What is the working mechanism of a fuse-switch combination device?
2026-01-29 15:39:53
Its working mechanism can be understood by breaking it down into three core parts: "normal operation," "fault protection," and "safety isolation."
I. Core Components
Disconnecting Switch (or Load Break Switch): Provides a visible and reliable electrical disconnection point, ensuring safety during maintenance. It cannot interrupt fault currents.
High-Voltage Fuse: Typically a current-limiting fuse, acting as a protective element. When an overload or short-circuit current flows through, the fuse wire melts rapidly and generates an arc. Under the cooling and deionization effect of quartz sand, the arc is quickly extinguished, interrupting the fault current.
Mechanical Interlocking Mechanism (the most critical part): Links the operating handle of the switch with the fuses. It achieves the following key functions:
Operating Procedure Interlock: Prevents operating the disconnecting switch under load.
Fuse Striker Interlock: When any phase fuse blows, it automatically triggers the switch to trip.
Three-Pole Interlock: Ensures that all three phases of the circuit are connected or disconnected simultaneously, preventing phase loss operation.
II. Working Mechanism and Process
1. Normal Closing and Opening Operations
Closing: The operator closes the disconnect switch by operating the handle (manually or via an electric mechanism). During the closing process, the interlocking mechanism ensures that the main contacts of the switch connect before the fuse holder, preventing arcing.
Opening (Manual): The operator manually operates the handle, first disconnecting the main contacts of the disconnect switch to interrupt the load current (if the switch is a load switch, it can interrupt the rated load current; if it is a disconnect switch, it must be operated under no-load conditions), achieving a visible disconnection of the circuit.
2. Fault Protection Action (Automatic Tripping)
This is the most crucial automatic protection function of the fuse combination switchgear. The process is as follows:
a. Fault Occurrence
When an overload or short-circuit fault occurs in the circuit, the fault current flows through the high-voltage fuse.
b. Fuse Operation
The fuse wire inside the fuse melts rapidly under the thermal effect of the fault current, generating an arc. The quartz sand inside the fuse divides, cools, and extinguishes the arc, thus interrupting the fault current. This process is usually completed within 10 milliseconds and has excellent current-limiting characteristics, significantly reducing the impact of short-circuit current on the system.
c. Striker Triggering
Most high-voltage fuses used in combination switchgear have a "striker." When the fuse wire melts, a spring-driven pin quickly pops out.
d. Interlocking Tripping
The ejected striker directly strikes the tripping mechanism of the switch. This tripping mechanism is connected to the operating handle, and under the impact force, it causes the disconnector switch to automatically trip open.
e. Formation of Visible Isolation Gap
After the switch trips open, a visible, safe air insulation gap is formed in both the faulty phase (the phase where the fuse has blown) and the non-faulty phases. This clearly indicates to the operator that the circuit has been disconnected due to a fault, and that the fuse in a particular phase has blown.
III. Core Advantages and Features of the Operating Mechanism
Integrated Protection and Isolation: It not only quickly interrupts fault current (fuse function) but also automatically provides a safe, visible isolation point (switch function), eliminating the need for manual intervention and greatly improving safety and power restoration efficiency.
Prevention of Phase Loss Operation: Even if only one fuse blows, the interlocking mechanism will force all three phases of the switch to trip, preventing damage to three-phase equipment such as motors due to phase loss.
Operational Safety: The interlocking mechanism is usually designed with a "make-before-break" logic and includes an anti-misoperation interlock to prevent operating the disconnect switch under load.
Economy and Compactness: Integrating the functions of two devices into a single compact unit saves space and cost, making it particularly suitable for the protection of distribution transformers (such as ring main units and compact substations).
Typical Application Scenarios
Distribution transformer protection: Used as a high-voltage side protection switch for pole-mounted transformers or compact substations in 10kV distribution networks.
Ring main unit: Used as an incoming and outgoing line unit in ring main units, providing protection and control functions.
Protection of small motors or capacitors.
Summary
The working mechanism of the fuse-switch combination device essentially utilizes the "electrodynamic force" of the fuse to quickly interrupt the fault current. Through a mechanical linkage mechanism, this force is converted into a "mechanical force" that trips the disconnector switch, ultimately achieving automatic and coordinated completion of both "fault current interruption" and "safe visible isolation."
It perfectly combines the advantages of good protection performance and low cost of fuses with the safety advantages of switches providing a visible isolation point, making it a simple, reliable, and economical key device in medium-voltage distribution networks.
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