15.1 Environmental Conditions
Electronic equipment shall be capable of operating in the environment specified, see "Outside Environmental Conditions for the State of California"
15.2 Electronic Cooling
15.2.1 Forced Air Cooling
Forced air cooling shall be used only when natural cooling is not adequate. Exhaust and recirculating fans and blowers shall be driven by ac brushless motors or by properly shielded dc motors. Air filters shall be provided for air intakes for fan and blower cooled units when required to protect internal parts. Filters, when used, shall be removable for cleaning without disassembly of the equipment. All ventilation openings shall be designed and located to comply with electromagnetic interference, undesired radiation and enclosure guidelines. Air exhaust shall be directed away from operating personnel.
15.2.2 External Source Cooling
For equipment designed for use with forced air cooling, and the air may contain entrained water or other contaminants (e.g., dust) detrimental to the equipment, precautionary measures shall be taken to avoid direct impingement on internal parts and circuitry by channeling, filtering, baffles or use of heat exchangers.
15.2.3 Other Cooling Methods
Prior approval of Caltrans shall be obtained when heat densities or other design requirements make the use of air for cooling impractical and alternate methods, such as liquid, evaporative, air conditioners, change of phase material, or heat pipes, are required.
15.2.4 Fan and Blower Characteristics
- The design factors which shall be considered in determining the required fan or blower characteristics include such factors as amount of heat to be dissipated, the quantity of air to be delivered at the pressure drop of the enclosed equipment, the allowable noise level, the permissible level of heat that may be exhausted into the surrounding environment, and other pertinent factors affecting the cooling requirements of the equipment. Induced drafts and ventilation by means of baffles and internal vents shall be used to the greatest practicable extent. When practicable, ventilation and air exhaust openings shall not be located in the top of enclosures or in front panels.
- When required, a visual or aural warning device shall be used to indicate failure of a cooling device.
- Equipment that requires forced cooling to operate shall have control features to prevent equipment damage due to failure of the forced cooling.
15.2.5 Cooling Air Temperature Difference
The difference between the exhaust air temperature (measured inside the cabinet or console in front of the exhaust air vent) and input air temperature (measured outside the cabinet or console directly in front of the input air vent) shall be less than 59Â° F with the equipment operating under normal service conditions.
15.2.6 Temperature Controlled Fans
Fans controlled by an automatic temperature switch shall not come on when main power is shut off to the unit. A sign shall be placed near any automatic temperature controlled fan (e.g."Caution fan may start at any time").