Last week, the U.S. Court of Appeals for the Federal Circuit issued an opinion in Littelfuse, Inc. v. Mersen USA EP Corp., No. 21-2013 (April 4, 2022), vacating a district court’s judgment of noninfringement because it was based on erroneous claim constructions.
Littelfuse sued Mersen for infringement of U.S. Patent No. 9,564,281 (the “’281 patent”) which, as stated in the Abstract, is directed to a “fuse cap for providing an electrical connection between a fuse and an electrical conductor.” As issued, independent claims 1 and 10 of the ’281 patent each require a mounting cuff, a terminal, and a fastening stem. Littelfuse had added the fastening stem limitation during prosecution to overcome the examiner’s rejection of the claims as anticipated by prior art. After the examiner found the independent claims patentable, Littelfuse rejoined dependent claims it had dropped earlier in response to a restriction requirement.
In the lawsuit, the district court construed “fastening stem” to mean a “stem that attaches or joins other components,” and construed “a fastening stem that extends from the mounting cuff and into the second cavity of the terminal that receives the conductor” to mean “a stem that extends from the mounting cuff and into the second cavity of the terminal that receives the conductor, and attaches the mounting cuff to the terminal.” The district court further clarified that, based on these constructions, independent claims 1 and 10 covered only multi-piece fuse end caps. The parties stipulated to judgment of noninfringement, and Littelfuse appealed.
The Federal Circuit vacated and remanded. First, the Court noted that several dependent claims “required that the end cap be formed ‘from a single, contiguous piece of conductive material.’” Applying the doctrine of claim differentiation, the Court noted that construing independent claims 1 and 10 to only cover multi-piece fuse end caps “would not merely render the dependent claims superfluous, but would mean that [the dependent claims] would have no scope at all.” Acknowledging that the presumption of differentiation in claim scope is not a hard and fast rule, the Court determined that nothing in the written description or prosecution history advised against its application here. The Court concluded that looking at the plain language of the claims a “fastening stem” is simply a stem that “attaches or joins,” and that neither claim 1 nor claim 10 is limited to a multi-piece fuse end cap.