A filter assembly is used in combination with a piece of electrical equipment having a vertical side wall formed with vent opening. The assembly has a housing secured to the side wall at the opening thereof and having a closed top side, a closed front wall, and a pair of closed end walls. The end and front walls form below the vent opening a downwardly opening mouth. A panel of air-pervious filter material in the housing has a back face turned toward the side wall at the vent opening and an opposite front face turned away from the side wall. A holder in the housing secures the filter panel therein with lower edge of its back face operatively engaging the equipment side wall below the opening and an upper edge of its front face engaging the housing front wall and with the panel tipped out away from the equipment side wall.