Electrical Control Circuit

Contact: two types
1. Normally Open (NO)
2. Normally Close (NC)
And
1. Electromechanical
2. Static / solid state

To make or break (Close or open) a contact it needs a force. Depends on force contact symbol will change.

Push / pull- switch
electromechanical attraction- relay / Magnetic contactor.
Pressure switch
Limit switch- axial
Timer

Basic question:
Why relay k powered up / latched / self hold by its contact k?..................................................................
No answer till now. Why?

Answer:
In electromechanical relay it has a mechanical movement and it takes time to change of state of auxiliary contact of relay. Thats why relay remain powered up by its auxiliary contact.

It will not happen if you give pulsating DC of duration greater than the mechanical movement time of auxiliary contact of ralay.