Introduction to Industrial Automation

Figure 2.9  Typical form of a general purpose relay without (a) and with (b) a mounting base.

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  Introduction to Industrial Automation

closed contact (NC) and a normally open one (NO). This type of electric contact is usually called 

“changeover two-way contact”, and all these types of electric contacts and their nomenclature are 

presented in Section 2.1.10.

The number of two-way contacts varies and depends on the type of the general purpose relay. 

For example, the relay in Figure 2.10 has three sets of changeover two-way contacts. The electric 

contacts of a general purpose relay are passing a current rated up to 15 A approximately, except 

from the very small ones (with dimensions of 1 × 1 × 2 cm for PCB boards) that have a nominal 

current of 1 A. The operation of a relay is normal and stable when the coil voltage is exactly the 

nominal one. Therefore, care is required to apply the correct coil voltage because if it is larger or 

smaller than the nominal one, the temperature rise of the coil or the insufficient holding force 

of the movable contact or other electromechanical malfunctions may be caused. Such situations 

reduce the relay contact life expectancy and tend to cause fusing of the contacts.