Differences between load switches, disconnectors and vacuum circuit breakers

What is a Load Break Switch?

A load break switch is a control device equipped with a simple arc-extinguishing mechanism, capable of interrupting and closing circuits under load. It can interrupt a certain level of load current and overcurrent, but cannot interrupt short-circuit currents. Therefore, it must be used in series with a high-voltage fuse, relying on the fuse to clear short-circuit faults.

Functions of a Load Break Switch:

• Interrupting and Making Function: Due to its limited arc-extinguishing capability, a load break switch can be used to interrupt and make load currents and overloads up to a certain multiple (typically 3-4 times). It can also operate unloaded transformers with capacities exceeding those allowed for disconnectors, longer unloaded lines, and sometimes large capacitor banks.
• Substitution Function: A combination of a load break switch and a current-limiting fuse in series can replace a circuit breaker. The load break switch handles interrupting and making smaller overloads, while the current-limiting fuse handles larger overloads and all short-circuit currents.
• Combined Device: The integrated assembly of a load break switch and a current-limiting fuse in series is termed a "Switch-Fuse Combination" in national standards. The fuse can be installed on the supply side or the load side of the switch. When fuse replacement is infrequent, installing it on the supply side is preferred, allowing the load break switch to function as an isolator, thereby isolating the voltage applied to the current-limiting fuse.

What is a Disconnector (Isolator)?

A disconnector is a control device without an arc-extinguishing mechanism. Its primary function is to isolate power sources to ensure the safe maintenance of other electrical equipment; therefore, operation under load is strictly prohibited. However, under specific conditions, it is permitted to connect or disconnect low-power circuits. It is one of the most widely and frequently used devices in high-voltage switchgear.

Functions of a Disconnector:

• Establishing a Safe Isolation Point: After opening, it creates a reliable insulating gap, clearly disconnecting the equipment or line under maintenance from the power source, ensuring the safety of personnel and equipment.
• Circuit Switching: To switch circuit connections according to operational requirements.
• Interrupting Small Currents: It can interrupt small currents in circuits, such as charging currents of bushings, busbars, connectors, and short cables; capacitive currents of switch equalizing capacitors; circulating currents during double-busbar transfer operations; and excitation currents of voltage transformers.
• Interrupting Unloaded Transformer Magnetizing Current: Depending on the structural type, it can interrupt the magnetizing current of unloaded transformers up to a certain capacity.

Classification:

• By Installation Location: Can be divided into outdoor high-voltage disconnectors and indoor high-voltage disconnectors.
  • Outdoor Type: Designed to withstand harsh environmental conditions such as wind, rain, snow, pollution, condensation, ice, and heavy frost, suitable for open-air installation.
• By Insulating Post Structure: Can be classified as single-column, double-column, and triple-column disconnectors. The single-column disconnector uses the vertical space directly beneath overhead busbars as the electrical insulation for the break, offering significant advantages: saving land area, reducing connecting conductors, and providing clear visual indication of open/closed status. In EHV transmission systems, the space-saving effect of single-column disconnectors in substations is particularly pronounced.

Note: Disconnectors are primarily used in low-voltage equipment, such as residential and building low-voltage final distribution systems. The description stating its main function is "interrupting and making lines under load" is incorrect; its primary function is power source isolation.

What is a Vacuum Circuit Breaker?

A vacuum circuit breaker derives its name from using high vacuum as both the arc-quenching medium and the insulating medium between contacts after interruption. It features a compact size, light weight, suitability for frequent operation, and maintenance-free arc interruption, making it widely used in distribution networks.

Vacuum circuit breakers are primarily used in indoor distribution installations for 3–10 kV, 50 Hz three-phase AC systems. They serve as protection and control elements for electrical equipment in industrial and mining enterprises, power plants, and substations, especially suitable for applications requiring oil-free operation, minimal maintenance, and frequent operation. They can be installed in mid-mounted switchgear, double-deck cabinets, or fixed cabinets as control and protection switches for high-voltage electrical equipment.

Working Principle of Vacuum Circuit Breakers

When the moving and fixed contacts are opened by the operating mechanism, an arc forms between them. The contact surfaces vaporize metal under high temperature. Due to the specially designed contact shape, the current generates a magnetic field that drives the arc rapidly along the tangential direction of the contact surface. Part of the metal vapor condenses on the metal shield (screen). When the current naturally passes through zero, the arc extinguishes, and the dielectric strength between the contacts recovers rapidly.

Function of Vacuum Circuit Breakers

Circuit breakers provide protection functions including overload, short-circuit, and undervoltage protection, effectively safeguarding circuits and power sources.

Differences Between Load Break Switches and Disconnectors

• Interrupting Current Capability:
  • A disconnector lacks an arc-extinguishing mechanism and can only interrupt no-load current, unable to handle load current or short-circuit current. Operation must be performed only when the circuit is completely de-energized and unloaded; operating under load is strictly prohibited to prevent safety incidents.
  • A load break switch has an arc-extinguishing mechanism, enabling it to interrupt rated load currents and overloads up to a certain multiple, but it still cannot interrupt short-circuit currents.
• Presence of Arc-Extinguishing Mechanism:
  • This is the fundamental difference. An arc-extinguishing mechanism aids in the opening and closing operations of the switch and effectively limits and extinguishes arcs, enhancing operational safety. Consequently, most switching devices (especially those designed to interrupt current) are equipped with arc-extinguishing mechanisms.
• Functional Purpose:
  • A disconnector is primarily used to establish a clear break point in high-voltage circuits, isolating energized sections from de-energized sections for maintenance, ensuring personnel safety.
  • A load break switch is used in fixed high-voltage installations to interrupt fault currents (overloads) and rated operating currents of equipment. Although both are used in high-voltage systems, their functional roles differ.

Differences Between High-Voltage Load Break Switches and Vacuum Circuit Breakers

• Interrupting Capacity: A high-voltage load break switch can interrupt under load and has self-extinguishing capability, but its interrupting capacity is small and limited.
• Structural Differences: High-voltage disconnectors typically cannot interrupt under load and lack an arc chute in their structure (a few specially designed disconnectors can interrupt small loads, but their structure is relatively simple).
• Isolation Function: Both high-voltage load break switches and high-voltage disconnectors can create a visible break point, whereas most circuit breakers do not have this function (except for a few circuit breakers with isolation capability).
• Protection Functions:
  • High-voltage disconnectors have no protection functions.
  • Protection for high-voltage load break switches typically relies on series-connected high-voltage fuses, providing only instantaneous and overcurrent protection.
  • High-voltage circuit breakers (such as vacuum circuit breakers) can be designed with very high interrupting capacities, primarily protected by current transformers in conjunction with secondary protection devices, offering multiple protection functions including short-circuit, overload, and earth leakage protection.
• Functional Positioning: A high-voltage load break switch functions between a high-voltage circuit breaker and a high-voltage disconnector. It is often used in series with a high-voltage fuse for controlling power transformers. It can interrupt load currents and overloads, but due to its inability to interrupt short-circuit currents, it must rely on fuses for short-circuit protection.