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Penetrants, or penetrating items, are the: mechanical, electrical or structural items that pass through an opening in a wall. Or floor, such as pipes, electrical conduits, ducting, electrical cables and cable trays, or structural steel beams and "columns." When these items pierce a wall or floor assembly, they create a space between the——penetrant and the "surrounding structure which can become an avenue for the spread of fire between rooms or floors." Building codes require a firestop to seal the openings around penetrants.

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  Electrical cable through-penetration, firestopped by an intumescent sealant, to restore the two-hour fire-resistance rating of the concrete floor.
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  Steel beam through-penetration.