The “Desk-Drawer Standard” Era

The earliest requirements used to evaluate industrial control panels were surprisingly informal by modern standards. According to historical accounts from individuals familiar with the program’s development, the first requirements consisted largely of handwritten notes prepared by a UL electrical engineer.

Rather than creating an entirely new safety philosophy from scratch, the engineer relied heavily on the existing requirements found in UL 508, the long-established Standard for Industrial Control Equipment. UL 508 already addressed many of the electrical safety principles relevant to industrial control devices, including spacing requirements, conductor sizing, overcurrent protection, component suitability, grounding, and construction practices. The engineer also borrowed requirements from the National Electrical Code (NEC) in existence at the time.

The engineer adapted these requirements to the evaluation of complete industrial control panels.

Because the requirements were informal internal documents rather than published standards, they became known throughout the industry as a “desk-drawer standard.” The name reflected the fact that the evaluation criteria were literally stored in engineers’ desks and were not publicly distributed documents.

Despite its unofficial nature, the desk-drawer standard proved extremely valuable. It gave UL field representatives and engineers a consistent basis for evaluating industrial control panels while allowing manufacturers to obtain approvals for custom-built assemblies.

For many years, this internal evaluation process served as the primary method for assessing industrial control panels in the United States.