Introduction to Industrial Automation

Figure 2.60  Timing bar chart of a five-cam timer.

R

S

T

N

Electronic

circuit

Three-phase

monitoring relay 

C

Load

(a)

(b)

Figure 2.61  Connection diagram of a three-phase monitoring relay (a), and a single phase 

monitoring relay from ABB Ltd. (b).

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  Introduction to Industrial Automation

is a protective action, since it is known that all electric devices can be damaged when they operate 

continuously under abnormal electrical conditions. As a load, it can be considered either a simple 

machine with one motor (this case corresponds to Figure 2.61a) or an entire department of an 

industrial production factory. In the second case, the relay shown in Figure 2.61a is substituted 

by the electromagnetic three-pole circuit breaker feeding the department, and particularly by 

the trip coil of the circuit breaker. When the three-phase monitoring relay is connected to the 

side of a power network, as shown in Figure 2.61a, then it is referred to as “network monitor-

ing”. Otherwise, it may be connected to the side of the load (three-phase line feeding the load) 

and in this case it is referred to as “load monitoring”. Obviously, an eventual interruption, due to 

the monitoring-relay activation in the case of “network monitoring”, will be global for the entire 

department, while in case of “load monitoring”, it will concern only the monitored load.

Several important parameters of a three-phase power network can be monitored reliably and 

continuously by a three-phase monitoring relay. For example, it can monitor a phase failure, a 

phase sequence, a phase imbalance, over- and under-voltage conditions, over- and under-current 

conditions, a power factor, a frequency deviation, a broken neutral wire, and combinations of 

them. Therefore, there is a wide variety of monitoring-relay types in the market, mainly depend-

ing on the number and kind of monitored electric parameters. In general, the utilization of such a 

multifunctional three-phase monitoring relay ensures a trouble-free production procedure, while a 

single-phase voltage monitoring relay is shown in Figure 2.61b used for the protection of sensitive 

equipment and critical control systems.