The PLC replaces electromechanical relays due to their following advantages
a. PLCs are more reliable and faster in operation
b. They are compact and can be expanded easily
c. They require less electrical power
d. They are less expensive when compared to Hardwired systems of same number of control functions
e. Hard-wired electromechanical relays lack flexibility.
For example, when system operation requirement change, then the relays have to be rewired.
f. PLCs have very few hardware failures when compared to electro-mechanical relays
g. Special functions such as time-delay actions and counters , can be easily performed using PLCs