What is the standard of correctness of a tribunal arbitrator's interpretation of common law?

The expert skill and knowledge exercised by an arbitrator in applying and interpreting a collective agreement does not usually extend to the interpretation of outside legislation or the common law. Accordingly, the arbitrator's decision on these matters is usually subject to the standard of correctness. An exception exists, however, where the legislation or principle of common law is . . . intimately connected with the mandate of the tribunal and is encountered frequently as a result. See: Toronto (City) Board of Education v. Ontario Secondary School Teachers Federation, District 15, supra, per Cory J. at para. 39.