A knife switch is a kind of isolation switch. A knife switch consists of an insulating bottom plate, a “knife” moving contact, a static contact, an arc extinguishing device, and an operating mechanism. In layman’s terms, we call it an isolation knife switch disconnect.
A knife switch provides a clear, manual disconnect by physically moving a blade into or out of fixed contacts on an insulating base. Originally used for simple low-voltage isolation, today’s knife switches often include arc chambers and safety interlocks to safely interrupt load currents and prevent accidental re-closure.
Modern designs incorporate visible blade windows, quick-make/quick-break mechanisms, and advanced materials (e.g., V0-grade nylon or silver-alloy contacts) to meet demands in renewable-energy and high-current applications.
Compliance with UL 98, IEC 60947-3, and NEMA/EN standards ensures safety and reliability. Proper selection, installation, and maintenance, along with understanding common faults, are key to maximizing performance and service life.
Knife Switch Types
Knife switches can be divided into many types according to the number of poles, conversion methods, and operation methods.
Among them, there are single-throw knife switches (HD series), double pole double throw knife switches (HS series), which are also knife changeover switches, fused knife switches （HR series）, etc., and we will not explain them one by one.
Among them, the fuse link as the moving contact, is called a fused knife switch, or a knife fused switch for short.
The copper knife switch used for power isolation generally does not need an arc extinguishing device, unless there is a requirement for the knife switch to break the load current, an arc extinguishing device, such as a fast blade or an arc extinguishing chamber, needs to be installed.
In the circuit, the industrial knife switch has to withstand the electric power and heat generated by the short-circuit current, so it must be ensured that the knife blade disconnect switch is in the circuit, and the movable contact knife will not bounce, weld firmly, or burn.
Type
Description
Key Features
Single-Pole Single-Throw (SPST)
Blade engages with a single fixed contact on an insulated base, providing straightforward on/off isolation for one circuit.
Simple on/off control; single circuit; visible operation
Double-Pole Double-Throw (DPDT)
Two blades mounted on one handle can engage with either of two sets of fixed contacts, enabling source switching between two circuits or power sources.
Two circuits or sources; changeover functionality; manual source transfer
Fused Knife Switch
Integrates a fuse link as the moving contact; the fuse melts under overload or short-circuit conditions, providing built-in overcurrent protection.
Combined isolation and protection; single-use fuse element
Non-Fused Knife Switch
Acts solely as a manual disconnect; requires external overcurrent protection (e.g., fuses or breakers) to safeguard the circuit.
Isolation only, reusable without internal fuse, cost-effective
How to use a knife switch?
⑴ When the operating knife switch is disconnected, in the opening and closing operation, it should act quickly to extinguish the arc as soon as possible;
⑵ Some knife switch disconnectors do not have arc extinguishers. When disconnecting the circuit, try to avoid load operation. Switches with loads should be turned off first, and then the knife switch should be turned on.
⑶ The disconnect knife switch should be placed outdoors or in a suitable place in the room where flammable, explosive, and corrosive items are stored, or use the corresponding explosion-proof and anti-corrosion switch.
⑷ Regularly check whether the knife of each switch is in good contact with the wire and the contact point of the knife, and whether the switch plastic box, porcelain seal,and handle are damaged.
⑸ In the neutral point grounding system, the single throw knife switch must be connected to the live wire to ensure the safety of electricity use.
⑹ When replacing the fuse, it must be done in the case of disconnection, and the fuse must be the same specification as the original.
⑺ When using the knife switch to disconnect the control power, the knife switch must be turned on to prevent a power failure. Otherwise, in the event of power restoration, the transmission will be activated, causing not only wasted power but other unexpected hazards as well.
Typical Applications
Industrial Distribution: Main and sub-distribution panels, emergency power transfer, and maintenance isolation.
Renewable Energy Systems: Photovoltaic arrays and wind-turbine controllers require a DC isolation switch with high withstand voltage. GRL has launched a new DC knife switch that can be used in PV power generation systems.
Battery Energy Storage: EV charging stations and grid storage modules, safeguarding against unintended discharge or short-circuit faults.
Lockout/Tagout Procedures: Machine tools and chemical plants use visible blade switches for “Lockout Line of Sight” compliance.
A Knife Switch Key Components and Materials
Blade Contacts: Typically copper or silver-plated copper to balance conductivity and corrosion resistance; some high-end models use self-cleaning contact profiles.
Arc-Extinguishing Chamber: Metal or ceramic enclosures that quench arcs when interrupting load currents, extending switch life, and improving safety.
Insulating Base: Constructed of V0-grade flame-retardant nylon, phenolic, or epoxy to withstand –35 °C to +70 °C and harsh environmental conditions.
Operating Mechanism: Options include direct-handle, panel-mounted, or door-mounted actuators, often with multi-position locking to prevent misoperation.
A Knife Switches Installation and Maintenance Best Practices
Selection: Match voltage, current, and interrupting capacity requirements; decide on fused vs. non-fused and arc-chamber needs.
Environment: For outdoor or corrosive settings, choose IP65/IP66 enclosures or explosion-proof variants
Inspection: Every 6 months, check contact surfaces for oxidation, tightness of connections, and integrity of insulation
Fuse Replacement: Always isolate the load, confirm the de-energized state, and replace with the same fuse type and rating
FAQ：
Q1: The contact is burnt, the reason may be that the action of pulling the electric knife switch is too slow, and the switch capacity is too small. A1: Repair or replace the contacts, improve the operation method, and replace the switch with a higher rated current.
Q2: The fuse blows when closing. The reason may be a short circuit behind the electric knife switch, and the size of the fuse is too small. A2: Eliminate the short circuit, and replace the fuse as required.
Q3: One or two phases are disconnected after the power is on. The possible reasons are poor contact between the input and output wires of the switch. Blown fuse or wrong connection, contact with oxidation or dust, loss of elasticity of the fixed contact, resulting in poor contact. A3: Check the knife switches‘ incoming and outgoing wires, and reconnect if there is a problem; replace the fuse or tighten the screw that presses the fuse; clean the contacts; pinch the fixed contacts, and replace them if you can’t pinch them.
