(a) On a date to be determined by the department following the effective date of this article, employers who are self-insurers as of that effective date shall pay an initial assessment of one-half per centum of the compensation paid by each self-insurer in the year preceding the assessment. Self-insurers who, prior to such effective date, were not self-insurers shall pay an assessment based on one-half per centum of their modified manual¹ premium for the twelve (12) months immediately prior to becoming self-insurers.

(c) A self-insurer which ceases to be a self-insurer shall be liable for any and all assessments made pursuant to this section during the period following the date its authority to self-insure is withdrawn, revoked or surrendered until such time as it has discharged all obligations to pay compensation which arose during the period of time said former self-insurer was self-insured. Assessments of such a former self-insurer shall be based on the compensation paid by the former self-insurer during the preceding calendar year on claims that arose during the period of time said former self-insurer was self-insured.