Basically, the CBEMA curve was originally derived to describe the tolerance of mainframe computer business equipment to the magnitude and duration of voltage variations on the power system. Also, the association designed the curve to point out ways in which system reliability could be provided for electronic equipment. Eventually, it became a standard design target for sensitive equipment to be applied on the power system and a common format for reporting power quality variation data. The best scientific interpretation of the curve can be given in terms of a voltage standard applied to the DC bus voltage of a rectifier load.

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This curve provides an AC voltage boundary that most information technology equipment ITE can tolerate or ride through without experiencing unexpected shutdowns or malfunctions. Information technology equipment could include single phase computers, printers, scanners etc.

After some minor modifications to the proposal, the ESC-3 working group approved this initial version of the curve, which remained unchanged until early in Throughout the next 20 years that the original version was published, it grew in stature from a simple curve describing the performance of mainframe computer equipment PCs were not available , to a curve that was used to define everything from specification criteria for electronic equipment to the basis of power quality performance contracts between electric utilities and large industrial customers.

Obviously, this is quite an extension from the initial intent of describing the power quality performance of typical mainframe computers.

Other voltages are not specifically part of this and it is the responsibility of the user to verify that the curve is applied correctly at other voltages. People tend to extrapolate these curves to V or even higher voltages and also as a general metric of incoming power quality. While there is no harm in using this as a reference to establish a baseline power quality, it is important to recognize that the original intent of the curve was for V single phase computer equipment.

The curve describes both steady state and transitory conditions. Most modern ITE equipment are powered by a switched mode power supply. The front end of the switched mode power supply has a bridge rectifier that rectifies the incoming AC to DC. The rectified DC is stored in the bus capacitor.

The DC voltage is further converted to the required voltage and is in turn used to power the various subsystems inside the ITE equipment.

During a power system disturbance, like a voltage sag, swell or a transient voltage for example, the DC bus voltage could go very low or very high and in turn affect the reliable operation of the ITE equipment.

The ITIC curve is essentially an input voltage vs duration performance plot that covers sags, swells, transients, interruptions and steady state voltage variation at the input terminals to the ITE equipment.

It should be noted that the individual manufacturers performance to input voltage fluctuation is difficult to quantify as each many use a different technology inside their switched mode power supply. The basis of this curve is supported by tests that were conducted on a representative sample of eight PC power supplies supplied by eight different manufacturers.

This is shown in the right portion of the curve. Low Frequency Decaying Ringwave: This region describes the decaying ringwave which results from capacitor banks switching. The frequency of transient may range from Hz to 5KHz. The magnitude of transient is expressed as a percentage of the peak of 60Hz nominal voltage not the RMS value. Transient is assumed to occur near the peak of the nominal voltage waveform.

The transient is assumed to be completely decayed by the end of half-cycle in which it occurs. The IEC terminology for this phenomenon is voltage dip. Voltage sags are most often caused by faults on the utility system although they may be caused by faults within the facility or by large motor starts. Dropout: Voltage dropout includes both severe RMS voltage sags and complete interruptions of the applied voltage, followed by immediate re-application of the nominal voltage.

Interruption may last up to 20 milliseconds 1. This transient could occur during a temporary fault in the power system followed by clearing of the fault. No Damage Region: Voltage sags, dropouts, and steady state voltages in this region are not expected to damage the ITE equipment. Normal functioning of ITE equipment is also not expected in region. There are many other variables that could affect the reliable operation of IT equipment.

Some of which are unequal ground potentials, Electromagnetic Noise Interference etc. The standard has been implemented by essentially all semiconductor fabs and has been a huge cost saver for the industry. Semiconductor wafer production is very sensitive to voltage sag voltage dip events and one voltage sag event could cost facility hundreds of thousands of dollars per voltage sag event. SEMI F47 sets out limits of voltage sag that the equipment needs to tolerate without creating any process upsets or shutdowns.

SEMI F47 suggests that semiconductor manufacturers may use this standard in their specification whenever they purchase equipment.

By doing so the manufacturer can make sure all the equipment used for semiconductor production has been tested and certified to SEMI F47 standard. SEMI F47 does not address product quality and the purpose of the standard is to keep the equipment running without any operator intervention when exposed to voltage sags above the tolerance curve.

Steady State Voltage Variation 2 Capacitor Switching Transient: The waveform below shows the capacitor switching transient captured inside a facility when the bank was switched on at the utility substation. Note that the event creates a peak transient followed by low frequency decaying ringwave. These types of transients could cause variable frequency drive over voltage trip, failure of surge protective devices etc. In the worst case if the switching creates a voltage transient that creates another zero crossing of the voltage waveform on the x-axis, then equipment that relay on zero crossing may malfunction.

This could include thyristor controlled heaters, some digital clocks etc. Read voltage notching for additional information on this type of disturbance. Capacitor Switching Transient 3 Voltage Sag: Voltage sags occur when then there is some fault on the system or during starting of large loads motors etc. Graph below shows a voltage sag event followed by recovery followed by another voltage sag.

Voltage Sag.


Voltage Disturbance

The intent was to derive a curve that can better reflect the performance of typical single-phase, V, 60 Hz computers and their peripherals, and other information technology items like fax machines, copiers and point-of-sales terminals. Also, it is used as a reference to define the withstand capability of various loads and devices for protection from power quality problems. This is because the curve is generally applicable to other equipment containing solid-state devices aside from being specifically applicable to computer-type equipment. However, one should be careful and should keep in mind that the ITIC curve is not intended to reflect the performance of ALL electronic-based equipment.


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