What should be used to protect NM cables passing through metal edges?

Using listed grommets is essential for protecting NM cables when they pass through metal edges. Listed grommets are specifically designed to provide a secure and safe transition for cables, preventing any potential abrasion or damage from sharp metal edges. They create a barrier that mitigates the risk of insulation wear and ensures that the integrity of the electrical installation is maintained.

The term "listed" indicates that these grommets meet certain safety and quality standards established by testing agencies, ensuring reliability in a variety of installations. Their use is recommended in the National Electrical Code (NEC) to promote safety and compliance in electrical wiring methods.

The other options, while they may provide some level of protection, do not meet the specific criteria set forth by the NEC as effectively as listed grommets do. For instance, plastic covers may not provide adequate sealing around the edges, and steel plates could potentially cause damage over time through wear. Rubber grommets, although they can offer some protection, may not be tested or rated for the specific applications required by electrical installations, which is why the listed variety is the superior choice.