IEEE Std C37.24 pdf free download

IEEE Std C37.24 pdf free download.Evaluating the Effect of Solar Radiation on Outdoor Metal-Enclosed Switchgear.
3. Operating limitations
Temperature limitations for metal-enclosed switchgcar are established to ensure a satisfactory lifespan of the insulation. Operation at higher temperatures than permitted by the appropriate standard will result in shortened life, but if the temperature of the insulation goes above normal for only a few hours during just a few days ot’the year, the effect on total life is hardly measurable. This is very different from the establishment of higher short-duration ratings that might be repetitive on a day-to-day basis. The hazard of immediate breakdown is not increased unless extreme temperatures are reached.
To exclude wind-blown dust and dirt, outdoor metal-enclosed switchgear is usually designed with more restricted ventilation than indoor switchgear. I3ecause of this design difference, outdoor switchgear has a somewhat increased internal temperature even when there is an equivalent number of circuit breakers and buses. These conditions have been recognized in designs. With this restricted ventilation, the effect of solar radiation on an enclosure will cause the air immediately under the roof to be relatively warm. Electrical parts located near the roof will operate in air considerably warmer than that surrounding equipment in the center or lower portion of the switchgear units. Some types of cable insulation may not be rated for the higher air temperature found immediately under the roof. If cables are located in the upper part of’ housings, insulation with appropriate thermal characteristics should be selected. Relevant standards state, in effect, that the temperature of the air surrounding insulated cables within any compartment shall not exceed 65 °C when the assembly is in an ambient air temperature of 40 °C. This temperature is based on the use of 90 °C insulated power cables. Use of lower temperature rated cables requires special consideration. In many cases the effect of solar radiation alone will raise the inside temperature above that permitted by the appropriate standard. The insulation and size of’ conductors in this location should be carefully considered by both manufacturers and users.
End units, particularly those exposed to the sun during periods of heavy loading, should be considered on a different basis from the others in determining loading, conductor sizes, and cable insulation.
In addition to the effect on insulation, high internal air temperatures will increase the total temperature of the conducting parts. If circuit breakers are properly maintained, no adverse effects will result if their temperatures exceed allowable limits by a few degrees for short intervals during a few days of the year. All primary connections in metal-enclosed switchgear are usually silver surfaced, or equivalent, and no trouble should be experienced with them if rated temperatures are exceeded under similar conditions. Here again, it is important to realize the difference between increased temperatures due to outside influences that occur infrequently as compared to higher short-duration temperatures that might be repetitive on a daily basis.
Caution must be used, however, in the application of temperature sensitive devices, such as devices with electronic components in outdoor metal-enclosed switchgear. Accelerated component aging and device failure can result from increased temperatures that occur infrequently. Maximum temperature limitations for temperature-sensitive devices should be determined, and those temperature limitations should be considered during equipment design.
It is noted that the use of electronic devices has become widespread in outdoor metal-enclosed switchgear. Common electronic devices employed include protective relays, power metering devices, transducers, SCADA systems, remote terminal units (RTUs), PLCs, as well as many other devices. These devices are generally designed in accordance with specific component standards, and may be subject to temperature limitations more restrictive than those applicable to the switchgear equipment.IEEE Std C37.24 pdf download.