HOW ELECTRIC CONTROL CIRCUITS ARE CLASSIFIED
There are seven basic electric control circuits, each of which has unique characteristics. These control circuits are identified by Series Numbers 10, 20, 40, 60, 70, 80, and 90 (Table 1). Series 10 and 20 are no longer used. Series 70 is electronic control and is covered in the Electronic Control Fundamentals section.
The construction of individual control devices conforms to the requirements for the basic series for which it is intended. However, there are many applications which incorporate controls of different series in the same control circuit.
Table 1 lists and describes the basic control circuits. These basic circuits are frequently expanded to provide additional features such as:
1. High-limit override or control.
2. Low-limit override or control.
3. Minimum/maximum positioning of dampers and valves.
4. Manual reset.
The following paragraphs illustrate and discuss some of the more common applications. To make it easier to understand the control circuits, the simplest circuits (i.e., Series 40) are presented first.