Hole plugs prevent damage to cable systems and circuitry from dust, debris, and water ingress. They work by slotting or 'snapping' into spare or unwanted openings in electrical cabinets and enclosures. In this essential guide, we'll unveil the 5 Must-Knows about knockout plugs, empowering you to tackle this simple yet critical task safely and correctly, transforming a potential hazard into a secure, code-compliant home. Image taken from the YouTube channel ElectricalAnswerMan, from the video. I've got an existing electrical “switch” box in a wall from which I removed one, no longer required, BX wire (there are two remaining required BX coming into the box specific for lighting in the room).