ASHRAE Journal - March 2015 - 72

COLUMN DATA CENTERS
Donald L. Beaty

New Research on Humidity and its Impact on Electrostatic Discharge

Are Data Centers Drying Up?

BY DONALD L. BEATY, P.E., FELLOW ASHRAE; DAVID QUIRK, P.E., MEMBER ASHRAE

Valuable new data on the relationship between humidity and electrostatic discharge
(ESD) in a data center environment has recently been published through ASHRAE
Research Project RP-1499. The relationship between ESD and humidity has long been
anecdotally known, but insufficiently quantified.
Everyone has had the experience of walking on a carpet and touching a metal surface soon afterwards - the
result is an electrostatic shock.
The voltage of the shock ranges from a couple hundred volts to many thousand volts, and if the item being
touched is sensitive electronics, there is a potential to
damage that equipment. Likewise, these shocks tend to
have much greater frequency (and magnitude) in the
winter when the humidity is lower than in the summer
when the humidity is higher.

Historical Trends in Data Center Humidification
IT equipment manufacturers have long known that
electrostatic discharges occur at low relative humidity
levels, and that these discharges can cause IT equipment
failures. Legacy data centers were designed with a narrow relative humidity deadband: 45-55% RH.
When ASHRAE TC 9.9 first published Thermal
Guidelines for Data Processing Environments in 2004, the
recommended RH was a little more relaxed to the
range of 40% to 55% RH, but introduced an Allowable
Range that was set at a much broader 20% to 80%.
In the second edition of Thermal Guidelines (2008), the
recommended humidity level was further relaxed to
5.5°C (42°F) dew point on the low side, and the lower of
60% RH and 15°C (59°F) dew point on the high side. Figure
1 plots the 2004 and 2008 recommended envelopes on a
psychrometric chart. The allowable range stayed the same
between the first and second editions, at 20% to 80% RH
and is plotted on a psychrometric chart in Figure 2.
In the latest (third) edition of the Thermal Guidelines,
published in 2012, the recommended range has stayed
the same, but two additional allowable ranges have been
added: A3 and A4 (Figure 3). Both of these ranges allow
for as little as 8% RH on the low side.
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Justification for Additional Research
Humidification can have a significant impact on
energy consumption that extends beyond the simple
consumption of the humidification conditioning equipment itself. For instance, a high minimum humidity
requirement in conjunction with an air-side economizer
could result in blocking out a significant number of
economizer hours and associated energy savings. Due
to the significant impact of high minimum-humidity
threshold requirements on energy consumption of data
centers, ASHRAE TC 9.9 applied and received approval
from ASHRAE for a research project to better understand the correlation between humidity levels and ESD.

The Research Project
While it may seem like a fairly straightforward
research project, the actual implementation of a study
to correlate ESD with humidity is complex. Part of
the setup for the research involved obtaining a better
understanding of exactly what mechanisms occur to
create these discharges.
For instance, a person walking will definitely build up
a charge (and the charging process was also studied in
this report), but how will this be discharged? Will the
person discharge by touching a piece of IT equipment
with their hand, or will contact be made by first touching the equipment with something metallic, such as a
flash drive? The magnitude and duration of these discharges will be different, and both instances were tested
as part of the research.
The type of shoe worn by the occupant matters as does
the type of flooring. At the least granular level, one can
Donald L. Beaty, P.E., is president and David Quirk, P.E., is vice president of DLB Associates
Consulting Engineers, in Eatontown, N.J. Beaty is publications chair and Quirk is the chair
of ASHRAE TC 9.9.

ASHRAE Journal - March 2015

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