An air circuit breaker includes two separable electrical contacts connected to electric current input and output terminals; and a chamber for quenching an electric arc, including a stack of splitter plates that are spaced apart from one another, and lateral walls placed on either side of the stack and including a thermosetting-resin impregnated polyamide woven and being devoid of glass fibres. The quenching chamber furthermore includes protective elements made of crosslinked polyamide, the elements being placed in junction zones between the lateral walls and holding plates, the protective elements covering corners of the splitter plates, which corners are adjacent to the lateral walls, so as to separate these corners of the splitter plates from the electrical contacts.