A rack mountable shelf supports a plurality of field replaceable units (FRUs) in the form of server cartridges, or blades. The blades are provided with an enclosure that includes a metal portion and a plastics portion. The provision of a blade enclosure for the blade can protect the components of the blade from physical damage and can also isolate them from electrical, static and other effects that could be potentially damaging to the blade components. The use of metal can provide strength and electromagnetic interference shielding with a very thin layer. The use of plastics can provide lightness, flexibilty in design, and the avoidance of metal to metal contact to facilitate insertion and removal of the blades.