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What are the normal service conditions for indoor medium voltage equipment according to IEC 62271-1 concerning ambient?

All MV equipment shall comply with their specific standards.
The IEC 62271-1 standard "Common specifications for high-voltage switchgear and controlgear" defines the normal service conditions for the installation and use of such equipment.

Ambient temperature:
The ambient air temperature does not exceed 40 °C and its average value, measured over a period of 24 h, does not exceed 35 °C.
The preferred values of minimum ambient air temperature are - 5 °C, - 15 °C and - 25 °C.
For instance, regarding pollution, humidity associated with condensation, the standard states:

Pollution:
The ambient air is not significantly polluted by dust, smoke, corrosive and/or flammable gases, vapours or salt.
The manufacturer will assume that, in the absence of specific requirements from the user, there are none.

Humidity:
The conditions of humidity are as follows:
  • The average value of the relative humidity, measured over a period of 24 h does not exceed 95%;
  • The average value of the water vapour pressure, over a period of 24 h does not exceed 2.2 kPa;
  • The average value of the relative humidity, over a period of one month does not exceed 90%;
  • The average value of water vapour pressure, over a period of one month does not exceed 1.8 kPa;
Under these conditions, condensation may occasionally occur.
NOTE 1: Condensation can be expected where sudden temperature changes occur in periods of high humidity.
NOTE 2: To withstand the effects of high humidity and condensation, such as a breakdown of insulation or corrosion of metallic parts, switchgear designed for such conditions should be used.
NOTE 3: 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.
 
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