In 1983, the U.S. Department of Commerce published a guideline summarizing the fundamentals of powering, grounding, and protecting sensitive electronic devices . The document, known as Federal Information Processing Standards (FIPS) Publication Number 94, was first reviewed by governmental agencies and then was sent to the Computer Business Equipment Manufacturers Association (CBEMA) for review. When the CBEMA group put its stamp of approval on the document, the data processing industry finally had an overarching guideline for power quality.
FIPS Pub. 94 was written to cover automatic data processing (ADP) equipment, which at that time constituted the principal equipment that was experiencing difficulty running on normal utility-supplied power. Since then, IEEE standard P1100 was issued, which applies to all sensitive electronic equipment. FIPS Pub. 94 is a guideline intended to provide a cost/benefit course of action. As a result, it can be relied upon to give the best solution to typical problems that will be encountered, for the least amount of money.
In addition to approving the FIPS Pub. 94 document, the CBEMA group provided a curve that had been used as a guideline for their members in designing power supplies for modern electronic equip- ment. The CBEMA curve from the FIPS document is shown in Figure 11.4. Note the similarity to the Key tolerance envelope shown in Figure 11.1.
The curve is a susceptibility profile. In order to better explain its meaning, the curve has been simplified and redrawn in Figure 11.5. The vertical axis of the graph is the percentage of voltage that is applied to the power circuit, and the horizontal axis is the time factor involved (in μs to s). In the center of the chart is the acceptable operating area, and on the outside of that area is a danger area on top and bottom. The danger zone at the top is a function of the tolerance of equipment to excessive voltage levels. The danger zone on the bottom sets the tolerance of equipment to a loss or reduction in applied power. The CBEMA guideline states that if the voltage supply stays within the acceptable area given by the curve, the sensitive load equipment will operate as intended.