Module 3 −− Introduction to Circuit Protection, Control, and Measurement
Figure 3-10. - Switch schematic symbols.
OTHER TYPES of Switches
You have learned that switches are classified by the number
of poles, throws, and breaks. There are other factors used to describe a switch such as the type of actuator and
the number of positions. In addition, switches are classified by whether the switch has momentary contacts or is
locked into or out of position and whether or not the switch is snap-acting.
Type of Actuator
In addition to the pushbutton, toggle, and knife actuated switches already described, switches can have
other actuators. There are rocker switches, paddle switches, keyboard switches and mercury switches (in which a
small amount of mercury makes the electrical contact between two conductors).
Switches are also classified by the number of positions of the actuating device. Figure 3-11 shows three
toggle switches, the toggle positions, and schematic diagrams of the switch. Figure 3-11(A) is a single-pole,
single-throw, two-position switch. The switch is marked to indicate the ON position (when the switch is closed)
and the ofF position (when the switch is open). Figure 3-11(B) is a single-pole, double-throw, three-position
switch. The switch markings show two ON positions and an ofF position. When this switch is ofF, no connection is
made between any of the terminals. In either of the ON positions, the center terminal is connected to one of the
outside terminals. (The outside terminals are not connected together in any position of the switch.) Figure
3-11(C) is a single-pole, double-throw,
two-position switch. There is no ofF position. In either position of
this switch, the center terminal is connected to one of the outside terminals.
Figure 3-11. - Two- and three-position switches.
Momentary and Locked Position Switches
In some switches, one or more of the
switch positions are MOMENTARY. This means that the switch will only remain in the momentary position as long as
the actuator is held in that position. As soon as you let go of the actuator, the switch will return to a
non-momentary position. The starter switch on an automobile is an example of a momentary switch. As soon as you
release the switch, it no longer applies power to the starter.
Another type of switch can be LOCKED IN or OUT
of some of the switch positions. This locking prevents the accidental movement of the switch. If a switch has
locked-in positions, the switch cannot be moved from those positions accidentally (by the switch being bumped or
mistaken for an unlocked switch). 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.
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
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.
Q10. What classification of a switch is used when you describe it as a rocker switch?
Q11. In describing a switch by the number of positions of the actuator, what are the two possible
configurations for a single-pole, double-throw switch?
Q12. What type of switch should be used to
control a circuit that requires a temporary actuation signal?
Q13. What type of switch is used if it is
necessary to guard against a circuit being accidentally turned on or off?
Q14. What is the common name used for an accurate snap-acting switch?
Switches are rated according to their electrical
characteristics. The rating of a switch is determined
by such factors as contact size, contact material, and
contact spacing. There are two basic parts to a switch rating-the current and voltage rating. For example, a
switch may be rated at 250 volts dc, 10 amperes. Some switches have more than one rating. For example, a single
switch may be rated at 250 volts dc, 10 amperes; 500 volts ac, 10 amperes; and 28 volts dc, 20 amperes. This
rating indicates a current rating that depends upon the voltage applied.
CURRENT RATING of a
The current rating of a switch refers to the maximum current the switch is designed to
carry. This rating is dependent on the voltage of the circuit in which the switch is used. This is shown in the
example given above. The current rating of a switch should never be exceeded. If the current rating of a switch is
exceeded, the contacts may "weld" together making it impossible to open the circuit.
RATING of a Switch
The voltage rating of a switch refers to the maximum voltage allowable in the circuit in which the switch is used. The voltage rating may be given as an ac voltage, a dc voltage, or both. The voltage rating of a switch
should never be exceeded. If a voltage higher than the voltage rating of the switch is applied
to the switch,
the voltage may be able to "jump" the open contacts of the switch. This would make it impossible to control the
circuit in which the switch was used.
Q15. What is the current rating of a switch?
is the voltage rating of a switch?
Maintenance and REPLACEMENT of Switches
Switches are usually a very reliable electrical component. This means, they don't fail very often. Most
switches are designed to operate 100,000 times or more without failure if the voltage and current ratings are not
exceeded. Even so, switches do fail. The following information will help you in maintaining and changing switches.
There are two basic methods used to check a switch. You can use
an ohmmeter or a voltmeter. Each of these methods will be explained using a single-pole, double-throw,
snap-acting, toggle switch.
Figure 3-15 is used to explain the method of
using an ohmmeter to check a switch. Figure 3-15(A) shows the toggle positions and schematic diagrams for the
three switch positions. Figure 3-15(B) shows the ohmmeter connections used to check the switch while the toggle is
in position 1. Figure 3-15(C) is a table showing the switch position, ohmmeter connection, and correct ohmmeter
reading for those conditions.
Figure 3-15. - Table of correct readings.
With the switch in position 1 and the ohmmeter connected to terminals 1 and 2 of the switch, the
ohmmeter should indicate (∞). When the ohmmeter is moved to terminals 2 and 3, the ohmmeter should indicate zero
ohms. With the ohmmeter connected to terminals 1 and 3, the indication should be (∞).
As you remember from
chapter 1, before the ohmmeter is used, power must be removed from the circuit and the component being checked
should be isolated from the circuit. The best way to isolate the switch is to remove it from the circuit
completely. This is not always practical, and it is sometimes necessary to check a switch while there is power
applied to it. In these cases, you would not be able to use an ohmmeter to check the switch, but you can check the
switch by the use of a voltmeter.
Figure 3-16(A) shows a switch connected between a power source (battery) and two loads. In figure
a voltmeter is shown connected between ground and each of the three switch terminals while the switch is in
position 1. Figure 3-16(C) is a table showing the switch position, voltmeter connection, and the correct voltmeter
Figure 3-16. - Table of correct readings.
With the switch in position 1 and the voltmeter connected between ground and terminal 1, the voltmeter
should indicate no voltage (OV). When the voltmeter is connected to terminal 2, the voltmeter should indicate the
source voltage. With the voltmeter connected to terminal 3, the source voltage should also be indicated. The table
in figure 3-16(C) will show you the correct readings with the switch in position 2 or 3.
REPLACEMENT of Switches
When a switch is faulty, it must be replaced. The technical manual for
the equipment will specify the exact replacement switch. If it is necessary to use a substitute switch, the
following guidelines should be used. The substitute switch must have all of the following characteristics.
• At least the same number of poles.
• At least the same number of throws.
• The same
number of breaks.
• At least the same number of positions.
• The same configuration in regard to momentary or locked positions.
• a voltage
rating equal to or higher than the original switch.
• a current rating equal to or higher than the
• a physical size compatible with the mounting.
In addition, the type of actuator (toggle, pushbutton, rocker, etc.) should be the same as the original
switch. (This is desirable but not necessary. For example, a toggle switch could be used to replace a rocker
switch if it were acceptable in all other ways.)
The number of poles and throws of a switch can be determined
from markings on the switch itself. The switch case will be marked with a schematic diagram of the switch or
letters such as SPST for
single-pole, single-throw. The voltage and current ratings will also be marked on the
switch. The number of breaks can be determined from the schematic marked on the switch or by counting the
terminals after you have determined the number of poles and throws. The type of actuator, number of positions, the
momentary and locked positions of the switch can all be determined by looking at the switch and switching it to
all the positions.
PREVENTIVE Maintenance of Switches
As already mentioned, switches do not fail very
often. However, there is a need for preventive maintenance of switches. Periodically switches should be checked
for corrosion at the terminals, smooth and correct operation, and physical damage. Any problems found should be
corrected immediately. Most switches can be inspected visually for corrosion or damage. The operation of the
switch may be checked by moving the actuator. When the actuator is moved, you can feel whether the switch
operation is smooth or seems to have a great deal of friction. To check the actual switching, you can observe the
operation of the equipment or check the switch with a meter.
Q17. What two types of meters can be used
to check a switch?
Q18. If a switch must be checked with power applied, what type of meter is used?
Q19. a double-pole,
double-throw, single-break, three-position, toggle switch is faulty. This switch has a momentary position 1 and is
locked out opposition 3. The voltage and current ratings for the switch are 115 volt dc, 5 amperes. No direct
replacement is available. From switches a through I, in table 3-1, indicate if the switch is acceptable or not
acceptable as a substitute. Of the acceptable switches, rank them in order of choice. If the switch is
unacceptable, give the reason.
Q20. What should you check when performing preventive maintenance on a switch?
Table 3-1. - Replacement Switches and Their Characteristics
A SOLENOID is a control device that uses electromagnetism to convert
electrical energy into mechanical motion. The movement of the solenoid may be used to close a set of electrical
contacts, cause the movement of a mechanical device, or both at the same time.
Figure 3-17 is a cutaway view
of a solenoid showing the solenoid action. a solenoid is an electromagnet formed by a conductor wound in a series
of loops in the shape of a spiral. Inserted within this coil is a soft-iron core and a movable plunger. The
soft-iron core is pinned or held in an immovable position. The movable plunger (also soft iron) is held away from
the core by a spring when the solenoid is deenergized.
When current flows through the conductor, it produces a
magnetic field. The magnetic flux produced by the coil results in establishing north and south poles in both the
core and the plunger. The plunger is attracted along the lines of force to a position at the center of the coil.
As shown in figure 3-17, the deenergized position of the plunger is partially out of the coil due to the action of
the spring. When
voltage is applied, the current through the coil produces a magnetic field. This magnetic
field draws the plunger within the coil, resulting in mechanical motion. When the coil is deenergized, the plunger
to its normal position because of spring action. The effective strength of the magnetic field on the
varies according to the distance between the plunger and the core. For short distances, the strength of
the field is strong; and as distances increase, the strength of the field drops off quite rapidly.
Figure 3-17. - Solenoid action.
While a solenoid is a control device, the solenoid itself is energized by some other control device such
as a switch or a relay. One of the distinct advantages in the use of solenoids is that a mechanical movement can
be accomplished at a considerable distance from the control device. The only link necessary between the control
device and the solenoid is the electrical wiring for the coil current. The solenoid can have large contacts for
the control of high current. Therefore, the solenoid also provides a means of controlling high current with a low
current switch. For example, the ignition switch on an automobile controls the large current of a starter motor by
the use of a solenoid. Figure 3-18 shows a cutaway view of a starter motor-solenoid combination and a section of
the wiring for the solenoid. Notice that the solenoid provides all electrical contact for current to the starter
motor as well as a mechanical movement of the shift lever.
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