power distribution - Siemens

18TECHNICALMolded Case Circuit BreakersUnusual Operating ConditionsHigh Ambient TemperaturesBecause standard thermal-magneticbreakers are temperature sensitive andcalibrated for a specific ambient of 40°C(104°F) (average enclosure temperature),the presence of an ambient higher willcause the breaker to carry less currentthan its name-plate rating — in otherwords, causing the breaker to “derate”(see Table 17.33). Similarly, the safecurrent carrying capacity of a circuitconductor is based upon an ambienttemperature of 30°C (86°F) (average airtemperature) and the presence of a higherambient will reduce its safe currentcarrying capacity, causing it to “derate.”Thus, it can be seen from Table 17.33 thatin the presence of a fluctuatingtemperature, a thermal-magnetic breakerwill derate nearly parallel with itsconnected circuit conductors and maintainclose circuit protection. If the applicationtemperature exceeds 40°C (104°F) and isknown, either a breaker especiallycalibrated for the higher ambients or oneoversized according to Table 17.33 may beselected. It should be noted that in a casesuch as this, the circuit conductors shouldbe oversized as well.Siemens Sensitrip ® III and Type SBEncased System Breakers are insensitiveto temperature changes. However, they doinclude circuitry to protect the componentsfrom abnormally high temperatures.Moisture — CorrosionFor atmospheres having high moisturecontent and I or where fungus growth isprevalent, a special preventive treatmentmay be required. Where the air is heavilyladen with corrosive elements, breakersmade with special corrosion-resistantfinishes are recommended.AltitudeReduced air density at altitudes greaterthan 6600 ft. (2000 meters) affects theability for a molded case circuit breaker totransfer heat and interrupt faults.Therefore, circuit breakers applied at thesealtitudes should have interrupting insulationand continuous currents derated asindicated in Table 17.34.Table 17.34 Altitude CorrectionApplicationNote: The information provided on this and the next page is intended for reference and recommendation only. Because severalvariables can act on a circuit breaker’s performance at the same time, the data below is based less on controlled testing, than onexperience and engineering judgment. Contact Siemens for further information on special conditions and treatment.Altitude in feet (meters)6600 (and below) (2000)8500 (2600)13000 (4000)RatingCorrection FactorContinuousCurrent10.990.96Voltage10.950.8Values for intermediate altitudes may be derived fromlinear interpolation. See Figure 1.Table 17.35Reference Relative Amperage Rating at:AmpereRating at40°C (104°F)25°C(77°F)50°C(122°F)60°C(140°F)15 17 13 1120 22 18 1625 28 23 2130 33 28 2635 39 30 2540 44 37 34 ED2Temperature Derating Data for Thermal-Magnetic Breakers50 55 46 4260 66 56 5270 77 65 6090 99 84 78100 110 94 87125 137 144 100150 165 136 120175 192 159 140200 220 182 160225 247 205 180250 275 235 220300 330 276 252350 385 325 301400 440 372 340500 550 468 435600 660 564 525700 770 658 613800 880 754 704900 990 828 7491000 1100 900 8251200 1320 1090 10001400 1540 1304 11481600 1760 1500 13201800 1980 1690 14852000 2200 1880 1650Figure 1 — Altitude Adjustment ChartSiemens Beaker TypesQJ2QJH2QJ2-HJXD2MXD6, MD6HMXD6HMD6CMD6ED4ED6HED4HED6CED6JXD6JD6HJD6HHJD6CJD6NXD6, ND6HNXD6HND6CND6FXD6FD6HFD6CFD6LXD6LD6HLD6HHLD6CLD6PXD6, PD6HPXD6, HPD6CPD6RXD6, RD6HRXD6, HRD618-22Siemens Power Distribution & Control, SPEEDFAX 2007-2008 Product Catalog