ASHRAE TC 9.9 asked the EMC laboratory at the Missouri University of Science and Technology (Dr. David Pommerenke, David Swenson) to investigate the risk of electrostatic discharge (ESD) related upsets or damage to electronics in data centers (project 1499). Emphasis was placed on the increase in risk with reduced humidity. Operators using different flooring and footwear combinations, cable charge and discharge, human discharge and charge creation caused by chairs and garments were investigated in a climate chamber in the range of 8% RH to 60% RH at temperatures from 5°C to 38°C.

Lower humidity not only increases the charge voltages, but also leads to longer charge retention and more damaging discharges; therefore, the following experiments were performed under various environmental, footwear and flooring conditions:

• Measurement of the voltages generated by people walking
• Measurement of the voltages generated by people sitting up from a chair or removing a garment
• Measurement of the voltages generated by dragging cables on different surfaces
• Measurement of the discharge currents and fields

The charging experiments were analyzed to obtain both the maximal voltage for each parameter combination and the effect of parameter changes, especially the humidity. Furthermore, an extrapolation was performed to obtain the probability of voltages larger than typical thresholds used for electronic system robustness. Here, 4kV and 8kV, derived from the IEC 61000-4-2 test method, and 500V for service conditions, were used as the limit.
Overall, the results can be summarized as follows:

• Reducing the absolute humidity may not always lead to an increase in the charge voltages. Below a dew point of -6 to 0 °C, the voltages may even decrease as the absolute humidity decreases.
• Reducing the relative humidity will increase the voltages.
• If a data center maintains the same user activity, footwear and flooring, but changes the relative humidity from, for example, 25% to 8%, then the X-factor will express the increase in ESD-related upset or damage. Going from 25% RH to 8% RH had an X-factor of 1.5-3, depending on the material pairing, and extreme values of 1-20 have been observed.
• The Relative Humidity Voltage (RHV) factor expresses the increase of the average voltages if the humidity is reduced. For a reduction from 45% to8% RHV was approximately 3.
• The RHV for a reduction from 25% to 8% was approximately 1.5.
• The variations in RHV were large, depending on the materials and environmental conditions. The RHV was between 1 and 20.
• The chair and sweater events caused very high voltages.
• The event voltage definition did not take into account the rapid discharge that occurs if there is a conductive path to ground. For the chair and sweater event voltages, the voltage was on a person only for a brief moment if the flooring and footwear were good. However, the high voltages measured during the chair and sweater experiment indicate that only ESD mitigating chairs should be used in data centers, and that operators need to be aware of possible high static voltages after removing garments, and after unwrapping foil wrapped material.
• From each walking experiment three types of voltages have been derived. The walking voltage, for walking in a repeatable fashion, the walking voltage from the more realistic random walking, and the voltage that is measured on a person after he or she walked, and then stands still for 2 seconds. The results for walking in a prescribed pattern underestimated the voltages, relative to random walking by about an average of 30%. On the other hand, people will, in many cases, pause for a moment before touching something. As some argue that walking in a pattern underestimates the voltage while others argue that it overestimates the voltage, we believe that walking in a pattern serves as a good basis for estimation of the ESD risk probabilities.
• All of the data found in the literature indicate that uncovered concrete floors are rather conductive.
• The ESD discharge will exhibit higher currents at lower values of humidity (for the same speed of approach of the hand to the grounded equipment), and a lower critical velocity of the approach between the person and the data center equipment (in other words, the velocity above the peak current value will increase). However, as this requires voltages with a high charge (e.g., 4kV), an approach between a handheld metal piece and the metallic equipment, and clean, rounded surfaces, the additional risk caused by this increased severity was considered insignificant.

Using ESD-mitigating flooring and footwear, the risk of ESD upset and damage can be reduced to an insignificant level, even if the humidity is allowed to drop to low values, such as 8%. In addition to using conductive footwear and flooring, other precautions should be taken, especially under low -humidity conditions, to avoid rapid removal of non-conductive plastic wrapping. Furthermore, all office chairs and carts selected for use in data centers should have ESD-mitigating properties.

The low increase in the ESD risk with reduced humidity indicates that a data center with a low incident rate of ESD-induced damage operating at 25% RH will maintain a low incident rate if the humidity is reduced to 8%. The increase in ESD-induced incident rates will range from 1.5-3. The concerns raised prior to the study regarding the increase in ESD-induced risk with reduced humidity are not justified. A standard set of ESD-mitigation procedures will ensure a very low ESD incident rate at all humidity levels tested.