9- 3.1 Instrument-Rack Structure

The instrument-rack structure is defined, for the pur poses of this chapter, as a structure in which the compo­nents of an instrument system are mounted This structure mav be a completely enclosed cabinet with the components mounted inside the enclosure or a panel with the compo­nents mounted in a cutout in the face of the panel It may be a rack used to mount and support the equipment but which does not function as an enclosure for the compo­nents

(a) Structural Materials 1 he structural materials used in an instrument-rack structure must have sufficient strength to support all equipment mounted in the struc ture Steel and aluminum, the materials most used for panel structures, are easily worked and readily available The thickness of the structural material used for the front panel depends on the type and weight of the instruments being installed, the structural material used, and the panel design Stiffening members may be required, however, they must not limit access to instruments and terminal points at the rear of the panels

Since instrument systems are generally assembled at a vendor’s plant rather than at the location where the system is to be used, the instrument rack structure must have sufficient strength to allow the handling of the structure with all the equipment mounted Lifting eyes should be provided to permit moving completed rack assemblies

(b) Standard Modular Enclosures Modular enclosures are made by several manufacturers to meet the require­ments of both stationary and mobile instrumentation systems They are designed to conform to an industry

standard, such as the Electronic Industries Association[20] (EIA) Mounting Standards, thus eliminating by the stan­dardization of parts the need for many special mounting devices. Because of the modular design, a series arrange­ment of almost any configuration can be developed. Manufacturers of modular enclosures also make many special features and accessories for the enclosures, such as radio-frequency interference (RFI) shielding, equipment­cooling blowers, and special enclosure trims.

Modular enclosures allow flexibility in panel configura­tion and equipment layout (see Fig. 10.3).

(c) Mounting Practices. Instrument cabinets are gener­ally designed to be free-standing structures which may or may not be fully enclosed. Instrument cabinets that are stationary are mounted on bases or curbs. The bases or curbs are either steel or concrete and are designed so that the instrument structure with all the equipment installed
can be placed on them and attached and held in place by bolts or clamps.

A general practice is to provide each rack group with a fabricated, 3- to 5-in. channel iron base. This allows for easy mounting to curbs with bolts and clamps sufficiently strong to withstand nominal seismic forces.