A knife switch is a kind of isolation switch, A knife switch consists of an insulating bottom plate, “knife” moving contact, static contact, arc extinguishing device, and operating mechanism, so in layman’s terms, we call him an isolation knife switch disconnect.
The definition of the national professional term electrical knife switch is “a switch with a knife-shaped moving contact, which is wedged with the static contact on the base in the closed position”.
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 switch (HD series), double pole double throw knife switches (HS series) which is also a knife changeover switch,fused knife switch（HR series）, etc., and 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 and burn
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 seat 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 replace the fuse with 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 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.
Problems and solutions that may be encountered by the knife switch:
Problem 1: 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.
solution: Repair or replace the contacts, improve the operation method, and replace the switch with a higher rated current.
Problem 2: 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.
Solution: Eliminate the short circuit, and replace the fuse as required.
Problem 3: One or two phases are disconnected after 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.
Solution: 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.
