Ask the Experts

Sep 1, 2003 12:00 PM, By John Dedad, Editorial Director

A Curve By Any Other Name Is Still a Curve

You say CBEMA, I say ITIC. What's the difference? And why should you care? Editorial Director John DeDad offers an explanation of these important industry references and explains the differences between these two performance curves.

Q. In discussions and articles on the performance of solid-state electronic equipment in unstable voltage conditions, I've heard and seen references to the CBEMA curve and the ITIC curve. What are these curves and why are they important to end-users? I thought electronic equipment manufacturers designed their equipment per industry standards for proper operation during unstable voltage conditions.

DeDad's answer: The CBEMA curve (Fig. 1) was derived by the Computer Business Equipment Manufacturers Association — thus, the acronym used in the name of the curve — as a guideline for the organization's members in designing their power supplies. Essentially, the association designed the curve to point out ways in which system reliability could be provided for electronic equipment.

The curve is a susceptibility profile, with the vertical axis representing the percent of voltage applied to the power circuit and the horizontal axis representing the time factor involved, measured from microseconds to seconds. In the center of the plot is an acceptable area. Outside this area is a danger area on top and bottom.

The danger zone at the top involves tolerance of equipment to excessive voltage levels, while the zone at the bottom sets the tolerance of equipment to a loss or reduction in applied power. If the voltage supply stays within the acceptable area, the solid-state equipment will operate well.

Computers, programmable logic controllers (PLCs), power distribution units (PDUs), instrumentation, telecom, and other solid-state systems will operate reliably when applied carefully. However, all these units have one thing in common: they're voltage- and time-sensitive. In other words, voltage sags and swells, as well as outages and transients, will seriously affect their operation.

For an end-user or power quality technician to use the curve, he or she must first determine the nature of the disturbances that are most prevalent in a facility. Disturbances associated with powering, grounding, and protecting solid-state devices can be measured, analyzed, and evaluated using test equipment specifically intended for digital logic systems. These instruments, when located near the suspected disturbance, or when measuring the unusual operation of the power distribution system, will provide data on voltage fluctuations, short- and long-term excursions, and the specifics on how the disturbance places the equipment at risk.

Once these measurements have been taken — preferably with recording-type instruments — the results can be analyzed in combination with the CBEMA curve to help understand the nature of the disturbances.

The ITIC curve (Fig. 2) was derived by a working group of CBEMA, which changed its name to the Information Technology Industry Council. This derivation was developed in collaboration with EPRI's Power Electronics Application Center (PEAC). The intent was to develop a curve that more accurately reflects the performance of typical single-phase, 60-Hz computers and their peripherals, and other information technology items like copiers, fax machines, and point-of-sales terminals. While specifically applicable to computer-type equipment (as with the CBEMA curve), the ITIC curve is generally applicable to other equipment containing solid-state devices.

Because some of the ITIC curve's data points are carefully negotiated, each one is a useful basis on which to test the performance of a given product. For the vertical percent-of-nominal-voltage point selected, if the output isn't affected until later than the value shown on the horizontal time scale for that point, the product has met the limit described by the curve.

Keep in mind that the ITIC curve isn't intended to reflect the performance of all electronic-based equipment. There are too many variables, such as power loading, nominal operating voltage level, and process complexity, to try to apply a “one-size-fits-all” ITIC curve.

Q. The requirements of Art. 645 in the 2002 NEC say that the grounding of information technology (IT) equipment should be done in accordance with Art. 250. I've been told that “daisy-chained” grounding is not suitable for this type of equipment. Why not? It provides the required safety. What method of grounding should I use?

DeDad's answer: When grounding IT equipment, you must consider proper operation as well as safety from shock. Yes, IT equipment grounding must comply with the NEC, and it will be safe if it meets these requirements. But there are several additional provisions you must include if the installation is to operate with minimum electrical noise problems from the grounding system.

If you install the IT equipment grounding the same way as most other electrical equipment grounding — “daisy-chained” — it might look somewhat like the configuration shown in the upper diagram in Fig. 3. Here, the grounding conductors are run with the power conductors from the source to each piece of equipment. But, there are also many pieces of equipment fed from other equipment, and many other metallic interconnections between the grounded metal enclosures of various units. This is perfectly safe and meets the equipment grounding requirements as set forth in the NEC. But the multiple points of grounding can have slight differences of potential, causing small ground currents to flow, especially under transient conditions. These ground currents appear as noise to the IT equipment and can cause errors in computation or component failures. Basically, this grounding arrangement produces multiple paths for the circulation of ground currents.

You should modify the grounding to the configuration shown in the lower diagram in Fig. 3, which will eliminate the possibility of ground loops. Each piece of equipment is fed separately and radially from the source and grounded by means of a metallic raceway, a green-wire ground, or any other grounding conductor to a single IT equipment ground point, noted as “G” in the diagram, at the source or distribution point. This radial grounding eliminates all ground loops because all points are at the same potential. An additional advantage is that radial grounding provides a convenient point for tying into any high-frequency reference grid.