In electrical work, what do the metal frames of units serve to enhance?

The metal frames of units in electrical work primarily serve to enhance employee safety. These frames are designed to provide structural integrity and protect users from potential electrical hazards. By enclosing components, they help prevent accidental contact with live parts, reducing the risk of electric shock. Moreover, metal frames often ground the unit, further enhancing safety by directing any fault current away from individuals and towards the ground.

While the other options may touch on relevant aspects of electrical units, the foremost purpose of metal frames is their contribution to safety. They do not specifically enhance device performance, equipment longevity, or environmental protection to the same extent as they do for safety.