INSTALLING CONTROL SYSTEMS
Use the number system on a schematic diagram to troubleshoot a circuit.
Wiring diagrams can be misleading in that they show all components grouped together. In actual practice, the control relays and motor starters may be located in one cabinet, the push buttons and pilot lights in a control panel, and pilot devices such as limit switches and pressure switches may be located on the machine itself (Figure 26 – 1). Most control systems use a relay cabinet to house the control relays and motor starters. Wiring is brought to the relay cabinet from the push buttons and pilot lights located in the control panel, and from the pilot devices located on the machine. All of the connections are made inside the relay cabinet.
Relay cabinets generally contain rows of terminal strips. These terminal strips are used as connection points between the control wiring inside the cabinet and inputs or outputs to the machine or control panel. Most terminal strips are designed so that connection points can be numbered. This type of system can be more costly to install, but it will more than pay for the extra cost in time saved when troubleshooting.
For example, assume that an electrician desires to check limit switch LS15 to see if its contacts are open or closed. Limit switch LS15 is located on the ma- chine, but assume that in the schematic one side of the limit switch is number 25 and the other side of the limit switch is number 26. If numbers 25 and 26 can be located on the terminal strip, the electrician can check the limit switch from the relay cabinet without having to go to the machine and remove the cover from the limit switch. The electrician will only have to connect the probes of a voltmeter across terminals 25 and 26. If the voltmeter indicates the control voltage of the circuit, the limit switch contacts are open. If the voltmeter indicates 0 volts, the limit switch contacts are closed.
Part of a typical ladder diagram is shown in Figure 26 – 2. Wire numbers have been placed on the schematic. Connections to the components using a terminal strip are shown in Figure 26 – 3. An indus- trial diagram showing terminal block connections is shown in Figure 26 – 4.
Review Questions
1. Define a schematic diagram.
2. Define a wiring diagram.
3. Are symbols in a schematic shown in their energized or de-energized condition?
4. Draw the standard NEMA symbol for the following components.
a. Float switch (NO)
b. Limit switch (NC)
c. Normally open push button
d. Relay coil
e. Overload contact
5. You are an electrician working in an industrial plant. Your job is to connect the components shown in Figure 26 –3 to the terminal strip. Should switch FS3 be connected normally open or normally closed? Explain your answer.
6. Should switch PS12 be connected normally open or normally closed? Explain your answer.