What is required for protecting cables laid in wood notches against penetration?

In the context of protecting cables laid in wood notches, the requirement is to prevent the cable insulation from being penetrated by the surrounding material. This can occur due to the edges of the notches where the cable is placed, leading to potential damage over time due to friction or movement.

Using a steel bushing, steel grommet, or steel plate provides a safeguard against these types of penetrations. Each of these materials is designed to provide a protective barrier between the cable and the wood. Steel bushings protect at the entry points where cables pass through surfaces, steel grommets can help cushion the cable from sharp edges, and steel plates can cover areas where cables might be exposed to rough surfaces or edges.

Since all three options effectively contribute to safeguarding the cables, the correct answer encompasses the idea that any of these protective measures can be utilized depending on the specific installation scenario. The requirement is thus fulfilled by the inclusion of any of these protective components, making the choice of all of them appropriate in this context.