CBEMA and ITI Curves
CBEMA curve  is one of the most frequently employed displays of data to represent the
power quality. A portion of the curve adapted from IEEE Standard 4469 that we typically use in our analysis of power quality monitoring results is shown in below.

In the CBEMA Curve, the axes represent magnitude and duration of the event. Points below the envelope are presumed to cause the load to drop out due to lack of energy. Points above the envelope are presumed to cause other malfunctions such as insulation failure, overvoltage trip, and overexcitation.

The upper curve is actually defined down to 0.001 cycle where it has a value of about 375 percent voltage. We typically employ the curve only from 0.1 cycle and higher due to limitations in power quality monitoring instruments and differences in opinion over defining the magnitude values in the subcycle time frame.

Computer equipment sensitivity to sags and swells can be charted in curves of acceptable sag/swell amplitude versus event duration. This curve was originally developed by CBEMA (Computer Business Equipment Manufacturers Association) to describe the tolerance of mainframe computer equipment to the magnitude and duration of voltage variations on the power system.

While many modern computers have greater tolerance than this, the curve has become a standard design target for sensitive equipment to be applied on the power system and a common format for reporting power quality variation data.

In the 1970s, the (CBEMA) developed the curve [2.5] of Figure 2.5 utilizing historical data from mainframe computer operations, showing the range of acceptable power supply voltages for computer equipment.

The horizontal axis shows the duration of the sag or swell, and the vertical axis shows the percent change in line voltage.

In addition, the IEEE has addressed sag susceptibility and the economics of sag-induced events in IEEE Std. 1346–1998 [2.6]. This document includes measured power quality data taken from numerous sites.

The original CBEMA curve was originally developed by the Computers Business Equipment Manufacturers Association (CBEMA) and adopted by IEEE Standard 446. CBEMA has been renamed as the Information Technology Industry (ITI) Council, and a new curve as shown in Figure 1.1 has been developed to replace the original CBEMA curve.

The modified curve has been developed that specifically applies to common 120-V computer equipment. The concept is similar to the CBEMA curve. Although developed for 120-V computer equipment, the curve has been applied to general power quality evaluation like its predecessor curve.

Outside the bounded tolerance region, in the no-damage region, the applied voltages are very low, and sensitive computer equipment will not function properly; however, no damage occurs to the equipment.

In the prohibited region, sensitive computer equipment will be damaged due to the occurrence of severe voltage swells.

Both the CBEMA and ITI curves were specially developed for use in the 60-Hz 120-V distribution voltage system. The guidelines expect users in 50-Hz 240-V distribution systems to exercise their own judgment when applying the CBEMA and ITI curves.

Although there is no legal requirement to conform to these curves, most original equipment manufacturers build equipment that meet or exceed the limits set forth by these curves, with the occasional exception.

That is the value of CBEMA and ITIC Curves.


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