When you think of a mad scientist's laboratory, you think of black and white film, a Jacob's Ladder sparking in the background, and a monster lying on a slab, waiting to be brought to life. And of course, the mad scientist must do this with flourish, so he goes to the wall and throws a huge knife switch to start the process.

A knife switch looks like a dull metal blade — usually made of copper or silver, or silver-plated copper — with an insulated handle on one end and a hinge on the other. It may have a set of contacts on one (single-throw) or both (double-throw) extremes of its travel which perform double-duty as conductors and a means to hold the switch in position. There are single-pole, double-pole, and triple-pole knife switches with one, two, or three blades respectively — in all cases the blades are connected by some insulating material and attached to the same handle so that they all make or break the electrical connections at once.

Knife switches are large, manual operation only, and slow to perform their switching function. For this reason they are mainly used to connect or disconnect equipment from its power supply — they are ill-suited to providing any kind of control switching. However, they are excellent for the task of disconnecting since they are typically very large, have a wide contact surface, and can be physically locked into an open position. These three attributes are desirable for switching large amounts of current even at high voltages, and power companies use them to switch power feeds at substations carrying thousands of volts.

Large blades mean the switches can carry large amounts of current. Thicker conductors have less resistance and can therefore carry more current without heating up. This is why the wires feeding the circuit breaker panel in your house are thicker than the wires distributing the power to your various lights and outlets.

Wide contact surfaces are more resistant to corrosion, abrasion, and damage than thinner contact surfaces. This is not only important for repeated use (every throw wears the contact surfaces a little bit), but also makes the switch more resistant to damage by electric arcing. Electric arcs are often a result of switching high-voltage or high-inductance loads, and the heat generated is enough to vaporize the metal contacts if they are too small.

Physically locking the switch in an open position is a necessary part of the OSHA lock-out procedure. By supplying a means to prevent the switch from being closed, you can ensure that the power is disconnected and will remain disconnected while you are performing maintenance, making modifications, or otherwise working on the equipment.

The kind of knife switches found in science surplus stores, Radio Shack, high voltage power distribution equipment, and old horror movies have their conductors and contacts completely exposed. Only the handle is insulated. This is very dangerous for most applications though. Power companies get away with it by placing them well out of reach. Disconnects found at ground level switching higher voltages (that is, higher than the 12 Volts DC a hobbyist might use) are almost always enclosed in grounded boxes to protect the user from coming in contact with energized parts.

Knife switch. (Elec.)

A switch consisting of one or more knifelike pieces hinged at one end and making contact near the other with flat gripping springs.


© Webster 1913.

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