Introduction to Industrial Automation

2.3   Timers, Drum Switches, and Special Components

This chapter is completed with the description of a series of specific devices used in industrial 

automation and control applications. Timer switches, drum switches, electric-network monitor-

ing switches, reed relays, solenoid actuators, counters, hour meters, and encoders are basic devices, 

usually of small size, performing metering, protection, actuation, and time-based processing in an 

industrial automation system. Although some of these devices (such as timers and drum switches) 

are contained in multiple numbers within a programmable logic controller (PLC), they are still 

available as autonomous devices because they can facilitate the development of either a small or 

economical automation system.