The applicability or non-applicability of the ESI Act, 1948, and the rules/regulations thereunder is a very vast subject. Hence, it is difficult to provide complete details in a few lines. If you desire, you can download the necessary literature, including the ESI Act, 1948, and the rules/regulations framed thereunder from the websites of ESIC. I hope you will find the detailed reply from the said literature.
However, if after going through the said Act, rules/regulations, etc., you still have any doubts, you can submit them in this thread by indicating your specific doubt, if any, and the facts of a particular case for comments/views of seniors and experts.