contact engagement to full closure is much higher than that of air circuit breakers.

Specific energy requirements include:

• 45 joules for a 40kA breaker (contact force: 4200N)

• 63 joules for a 50kA breaker (contact force: 6200N)

Thus, the operating mechanism must be significantly reinforced to meet these demands. For a 100kA low-voltage application, the energy required by a vacuum interrupter exceeds the capacity of standard low-voltage operating mechanisms.

A complete upgrade is necessary—larger energy storage springs, increased spring compression stroke, etc. Some existing mechanisms have minimal compression (e.g., only 25mm), and even increasing spring stiffness cannot deliver sufficient energy. Instead, mechanisms with longer stroke are required. As seen in medium-voltage vacuum breakers, cam-driven springs often extend over 50mm, enabling sufficient energy storage. Additionally, the overall mechanical strength, hardness, and rigidity of the operating mechanism must be enhanced to handle the high forces involved.