If your first impression of a typical control system schematic drawing is complete confusion, don't worry: you're not alone. All of those strange symbols really stand for something you should know, right? Throw in a PLC, and the confusion grows. However, if you approach these drawings with a solid understanding of the basics, the mystery fades away.
Do you approach a set of control system drawings as an archeologist trying to decipher hieroglyphics? Sure, some systems can be quite complex. But you can transform this tedious task from decryption to analyzation. All you need is practical knowledge of control symbols, what they denote, and how various control components work. It's not that difficult.
Decrypting the symbols. As this author discovered early in his career, drawings for industrial control systems use a unique set of symbols. Although some are similar to those used in other electrical drawings, you'll find many only in the industrial control area.
Control systems use many different types of switches and contacts used as input devices; Fig. 1 (original article) shows some of the more common ones. A human operator actuates these switches.
Notice how you can draw each of these devices in either the normally open or normally closed configuration. This is true of many control symbols. Normally open devices are open (not conducting) when you don't actuate them and closed (conducting) when you do actuate them. On the other hand, normally closed devices remain closed when you don't actuate them and open when you do actuate them.
The toggle switches shown in Fig. 1 (original article) may or may not return to the "normal" position after you actuate them. Therefore, all control system drawings should specify if a toggle switch maintains its last position, or if it has a spring to return it to the normal position. The other devices shown in Fig. 1 (original article) typically return to the normal position when not actuated.
Fig. 2 (original article ) shows switches actuated by a process, product, or machine (rather than a human operator). Note: We show limit switches in both their normal (not actuated) positions and "held" positions. This is because some designers chose to show limit switches in their true normal state, which can be either normally open-held closed or normally closed-held open. Suppose you use a limit switch as a safety interlock on a cabinet door. Here, it might be a normally open switch held closed by the door when the machine is in normal operation. The other switches shown in Fig. 2 (original article) detect a change in some value of a process. With these types of devices, the normally open/normally closed distinction can sometimes be confusing. No matter what's normal for the process, the measured value actuates these switches when it rises. For example, a normally open liquid level switch closes on rising level and opens on falling level. A normally closed pressure switch, on the other hand, opens on rising pressure and closes on falling pressure. If it better reflects normal operating conditions, you can draw these devices in their "held" position.
One more thing to watch out for: Many often draw switches that measure vacuum as pressure switches, but vacuum is actually negative pressure. Here, you must be careful in determining the proper behavior of the switch. Does a normally open device close on rising vacuum or on rising pressure, which is actually falling vacuum? See what we mean? You're going to have to test these devices if you're not sure of their function (as noted on the drawings). You may run into drawings that include notes such as "switch closes at 15 in. Hg vacuum" to eliminate confusion.
Fig. 3 (original article) shows devices actuated by other devices in the circuit. For example, a relay coil, when energized, will actuate its contacts. Normally open contacts will close and normally closed contacts will open. When not energized, the contacts are in their normal positions.
Take timer contacts as another example. Special relays that perform a timing function actuate them. When you energize a timer contact, an on-delay timer begins timing immediately and actuates its associated contacts after the preset time expires. When you de-energize it, the timer's contacts return to their normal positions immediately with no delay. Most drawings designate on-delay timer contacts as normally open-timed close (NOTC) or normally closed-timed open (NCTO).
An off-delay timer has the opposite behavior: Its contacts change state immediately when you energize the timer but remain in the actuated state when you de-energize the timer until the preset time expires. Most drawings designate off-delay timer contacts as normally open-timed open (NOTO) or normally closed-timed closed (NCTC).
Fig. 4 (original article) shows many other symbols commonly found in control diagrams. The motor shown is the common 3-phase squirrel cage type, but you'll see different symbols for other motor varieties like wound rotor and synchronous motors.
Almost all control drawings use the coil symbol, and you may see it labeled to further identify its purpose. Designers use this symbol for control relays (CR), motor contactors (M or MC), and timers (TR), among other devices.
The lamp symbol often contains a letter to designate color, such as the letter "R" for red.
Designers use the magnetic coil or solenoid symbol to denote devices like solenoid-operated valves, electric clutch or brake coils, and magnetically operated circuit breaker coils. (Some additional notation usually clarifies this.) Shown are values or ratings for resistors, capacitors, inductors, fuses, and circuit breakers.
Of course, we all associate the thermal element symbol with thermal overload trip devices, which are used for motor protection.
The transformer shown is a single-phase control transformer; you'll usually find the size and input/output voltage ratio noted on the drawing.
We use the very familiar earth ground symbol to represent grounds actually referenced to an earth-contact ground rod or grid in a power distribution system.
The chassis ground symbol basically represents a connection to a common chassis that's not earth grounded, such as the electrical system in a vehicle or airplane. The connector symbol can represent any type of plug-in connection.
Decrypting PLC inputs. So now you have an understanding of symbols and their meaning. But what about that intimidating looking black box called a PLC? Interpreting the state of inputs to PLCs can be particularly confusing. Why? Because you can program a normally open or normally closed input device as either an examine-on (normally open contact) or examine-off (normally closed contact) instruction in the PLC. So what's "examining" what?
A PLC interprets an examine-on instruction as conducting if its associated input circuit is energized. It interprets an examine-off instruction as conducting if its associated input circuit is de-energized. Whether or not an input is electrically energized depends on the state of the process and type of connected device.
The table (original article) shows all possible combinations of devices (normally open or normally closed), actuation, and program instructions (examine-on or examine-off), and how the PLC program interprets each combination.
Why are there two types of diagrams? You'll run into two types of diagrams that describe electrical control systems: the schematic (sometimes called elementary) diagram and the wiring (sometimes called connection) diagram. Even moderately complex systems use them.
Schematic diagram. This diagram describes the electrical function of a circuit or system. Fig. 5 (original article) shows a schematic diagram for a 3-phase motor starter. Designers use this type of diagram to make following current or logic flow easy. As you can see, they lay out these various devices for convenience and clarity. Their locations in the drawing don't represent their actual physical locations.
For example, the coil, motor starter contacts, and auxiliary contact (all labeled "M") are physically parts of the same device, but we show them in three different locations to simplify the drawing.
Wiring diagram. Fig. 6 (original article) shows a wiring diagram for the same motor starter. Here, we see the approximate physical locations of the various components as well as the actual physical wiring connections between them. Note that you now group the motor starter coil and contacts together in the approximate physical relationship you would see in the actual starter. The wiring diagram also clearly shows how the wiring routes between the devices and how you make the interconnections.
For example, it's obvious the start and stop buttons and red light are actually located in a remote pushbutton station. It's also obvious a four-conductor cable containing wire numbers 1A, 2, 3, and 4A connects that station to the motor starter, which is located in a motor control center.
As you can see, it's easier to make an estimate of material and labor from a wiring diagram than from a schematic diagram. In the latter, you can't tell if the pushbutton station and pilot light are together, separate, remote, or part of the starter.
Troubleshooting requires both types of drawings. If you're troubleshooting a control system, make sure you use both the schematic and wiring diagram (if they're available). You can use the schematic diagram to follow the sequence of events and the wiring diagram to see where you make your measurements.
Let's suppose you're troubleshooting the motor circuit shown in Figs. 5 and 6 (original article). The motor won't start when you push the start button. Looking at the schematic drawing (Fig. 5), you decide the start button will not work unless there is power on Wire 2. The wiring diagram (Fig. 6) shows you can find Wire 2 on the top side of the motor auxiliary contact in the MCC or out in the remote pushbutton station. A check indicates there's no power at the auxiliary contact, so you look at the schematic diagram again. It shows power flows through the stop button from Wire 1A, and the wiring diagram shows the stop button in the remote station. You go to the remote station and find Wire 1A is broke at the stop button. You make the repair and return the motor to service.
As you can see, this troubleshooting process is much more straightforward using both drawings than working with either one or the other alone.
It's necessary to have a thorough understanding of the symbols, notations, and conventions used on these drawings to read them properly. A good understanding of such symbols ensures you interpret logic correctly, and the effective use of schematic and wiring diagrams will make troubleshooting control systems easier and faster.