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:
Dual inline position (DIP)
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.
Contact configurations and switch mechanisms
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 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.
Acknowledgements PLANT ENGINEERING magazine extends its appreciation to Eaton Corp., Eaton | Cutler-Hammer, and Omron Electronics for the use of their materials in the preparation of this article.
Poles and throws
Switch poles and throws are typically abbreviated. The following list is a sample of these acronyms:
SPST — Single pole, single throw
SPDT — Single pole, double throw
DPST — Double pole, single throw
DPDT — Double pole, double throw
Actuator — A movable part of a switch which causes a change in the electrical configuration of the switch. Examples of an actuator include toggle, rocker, slider, trigger, plunger, paddle, shaft, and button.
Arcing — A visible electrical discharge between separated contacts.
Bounce — The repeated rebounding of the movable contact during the transfer from one throw to the next — measured in msec.
Break before make — Interrupting one circuit of a pole before completing another of the same pole (nonshorting contact).
Capacitive load — A load in which the initial current on make is higher than steady state.
Contact resistance — The resistance across a pair of closed contacts in series with the load. Increases with the age of the switch at a rate varied by environment, frequency of use, voltage, and load conditions. Measured in milliohms.
Current rating — The maximum electrical load the switch is designed to handle at a given voltage.
Double-break — Having two pairs of contacts that open the circuit at two places. Having this added contact material improves heat dissipation and increases life. Desirable in DC circuit applications.
Dry circuit — A low-energy circuit condition where no arcing, melting or softening of the contacts occurs during contact switching. Typically requires gold contacts for reliable operation. For example, 0.4 VA maximum @ 28V ac/dc maximum.
Inductive load — A load in which the initial current on make is lower than steady state and upon break is greater than steady state. The long arcing time, due to stored energy in the inductor at the time of breaking, is severe on the switch contacts. Motors are the most common inductive loads.
Inrush — The initial, transitory high-level of current through contacts upon making (closing). Can cause severe degradation of contacts. Applicable to resistive and capacitive loads.
Insulation resistance — The electrical resistance between two normally insulated parts; measured at a specific high potential; usually greater than 1 Mohm.
Lamp load — Most notably characterized by the high inrush current at make (approximately 10 to 16 times the steady state).
Load — The amount of current being carried in a given circuit.
Maintained action — Remaining in a given circuit condition until actuated to the opposite circuit condition where it is again maintained. Maintained action is the opposite of momentary action.
Make before break — Completing one circuit of a pole before interrupting another of the same pole (shorting contact).
Momentary action — Mechanically returning from a temporary circuit condition to the normal circuit condition as soon as the actuating force is removed.
NC — Normally Closed contacts. Circuit is closed when actuator is in relaxed or normal position.
NO — Normally Open contacts. Circuit is open when actuator is in relaxed or normal position. (NC and NO apply to momentary or alternate action switches.
Pole — A completely independent circuit within a switch. In other words, a single pole controls one circuit, a double pole controls two circuits, etc.
Position — The mechanical detents or stops of a switch actuator.
Power factor — A measure of the inductive or capacitive character of an electrical load.
Resistive load — The easiest load to switch because current and voltage are in a steady state on make, and drop instantly to zero on break. Produces minimal arcing which maximizes contact life.
Single-break contacts — A contact mechanism using one set of contacts to make or break a given circuit. Typical of electronic or low power switches.
Snap-action — The abrupt transfer of contacts from one position to another; this action is relatively independent of the speed of actuator travel.
Terminal — The metal portion of the switch, exterior to the body, that is used to connect the switch to an electrical circuit. Examples include printed circuit (PC), wire lug, turret, quick-connect, screw, and wire-wrap.
Throw — The number of electrical circuits (outputs) within a pole.
Travel — The total distance the actuator moves to change electrical position.
Voltage breakdown — A buildup of electrical potential across the movable and stationary contacts causing an arc at the air gap that shorts the circuit.