Circuit control, in its simplest form, is the application and removal of power. It can also be expressed as turning a circuit on and off or closing and opening a circuit.
If a circuit develops problems that could damage equipment or endanger personnel, it must be possible to remove the power from the circuit. The circuit protection devices discussed in Chapter 10 will remove power automatically if current or temperature increases sufficiently. Even with this protection, a manual means of control is needed so the operator can start and stop electrical equipment as he chooses.
When working on a circuit, it is often necessary to de-energize the circuit to install test equipment or replace components. When power is removed from a circuit for servicing, be sure to tag out that circuit breaker that supplies power to those components. When work has been completed, restore power to the circuit. Check the circuit for proper operation before placing it back in service. After the circuit has been checked for proper operation, remove the tag and log the work.
Many electrical devices are used only part of the time. These controlling devices can allow a programed sequence of events to take place or to repeat cycles of specific operations. The air conditioner is a good example. The compressor motor cycles on and off automatically, controlled by the thermostat switch. As the temperature increases, the thermostat switch closes the circuit, and the air conditioner starts. When the temperature drops to the predetermined level, the thermostat opens the circuit and shuts the compressor off.
Multimeters and televisions use circuit control devices to select a specific function or circuit. The separate control of a specific part of a circuit can be made either automatically or manually by the operator.
Circuit control devices have many different shapes and sizes (Figures 11-1 and 11-2). There are three basic groups of circuit control devices: manual, magnetic, and electronic.
There are many ways of physically positioning electrical control devices. The toggle switch and push button comprise the largest concentration of manual controls. Other manually activated controls are those operated by an outside physical force, such as pressure operating a pressure switch or a water level operating a float switch. Even with the varied number of switching devices, all have one thing in common. They all have contacts.
Copper and silver alloy are the two most common types of contact materials. A contact is usually a circular or rectangular surface designed to carry and interrupt the flow of current. Figure 11-3 shows contacts both normally closed (view A) and normally open (view B). Contacts are found in pairs. One contact is permanently fixed in position. The other contact is affixed to a movable arm or plunger. When the switch is closed, both contacts come together and complete the circuit. When the switch is opened, the contacts are separated, and the circuit is broken. The contacts and their terminal connections are insulated from the switch housing and actuator handle. The contacts are always in series with the components they control.
When current flows in only one direction through a set of contacts, a problem known as cone and crater may develop. The crater is formed by the transfer of metal from one contact to the other contact. Figure 11-4 view A shows this condition. If this condition is present, replace the contacts.
Some contacts are formed in a ball shape. In many applications, this type of contact is superior to a flat surface. View B shows a set of ball-shaped contacts. Dust or other substances are not easily deposited on a ball-shaped surface. Also, a ball-shaped contact penetrates film more easily than a flat contact. When cleaning or servicing ball-shaped contacts, be careful to avoid flattening or otherwise altering the rounded surfaces. The contacts can be damaged by using sandpaper or emery cloth. Only a burnishing tool should be used for this purpose (Figure 11-5). Do not touch the surfaces of the burnishing tool. After the burnishing tool is used, it should be cleaned with alcohol.
Maintain contact clearances or gap settings according to the operational specifications of the component. If the contact gap needs to be adjusted, bend the contact arm with a point bender (Figure 11-6). Any other tool can cause the relationship between the two mating contact surfaces to distort. This would necessitate the replacement of the entire relay assembly.
After servicing the contacts, verify their operation with an ohmmeter. Ensure the circuit is de-energized. Disconnect at least one of the leads to the contact surfaces. This is necessary to ensure that other parallel circuits are not read by the ohmmeter. Connect one lead of the ohmmeter to one side of the contacts. Connect the other ohmmeter lead to the other contact (Figure 11-7). Physically open and close the contacts and observe the ohmmeter readings. The ohmmeter should read zero resistance when the contacts are closed and an infinite resistance when the contacts are open.
Chapter 10 discussed how the contacts open the electrical circuit during overcurrent conditions. Circuit breakers, circuit control devices, and switches have contacts of either copper or silver alloy materials. The National Electrical Manufacturers Association (NEMA) rates contractors according to the size and type of load. How contacts are rated and what they are made of depend on their physical size, current, voltage capacities, and particular application.
Table 11-1 lists some common ratings for AC contractors.
The overall size of a single-break contact device can be reduced by making it a double-break contact. Figure 11-8 shows a single- and double-break switch. A double-break contact can carry a much higher current in a smaller space because it interrupts the circuit in two places at the same time. It can further be reduced by making it out of silver alloy. Silver alloy is an excellent conductor with better mechanical strength. Contacts made of silver alloy will allow many more operations than a contact of copper construction. Copper is a good choice for large contacts because it is a good conductor and is relatively inexpensive.
The following rules apply to switch and contact symbols:
For example, Figure 11-9 shows some of the symbols for a toggle switch. View B shows a double-pole, single-throw (DPST) switch. This means that when the toggle switch is moved, two paths for current to flow will be completed. The dotted lines connecting the two poles indicates that they are mechanically connected. They are both activated with a single motion. This is known as mechanical interlocking.
Figure 11-10 view A shows a DPST normally open switch. View B shows the same configuration except that the switch is normally closed. View C shows a symbol for the double-pole, double-throw (DPDT) switch. In this situation, there is a possibility of four paths for current to flow. This type of switch has contacts that are normally opened and normally closed. At any given time, at least two circuits will have power available.
A common manual switch is the push button (Figure 11-11). The push button, like all the schematic symbols, has a standard that governs its drawn position on diagrams:
Figure 11-11 view B shows the symbol for the normally open, double-break push button. This switch, normally used for a start push button, can be physically depressed to touch the contacts or circles. When the finger is removed, the push button is spring-loaded and returns to its NO position.
Figure 11-11 view A shows the normally closed push button in contact below the contacts. This normally closed switch is generally used for a stop push button. When pressure is applied to the button, the pole moves away from the contacts. This push button is also spring-loaded and will return to its normally closed position.
If the stop push button's pole was placed above the contacts (circles), it would be impossible to imagine pushing the electrical diagram out of the way to open the circuit. This is a very important concept when dealing with more complex symbols. The way the switch is illustrated represents the manner in which the switch is constructed.
Selector Switches
A selector switch is rotated by the operator to a desired position to energize a specific circuit. Figures 11-12 and 11-13 show a two- and three-position selector switch positioned in a diagram. The target table used to determine the exact switch position and the circuit combination is in Figure 11-12. Each contact position on the line diagram is identified. The contacts are lettered, and the switch positions are numbered. The target table is identically marked.
In Figure 11-12 view A, the selector switch is in position 1. In the target table, position 1 has an X under the contact column a. This indicates that the circuit labeled "a" now can energize coil number 1.
Another way to indicate the position of the selector switch is to use differentiating lines. Figure 11-13 uses solid, circular, and dashed lines to indicate the three positions of the selector switch. In the solid line configuration, the selector points to the 1 position, and the topmost circuit is energized. In the circle configuration, the OFF position selection is made, and the pole is positioned between the top and bottom circuit contacts. This position leaves Ml and M2 de-energized. In the 2 position, M2 is energized, and Ml is de-energized.
Snap-Action Switches
A snap-action switch keeps the movement of the contacts independent from the physical activation of the switch. In a toggle switch, for example, no matter how fast or how slow the toggle is moved, the actual switching of the circuit takes place at a fixed speed. The snap-action switch is constructed by making the switch mechanism a leaf spring so that it snaps between positions. Increasing the contact closing speed decreases the time arcing can take place. A snap-action switch cannot be between positions.
Microswitch
A microswitch is a precision snap-action switch in which the operating point is preset and very accurately determined (Figure 11-14). The operating point is the point at which the plunger causes the switch to switch.
The microswitch in Figure 11-14 is a two-position, single-pole, double-throw, single-break, momentary-contact, precision, snap-action switch. The terminals are marked "C" for common, "NO" for normally open, and "NC" for normally closed.
In Figure 11-15, the common terminal is connected through the NC contact terminal. In this position, with this simple wiring circuit, bulb A lights. When the plunger is depressed, the spring will snap into the momentary position, and the common terminal will be connected to the NO terminal. In this position, the NC contact opens, and bulb A is off. The NO contact closes, and bulb B is lit. As soon as the plunger is released, the spring will snap back to the original NC position.
This switch can be used in four ways: normally open, normally open held closed, normally closed, or normally closed held open. Figure 11-16 shows the four limit switch positions. By arranging them according to the circuit and the response wanted from the circuit, this two-position switch will provide a wide range of safety options.
Pressure Switch. Pressure switches are control devices that react to pressure changes in water, oils, gases, and so forth. Figure 11-17 shows a cutaway portion of the pressure switch. A normally closed pressure switch is used to maintain the correct water pressure in a potable water system (view A). As the pressure from the pump increases, the pressure switch contacts will open and disconnect the pump motor from the circuit. As the water is used and the pressure drops, the switch closes, and the pump starts to replenish the reservoir.
The normally open switch in view B cannot be used in this situation. With water pressure at below acceptable standards, the pump would continue to remain idle because there was no outside force acting on it to close the contacts. If the contacts could be held closed until they stayed closed under pressure, then the pump would not shut off. The NO pressure switch is used to maintain inches of mercury (vacuum) in the sewage systems. When the vacuum is lost, the pressure increases (toward atmospheric pressure of 14.7 psia). This increase in pressure (or loss of vacuum) closes the switch. The vacuum pumps then pull out the air to maintain the correct pressure in inches of mercury.
An NO pressure switch needs an outside force to close its contacts. An NC pressure switch needs an outside force to open the contacts.
Temperature Switch. A bimetallic control device responds to changes in temperature. Two dissimilar metal stripes are attached together. The fusion of two dissimilar metals, one material on top of the other material, is called a bimetallic strip. The bimetallic strip has a contact surface at one end. As long as the bimetallic strip remains cool, the contacts remain together, completing a circuit. As the temperature increases, each of these two metal strips expand at a different rate. The faster expanding metal curves toward the slower expanding material. When the bimetallic strip distorts sufficiently, it curves away from the other contact, opening the circuit (Figure 11-18).
Figure 11-19 illustrates a capillary tube control device. The temperature switch is far removed from the bulb sensor, separated by a long capillary tube. The temperature switch can be placed in a convenient location, while the sensing bulb is positioned for the most effective temperature measurement. A volatile liquid or gas within the bulb and capillary tube reacts proportionally to temperature changes. As the ambient temperature surrounding the bulb rises, the bulb's internal volative gas expands with a resulting increase in pressure. The pressure within the bulb is transmitted through the capillary tube acting on the remotely located switch. As the temperature surrounding the bulb is reduced, so is the internal pressure of the volatile gas.
Switches are usually very reliable electrical devices. 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 troubleshooting switches.
Two meters can be used to check a switch: an ohmmeter or a voltmeter. The method employing these meters is explained below using a single-pole, double-throw, single-break, three-position, snap-acting toggle switch.
Figure 11-20 shows the method of using an ohmmeter to check a switch. View A shows the toggle switch positions and schematic diagrams for the three-switch positions. View B shows the ohmmeter connections used to check the switch while the toggle switch is in position 1. View C is a table showing the switch position, ohmmeter connection, and correct ohmmeter reading for those conditions.
Figure 11-21 shows the method of using a voltmeter to check a switch. View A shows a switch connected between a power source (battery) and two loads. View B shows a voltmeter connected between the battery terminal negative node and each of the three-switch terminals while the switch is in position 1. View C is a table showing the switch position, voltmeter connection, and the correct voltmeter reading.
With the switch in position 1 and the voltmeter connected between the battery terminal negative node and terminal 1, the voltmeter should indicate no voltage (0V). 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 view C shows the correct readings with the switch in position 2 or 3.
Replacing 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, it must have all of the following characteristics:
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 the number of poles and throws have been determined. The type of actuator and the number of positions of the switch can be determined by looking at the switch and switching it between positions.
Whenever component substitutions are made, the correct replacement must be installed as soon as possible. Vessel configuration must be maintained, and unauthorized modifications are prohibited.
Performing Preventive Maintenance of Switches
Switches do not fail very often. However, there is still a need for switch preventive maintenance. Switches should be checked periodically for corrosion at the terminals, smooth and correct operation, and physical damage. Any problems found need to be corrected immediately.
Most switches can be inspected visually for corrosion and 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, observe the operation of the equipment or check the switch with a meter.