let's analyze in detail and systematically the design and functional positioning of disconnect switches in terms of their two core capabilities: "current breaking" and "arc extinguishing."
Key Conclusion Reiterated
Disconnect switches do not possess any meaningful load current breaking capability or dedicated arc-extinguishing capability. Using them to interrupt load current is a serious misoperation that can lead to catastrophic consequences.
Detailed Analysis
1. Core Design Purpose: Electrical Isolation
The primary and fundamental function of a disconnect switch is to provide a visible and reliable air-insulated break to ensure complete electrical isolation between electrical equipment and the power supply side during maintenance, ensuring personnel safety.
• "Visible": After operation, the contacts should be clearly visible to the naked eye, forming a distinct break point.
• "Reliable": The insulation distance between the breaks is sufficient to withstand potential overvoltages in the system, ensuring no current flows.
2. Regarding "Interrupting Current Capacity"
• Standard Definition: The "interrupting current capacity" on the nameplate of electrical equipment usually refers to the maximum fault current (such as short-circuit current) or rated load current that it can safely interrupt.
• The Truth About the "Interrupting Current" of a Disconnect Switch:
It does not have a rated load current interrupting capacity. Any attempt to use it to interrupt normal loads (such as motors, transformers, lighting circuits) will generate a strong electric arc at the moment of opening, causing contact melting, phase-to-phase short circuits, or even arcing and short-circuit explosions.
The current it allows to operate with is limited to the small capacitive or inductive current under no-load conditions, such as:
▪ The charging current of disconnecting an unloaded busbar (within a few amperes).
▪ Disconnecting small-capacity equipment such as voltage transformers and surge arresters (typically requiring a current less than 0.5A).
▪ Performing equipotential switching operations (e.g., switching operations in a double-busbar system, ensuring the current path has been transferred in parallel through other switches).
These allowable operating currents are extremely small, and their arc energy can be extinguished by natural air stretching and cooling. However, this does not mean it possesses the designed "current-breaking capacity."
3. Regarding "Arc Extinguishing Capability"
• Arc Extinguishing Principle: To safely interrupt current, the arc generated when contacts separate must be rapidly cooled and extinguished. Circuit breakers employ complex arc-extinguishing chambers, utilizing vacuum, SF6 gas insulation and arc blowing, or oil medium, compressed air, etc., for powerful arc extinguishing.
Structure of Disconnecting Switches
Their structure is extremely simple, typically just a set of knife switches exposed to air. There are no dedicated arc-extinguishing devices (such as arc-extinguishing chambers, arc-extinguishing grids, compressed air pistons, etc.). The contact separation speed is very slow (manual operation), which is precisely the biggest taboo in arc extinguishing-slow separation will maintain arc burning for a long time.
The arc generated during interruption relies solely on natural air convection, thermal buoyancy, and electromagnetic force for stretching and cooling. Its arc-extinguishing capacity is extremely weak and unreliable, completely incapable of handling any meaningful load current.
Functional Coordination with Circuit Breakers (Typical Operating Sequence)
In power systems, disconnect switches must be used in series with circuit breakers to complete the entire process of "breaking" and "isolating" the circuit.
Products Description
Their structure is extremely simple, typically just a set of knife switches exposed to air. There are no dedicated arc-extinguishing devices (such as arc-extinguishing chambers, arc-extinguishing grids, compressed air pistons, etc.). The contact separation speed is very slow (manual operation), which is precisely the biggest taboo in arc extinguishing-slow separation will maintain arc burning for a long time.
The arc generated during interruption relies solely on natural air convection, thermal buoyancy, and electromagnetic force for stretching and cooling. Its arc-extinguishing capacity is extremely weak and unreliable, completely incapable of handling any meaningful load current.
Functional Coordination with Circuit Breakers (Typical Operating Sequence)
In power systems, disconnect switches must be used in series with circuit breakers to complete the entire process of "breaking" and "isolating" the circuit:
1. Breaking Current: First, the circuit breaker operates, cutting off all load current and fault current. At this point, there is no current in the circuit, but the circuit breaker contacts are not visible or do not meet the safe isolation distance.
2. Establishing Isolation: After confirming that the circuit breaker has opened, open the disconnect switches on both sides to form a visible and safe electrical isolation point.
3. Grounding Protection: If necessary, close the grounding switch (a special disconnect switch) on the isolated maintenance equipment side to release residual charge and prevent accidental energization.
Restore Power Supply:
1. Remove Grounding: Disconnect the grounding switch.
2. Release Isolation: Close the disconnect switch.
3. Reconnect Circuit: Finally, close the circuit breaker to restore power supply.
Summary Comparison Table
|
Characteristics |
Disconnector |
Vacuum circuit breaker |
|
Core functionality |
Electrical isolation, providing a visible break point |
Protection and control, capable of interrupting and making up loads and fault currents |
|
Flow interruption capability |
None. Strictly prohibited to operate under load. |
Yes. Capable of interrupting rated current and short-circuit current. |
|
Arc-extinguishing device |
None |
(Vacuum, SF6, oil, etc. arc extinguishing chambers) |
|
Operating speed |
Operating speed Slow (manual or electric) |
Extremely rapid (spring or hydraulic mechanism) |
|
Electrical symbols |
Typically a simple symbol with a notch |
Add a 'circle' symbol to the isolator switch symbol to denote the arc-extinguishing function. |
|
Operating sequence |
Must be operated after the circuit breaker has been opened (when no current is present). |
Capable of operating under current, it serves as the primary switching device for energising and de-energising circuits. |
In summary
Final summary: You may consider circuit breakers as the 'intelligent switches' within an electrical circuit, responsible for conducting or interrupting current flow. Disconnectors, however, function as 'safety blades', providing a visible and reliable "lock" after disconnection to ensure the power supply cannot be re-energised during maintenance. Under no circumstances should a 'safety blade' be used to perform the duties of an 'intelligent switch'.
Products Description
VTZ-12 vacuum circuit breaker (special model of Tongzhong Electrical Co., Ltd.) is an indoor switchgear for rated voltage 12kV, AC 50/60Hz. It adopts a modular operating mechanism and is suitable for various industrial and mining enterprises, as well as power grid equipment. It can be used as a handcart unit for use with KYN28A-12 switchgear, or as a fixed unit with relevant mechanical interlocking, making it suitable for XGN2 and other fixed cabinets.
contact us
Contact: Ms.Grace Liu (Director of Sales Department)
Tel: +86 917 3661109 Fax: +86 917 6739416
Email:xdtz04@westpowerelectric.com
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Add: Nanpo Village, Chencang Avenue Jintai District Baoji City, Shaanxi Province, China.
