section 154 is applicable to all Companies, public and private. there are two aspects to section 154:
1) the closure of the register of members/debenture holders is not mandatory. the company may or may not choose to close its register.
2) however, if the company chooses to close the register, compliance of the section as regards advertisement and closure period becomes mandatory and cannot be avoided. there are a few case laws supporting this view as well. therefore, an unlisted public company choosing to close its register must comply with sec 154.
to escape this, you may consider fixing a single record date instead of closing the register for a period. this will not require compliance of sec 154.
the penalty for non-compliance of 154(1) is detailed in 154(2).