With ASHRAE determining that electrostatic discharge (ESD) is a function of dew point or absolute humidity and not relative humidity, TIA-942-A has been updated to modify lower operating ranges for humidity based on dew point rather than relative humidity.
The result is that TIA-942-A states that computer rooms shall comply with class 1 of TIA-569-C, which is harmonized with ASHRAE. Specifically:
- Temperature: 18-27°C (64-81°F) dry bulb
- Maximum relative humidity: 60%
- Maximum dew point: 15°C (59°F)
- Maximum rate of temperature change: 5°C (9°F) per hour
The upper limits have been specified to for relative humidity and dew for the purpose of reducing failures caused by conductive anodic filament growth on printed circuit board laminate materials.
The upper limits have changed from 25 to 27 C (77 to 81 F) . Be aware that this can have a detrimental effect on equipment acoustical noise and fan power, since inlet temperatures increase can cause dramatic increase in equipment fan utilization.
The lower temperature limit has been reduced to improve the efficiency of HVAC systems by avoiding the need to mix hot return air to maintain the previous 20 C (68 F) limit. However, it should also be noted that there is a risk of freezing the coil of direct expansion air conditioning units that deliver air temperatures below 20 C (68 F).
