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 SWITCH
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.
VOLTAGE 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.
Q.15 What is the current rating of a switch?
Q.16 What 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 ohm meter or a voltmeter. Each of these methods will be explained using a single-pole, double-throw, single-break, three-position, 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 3-16(B), 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 reading.
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.
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.
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.
Q.17 What two types of meters can be used to check a switch?
Q.18 If a switch must be checked with power applied, what type of meter is used?
Q.19 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.
Q.20 What should you check when performing preventive maintenance on a switch?