(a) General rule. Except as provided in § 4207.9 (d) and (e) (pertaining to acquisitions, mergers and other combinations), an eligible employer that completely withdraws from a multiemployer plan and subsequently reenters the plan shall have its liability for that withdrawal abated in accordance with § 4207.3(c) if the employer resumes covered operations under the plan, and the number of contribution base units with respect to which the employer has an obligation to contribute under the plan for the measurement period (as defined in paragraph (b) of this section) after it resumes covered operations exceeds 30 percent of the number of contribution base units with respect to which the employer had an obligation to contribute under the plan for the base year (as defined in paragraph (c) of this section).

(b) Measurement period. If the employer resumes covered operations under the plan at least six full months prior to the end of a plan year and would satisfy the test in paragraph (a) based on its contribution base units for that plan year, then the measurement period shall be the period from the date it resumes covered operations until the end of that plan year. If the employer would not satisfy this test, or if the employer resumes covered operations under the plan less than six full months prior to the end of the plan year, the measurement period shall be the first twelve months after it resumes covered operations.

(c) Base year. For purposes of paragraph (a) of this section, the employer's number of contribution base units for the base year is the average number of contribution base units for the two plan years in which its contribution base units were the highest, within the five plan years immediately preceding the year of its complete withdrawal.