By 20 August 2014

Knife switches are used for low-voltage circuits. These are mounted in front of the board or panel, and are operated by hand. Knife switches should be mounted for a vertical throw, with the blade side of the switch either dead or disconnected from the source of the power when open. This is to minimize the risk of an accidental contact.

Originally, all switchgears consisted of open knife switches. Protective devices such as fuses were mounted close to the switch. The use of high-voltage AC and the great increase of the total power in a system necessitated the use of oil-break, air-break, vacuum, air blast, or SF6 switchgear.

In LV installations knife type switches are of metal enclosed or cubicle mounted, double break type complete with arc chutes. The off-circuit LV isolators have been largely replaced by switches of either load break or load-break fault-make capabilities. In some applications, open-type boards are installed, but generally, most of the switchgear today is enclosed. Knife switches are usually spring controlled, giving a quick make and break with a free handle action. This makes the operation of the switch independent of the speed at which the handle is moved.

In all cases, it is impossible to open the cover with the switch in the on position. The rated current capacities of LV cubicle type switches with independent manual operation are limited to 630 A with some vendors even offering switches of 800 or 1000 A on request.

Copper-brush switches substitute a leaved copper brush with a wiping contact for the knife-blade contact, and make use of an auxiliary break, between the carbon blocks, to prevent burning of the copper leaves due to arcing. This type of a switch has been used as a circuit breaker, particularly in MV range with remote action by the addition of tripping coils, though closing is not done remotely. Switches with integral MV fuses also have the provision to open the switch on fuse blowing.