can MCB be used as normal on/off switch?
What's an MCB? What is the context of your question?
Assuming Molded Case (Circuit) Breaker - then yes, but it is clunky, heavy to switch. In a larger application the MCB can be the main disconnect switch.
MCB is common terminology here in England for Miniature Circuit Breaker and modern MCB's that comply to BS60898 can be used as functional switching for say lights but in the capacity of design of the installation this would be regarded as poor design and practice. Older MCB's you will often find are not designed as a functional switching device and the repeated switching will reduce its lifespan dramatically.
As the BS number I have quoted is a British Standard you will need to relate to your own countries coding system for the MCB you are discussing then, I believe America using a UL code then look up whether your particular device is compliant for frequent functional switching.
There's no Yes or No answer here as its down to several factors some of which I've touched on.
This is the best answer I can give with the info supplied.
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