If the switch has locked-out positions, the switch cannot be moved into those positions accidentally. Figure 3-12 shows a three-position, locking switch.

Figure 3-12. - Three-position locking switch.

Snap-Acting Switches

A SNAP-ACTING switch is a switch in which the movement of the switch mechanism (contacts) is relatively independent of the activating mechanism movement. In other words, in a toggle switch, no matter how fast or slow you move the toggle, the actual switching of the circuit takes place at a fixed speed. The snap-acting switch is constructed by making the switch mechanism a leaf spring so that it "snaps" between positions. A snap-acting switch will always be in one of the positions designed for that switch. The switch cannot be "between" positions. A two-position, single-pole, double-throw, snap-acting switch could not be left in an OFF position.

Accurate Snap-Acting Switches

An ACCURATE SNAP-ACTING SWITCH is a snap-acting switch in which the operating point is pre-set and very accurately known. The operating point is the point at which the plunger causes the switch to "switch." The accurate snap-acting switch is commonly called a MICROSWITCH. A microswitch is shown in figure 3-13.

Figure 3-13. - Accurate snap-acting switch (microswitch).

The full description of the microswitch shown in figure 3-13 is a two-position, single-pole, double-throw, single-break, momentary-contact, accurate, snap-acting switch. Notice the terminals marked C, NO, and NC. These letters stand for common, normally open, and normally closed. The common terminal is connected to the normally closed terminal until the plunger is depressed. When the plunger is depressed, the spring will "snap" into the momentary position and the common terminal will be connected to the normally open terminal. As soon as the plunger is released, the spring will "snap" back to the original condition.

This basic accurate snap-acting switch is used in many applications as an automatic switch. Several different methods are used to actuate this type of switch. Some of the more common actuators and their uses are shown in figure 3-14.

Figure 3-14. - Common actuators and their uses for accurate snap-acting switches.