Things to Consider When Choosing an Enclosure for an OSP Application

Selecting the right enclosure for an outdoor application for the first time can be a daunting task and should be addressed with a systematic approach in order to minimize problems in the field. Once the enclosure has been placed in the field the number of options becomes limited and often expensive to correct. This document is intended to provide a check list up front for the initial design to eliminate as many issues prior to installation as possible.

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Moisture and/or Condensation is a Major Concern

Moisture and/or condensation is a major concern to any outdoor enclosure. Typically condensation and the chemicals that adhere to the condensation are a major cause of system faults in electronics equipment.

The key to keeping moisture from forming in an outdoor cabinet is to insure that the cabinet inside temperate is above the dew point. As the humidity outside of the cabinet is at or above RH 80 % and ambient temperature are above 80 degrees F the separation between the dew point and the cabinet inside temperature become very close about 5 to 7 degrees F.

Remember that NEMA-4 is dust tight but not air sealed and moisture is a gas contained within the air and will eventually penetrate the cabinet.

With direct air cooled cabinets the recommended temperature range is 90 to 104 degrees F to prevent condensation from forming. Air conditioned cabinets are a different story since the evaporator coil in the air conditioner will remove the moisture in the cabinet due to the temperature of the evaporator coil being below the dew point. This condensation is then routed outside of the cabinet via a drain tube.

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