Any affected employer may apply to the Director for a permit for a permanent variance from a rule or regulation promulgated under this article. Affected employees and their bargaining representative, if any, shall be given notice by the employer of each such application and shall be furnished an opportunity to participate in a hearing. The Director shall issue such permit if he determines on the record, after opportunity for an inspection where applicable and a hearing, that the proponent of a variance has demonstrated by a preponderance of the evidence that the conditions, practices, means, methods, operations or processes used or proposed to be used by an employer will provide employment and places of employment to his employees which are as safe and healthful as those which would prevail if he complied with the rule and regulation. The permit so issued shall prescribe the conditions the employer must maintain and the practices, means, methods, operations and processes which he must adopt and utilize to the extent they differ from the rule or regulation in question. Such a permit may be revoked or modified upon application by an employer, employee or by the Director on his own motion, in the manner prescribed for its issuance under this section at any time after six months from its issuance.