Introduction to Industrial Automation

2.3.3   Three-Phase Monitoring Relays

Three-phase monitoring relays are electronic devices that monitor various basic electrical quanti-

ties of the three-phase power network. A typical utilization is for the measurement of the current 

values of the electrical quantities and their deviations from the nominal values, where the monitor-

ing relay is able to activate their output(s) when these deviations exceed a predefined limit. Their 

output is usually an SPDT contact, suitably inserted in the automation circuit in order to cause 

the interruption of the load operation, as shown in Figure 2.61a. The interruption of the operation 

0°

90°

180°

270°

360°

Cam 1 

Cam 2 

Cam 3 

Cam 4 

Cam 5 

135°

135°

135°

90°

90°

90°

90°

C

ont

act

 pos

itio

n

Time (depending ton timing cycle)

Time bar chart

=The NO contact of the i

th

 cam-operated switch is closed