In the various standards there will be found a number of different situations regarding Usual Service Conditions. The following three are typical.
1. The group where the maximum ambient temperature is specified. The Standards for Bushings, Fuses, Lightning Arresters, Protector Tubes and Potential Devices where 40 C ambient is used and Marine Equipment where 50 C ambient is used are typical of this group.
2. The group where both the maximum and daily average temperatures of the cooling medium are
specified. Standards for Transformers, Regulators and Neutral Grounding Devices are typical of
3. The group where a higher ambient temperature is specified for silver contacts than for
copper contacts. Standards for Circuit Breakers, Air Switches and Switchgear Assemblies are typical of this group.
It is suggested that a selection of one of the following groups can be made for the statement of Usual Service Conditions in most of the standards for specific equipment.