IEEE Std C37.21-2017 pdf download – IEEE Standard for Control Switchboards
dual switchboard: A control switchboard with front and rear panels separated by a comparatively short distance (no aisle) and enclosed at both ends and top. The panels on at least one side are hinged for access to the panel wiring. See also: duplex switchboard. duplex benchboard: A combination assembly of a benchboard and vertical switchboard placed back-to-back and enclosed with a top and ends (not grille). Access space (aisle) with entry doors is provided between the benchboard and vertical switchboard. See also: benchboard and dual benchboard. duplex switchboard: A control switchboard consisting of panels placed back-to-back and enclosed with a top and ends (not grille). Access space (aisle) with entry doors is provided between the rows of panels. See also: dual switchboard. enclosed switchboard: A type of switchgear assembly that consists of one or more panels with electric devices mounted thereon, and associated framework. fixed rack (cabinet): An assembly enclosed at the top and sides, either open or with door(s) for access, with a top-to-bottom front panel opening for equipment mounting [(for example, 486 mm (19 in) wide chassis and subpanel assemblies]) . See also: swing rack cabinet.
d) The ambient air is not significantly polluted and would be classified as having pollution level I “light” according to IEEE Std C37.100.1. e) The conditions of humidity are as follows: 1) The average value of the relative humidity measured over a period of 24 h does not exceed 95%. 2) The average value of the water vapor pressure, over a period of 24 h does not exceed 2.2 kPa. 3) The average value of the relative humidity over a period of one month does not exceed 90%. 4) The average value of the water vapor pressure over a period of one month does not exceed 1.8 kPa. For these conditions, condensation may occasionally occur. NOTE1-Condensation can be expected where sudden temperature changes occur in periods of high humidity. NOTE 2—Condensation may be prevented by special design of the building or housing, by suitable ventilation and heating of the station or by the use of dehumidifying equipment. Other options include heaters with thermostats/humidistat inside the switchboard. Condensation may also be due to ground level rainwater or for underground applications, from incoming cable raceways connected to switchboard.