A seal apparatus having a seal member and a plurality of fasteners is mounted on an arc hood of a cassette in a switchgear cabinet and resists leakage of arc gases through a gap between the arc hood and a circuit breaker mounted in the switchgear cabinet. The seal member is flexibly received against the circuit breaker in a racked position and is preferably at least nominally deflected thereby to resist the flow of arc gases in a leakage direction through the gap. The seal member thus resists the arc gases from coming into contact with a plurality of line conductors at the rear of the circuit breaker until the arc gases have at least somewhat cooled, deionized, and/or slowed. The seal apparatus thereby resists arc gases from inducing arcing between adjacent line conductors, whereby the circuit breaker can be operated at substantially higher voltages.