Basics of electrical switches

A switch is an electromechanical device that completes or breaks a current path within a circuit, or sends current over a different path.

Despite the many switch types, they have basic components in common. The operator initiates switch operation. The low-resistance metal contacts make or break the electrical circuit. The switch mechanism is linked to the operator and opens and closes the contacts.

All switches, regardless of type, require some type of actuation. Many switches are designed for human actuation. However, machines can actuate switches as well. For example, limit switches typically detect limit of travel for a mechanical device.

Common switch actuation types include:

• Toggle
• Rocker
• Pushbutton
• Dual inline position (DIP)
• Slide
• Rotary
• Keylock
• Knife
• Limit

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:

• Resistive
• Inductive
• Motor
• Lamp

Resistive loads contain little or no inductance. When resistance only is present, ac loads are also called resistive. Electric heating elements are resistive loads. DC resistive loads are the easiest to switch because the steady state, or continuous, current is instantaneous upon switch closure. Also, the current drops almost instantly to zero when the switch is opened. Resistive loads are less severe on switch contacts. There is a greater electrical life expectancy for a given switch in resistive load applications.

Inductance significantly affects current whenever it changes, regardless of whether it is ac or dc. Inductive circuits are more severe on switch contacts than resistive circuits because inductance opposes a change in current. The self-induced voltage that is set up by a rapidly collapsing magnetic field can be much higher than the normal supply voltage. This is because the rate of change of the decreasing current on break is very high. And since the induced voltage is proportional to the rate of current change, this voltage (inductive kick) can be great — accounting for the arcing upon opening of switch contacts.

Motor load is inherently an inductive load. However, an inductive load can apply to any circuit containing coiled conductors, such as electromagnets, and solenoids.

Motor inrush current can be six or more times the steady state or continuous current. Peak inrush is equal to the locked motor current. High inrush can be attributed to typically low armature resistance and the initial absence of counter electromotive force.

Incandescent lamp loads are similar to motor loads because they also have heavy inrush currents — typically 10 times the steady-state current. The reason for the high inrush current for incandescent lighting is the tungsten filament, which has a very low cold resistance. But once the filament becomes hot, the resistance increases significantly.