Document ymG8pE88grJqenkxXbOMDK3bD

PROTECTION ON PARADE All electrical circuits use over- iotor protection. The einple trioal Code states that elrco: current protective devices to guard theroal overload relay osed In a breakers used for aotor protects against short circuit and overload. aotor starter does not as s rule oust bave a continuous curril Strictly speaking, they do not pro* provide ehort-elreult protection. rating of not -lees than 116% of tJ teet against abort ciroult since A fuse or circuit breaker should be full-load rating of tbe aotor. ]! seas daaage has been done at the Installed ahead of tbe starter to In eeleotlng a circuit breaker^ trouble point by the tine the fuse operate on short circuit. For one protect a feeder circuit supply!! or circuit breaker- opens, la alternatiag-eurreot aotor tbe several ootors, the iostantoasTM applying protective devices in inrush or transient current is In trip point of the breaker electrical circuits, tbe ala is to tbe oelghborbood of 140 to'160% of eight to nine tloes the localise the fault quickly, and the locked-rofcor eurreot. Locked* of the full*load curren. . hence confine or reduce any possible rotor correct ia that value vbicb rotors; this is a generally accept! dasage. vould flov sere tbe full voltage rule when full-toad current of .*: then selecting a device for applied vhlle tbe rotor sas held in ootors on this circuit is ebort-olreult protection, the uolt a standstill position. greater than 100 sop. there best suited is tbe one that opens Tbe locked rotor current of full-load current Is between 190-jl tbe circuit quickest. Tbe ordinary alternatlag*current rotors is fre .660 anp, tbe tripping point of " oonrenevable fuse and tbe ainple quently about 790% of tbe full-load breaker should be six to e*! magnetic device sill act fastest on current. Hence, select a breaker tlaes tbe full-load value. JS tbe floe of abort-clrcuit current. tbat has an ioatantaneoue tripping Tbe starting current of dlree] The theraal elenent on tbe other point of 140 to 160% of tbe locked current ootors is generally 1! hand baa an Inherent tlae-lag vbicb rotor current. Generally speaking, than tbat of alteroatlng-currtg ' aakes it suitable for overload but for noraal*torque aotora the ootors. It is recomended that not ahort-olreult. Ciroult breakers instantaneous tripping point of tbe lnstantaoeous trlpplog point of, equipped eltb both tbernal and breaker under consideration should circuit breaker, vbicb is to sup. oagnetle tripe offer suitable be 13 to 14 tlaes tbe foll*load short-circuit protection, be eet protection against both conditions. eurreot rating. Tbe Natloosl Bleo- to seven tiros tbe full-load cori POWER - July 1941 load current for several ootors. there there are several ootors on one feeder, it ie aeeuoed they will oot all be started at the saae tiro. Cireett Breakers. An air circuit breaker oust have an interrupting rating equal to or greater than the aailsuo. short-circuit current that uay flow at that point in the cir cuit where the breaker Is to be installed. Por exaaple, when a nlngle circuit breaker la placed between the power source and tbe load, its interrupting rating oust onual to or greater than the Jerage total calculated current one-half cycle froo tbe tiae hort circuit vaa applied. V1, * ! the conventional appllntiea, and is the rule when a clrcul* breaker is placed load p0,er 80o*ee-ond tbe ti ^scMlog. Consider a distrlbueu^..Va.teo consisting of a oalo bit breaker a feeding a bua to breakers B. each of which in turn feeds several eubfeeder breakers c. If the tripping points of these circuit breakers ore property co ordinated, breakers B and C aay be applied beyond their interrupting ratings. Such a sequence of proper ly coordinated breakers is known `m a cascade. Proceeding froa the source toward tha load, breakers in cascade bave a progressively lower standard Interrupting rating. Tbe central Idea in cascading ia to have breaker a set to trip at 80% of the aaxloua interrupting rating of B. Clrcolt breaker B in turn should trip at ao% of the aaxloua interrupting rating of C. All break ere so arranged will trip Instan taneously at their set value of current. The conventional application of olrcult breakers is recoaaeoded in all cases, but oay oot be econoal- eatly justifiable on all installa tions. there all breakers are lo- fOWU Ju)y 10^6