Fig. 1. Switch function refers to the number of poles and throws it has. Schematic symbols are shown for SPST, SPDT, DPST, DPDT, and rotary switches.
Three terms — pole, throw, and break — describe a switch’s function (Fig. 1.). “Pole” refers to the number of circuits that can be controlled by a switch. A single-pole switch is capable of interrupting the current in a single circuit; a double-pole switch is capable of simultaneously interrupting the current in two separate circuits.
“Throw” indicates the number of conductors or paths the switch can control. The movable contact member of a single-throw switch completes a circuit to only one conductor. However, a double-throw switch permits its movable contact element to alternately complete two paths. (See the sidebar titled “Poles and throws”)
“Position” refers to the number of stops a switch actuator makes when moved from one extreme position to the opposite position. For example, an “on-none-off” is a two-position switch; an “on-off-on” is a three-position switch. Position is particularly important in rotary switches. A rotary switch can have many positions.
The terms “normally open” (NO) and “normally closed” (NC) refer to the physical position of the contacts in reference to each other. In an NO switch, the contacts are separated. The circuit is open and no current can flow through the switch. A typical example is an NO pushbutton switch. Pressing the pushbutton causes the contact element to move to the other of its extreme positions and close the circuit (Fig. 2). In an NC switch, the contacts are closed, thereby making electrical contact. Operating the switch causes the contact element to move and open the circuit.
Fig. 2. Slow-make/slow-break pushbutton switches can be normally open (a) or normally closed (b).
The two basic switch mechanisms are the slow-make/slow-break and the quick-make/quick-break devices. Other mechanisms are simply variations of these.
Fig. 3. An ac switch features a slow-make/slow-break contact mechanism as depicted by this SPDT toggle switch.
The slow-make/slow-break mechanism is usually associated with ac applications because its slowness of operation provides a slight time delay, permitting the ac wave to go through its zero energy level (Fig. 3). The mechanism can be operated by toggle, slide button, rocker button (Fig. 4), or pushbutton, to name a few.
Fig. 4. This rocker-type ac switch also has a slow-make/slow-break mechanism. It differs from the toggle switch in Figure 3 only by its actuation method.
The quick-make/quick-break mechanism has a snap-acting mechanism that virtually eliminates contact teasing. Contact teasing occurs when a switch “bounces,” or makes rapidly repeating closures when only one was intended. A quick-make/quick-break mechanism also has wiping contacts.
Fig. 5. A switch that employs a quick-make/quick-break mechanism can be used for either dc or ac. The snap action of this switch type provides circuit closure reliability and self-cleaning contacts.
The quick-make/quick-break mechanism uses a compression-type spring, which provides the mechanical force to produce the snap action (Fig. 5). Movement of the switch operator compresses the spring, causing it to move from its end position to the trip position. During this change of position, the movable contact physically wipes across the stationary contact. The resultant abrasive action cleans the contact surface, thereby minimizing contact resistance.
The rating is an indication of the maximum electrical load that a switch is capable of handling. A switch may be rated in either current or in horsepower. Often both ratings are provided, along with operating voltages.
According to Underwriters Laboratories (UL), switches with a current rating only will have an overload test capability of 150% rated current if the switch rating is 10 amp or less, and a capability of 125% rated current if the switch rating is greater than 10 amp.
To be meaningful, the rating must be associated with the type of load. These loads consist of:
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