Eaton Cutler-Hammer Digitrip RMS 510 Instructions Manual

Also see for Cutler-Hammer Digitrip RMS 510: Instructional Leaflet Instructions Instruction leaflet

AD 33-855-4 Page 75SECTION 14 EXPLANATION OF TIMECURRENT CURVES14.1 Explanation of the Time GurrentCurves (LSl)Digitrip RMS Trip Units and Digitrip Retrofit Kitshave been designed to upgrade existing Breakersto meet the same protection requirements of themost modern Power Circuit Breakers. ln fact, theTrip Unit and Direct Trip Actuator (DTA) are electri-cally identical to those found on current DS Break-ers. They are built in the same manufacturingfacilities, with the same high level of Quality Con-trol, as those on the DS Breakers. ln this way, weassure that the Retrofit Designs are as up to dateas our current production.All Digitrip RMS Trip Units are designed to meetthe protection requirements of a modern powerdistribution system. As is shown in Figure 14-1 ,Application Current Levels can be broken intothree typical regions:. NORMAL currents, the magnitude of which iswithin the system continuous current rating;. OVERLOAD currents, which exceed the systemcontinuous current rating and where conductorheating is the primary concern (such as with anoverloaded motor, for example);. FAULT currents, which are usually caused byan unintended current path (either phase tophase or phase to ground) and can lead tocurrents as high as the maximum short circuitavailable from the system.Digitrip RMS Trip Units, when set up according tothe results of a Power System study of the specificdistribution system on which they are applied, willrespond to various current levels to provide accu-rate system protection as well as maximum systemstability by properly coordinating with upstreamand downstream devices (Figure 14-1).. For NORMAL currents, the Trip Unit willaccurately monitor the current, regardless ofcurrent waveform (due to its true RMS sensingsystem), assuring that up to full rated currentmay flow through the system without falselytripping the breaker. This is shown as theNORMAL OPERATING CONDITION inFigure 14-1 .. For OVERLOAD currents, the Trip Unit willaccurately assess the true heating value of thecurrent (again a distinct advantage of its trueRMS sensing system), and trip out followingthe LONG DELAY TRIP TIME curve in Figure14-1 . There are actually eight different LONGDELAY TRIP TIME curves available on the Trip.Unit (selected by the Long Delay Time switchsetting), and each curve follows a constant lztslope for excellent heating protection andcoordination with other devices. The lowestlevel of OVERLOAD current to bring on thisresponse is selected by the Long Delay Pickupsetting on the Trip Unit.. For FAULT currents, the Trip Unit will respondaccording to either the SHORT DELAY TRIPTIME or the INSTANTANEOUS TRIP TIMEshown in Figure 14-1 , depending on the faultcurrent magnitude and whether the Short Delayfunction or lnstantaneous function or both arepresent on the Trip Unit.Efiective 9/01 FlT.il

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