Thisinterruptingcurrentis,ofcourse,lowerthanthemomentary current and depend on the speed of the breaker,such as 8,5,3 or 2 cycles, which is a measure of the time fromthe occurrence of the fault to the extinction of the arc.
Professor Arturo Bretas | EEL 4251 - PSAThe Selection of Circuit Breakers60Breakers are identified by nominal-voltage class, such as 69 kV.Among other factors specified are rated continuous current, rated maximum voltage, voltage range factor K, and rated short-circuit currentat rated maximum kilovolts.
Professor Arturo Bretas | EEL 4251 - PSAThe Selection of Circuit Breakers61The rated maximum voltage of a circuit breaker is the highestrms voltage for which the circuit breaker is designed.The rated voltage range factor K is the ratio (rated maximumvoltage÷the lower limit of the range of operating voltage). Kdetermines the range of voltage over which the product (ratedshort-circuit current× operating voltage) is constant.
Professor Arturo Bretas | EEL 4251 - PSAThe Selection of Circuit Breakers62In the application of circuit breakers it is important not to exceedthe short-circuit capabilities of the breakers.A breakerisrequiredtohaveamaximumsymmetricalinterrupting capability equal to K×rated short-circuit current.
Professor Arturo Bretas | EEL 4251 - PSAExample 3:63A 69-kV circuit breaker having a voltage range factor K of 1.21 and a continuouscurrent rating of 1200 A has a rated short-circuit current of 19.000 A at the maximumrated voltage of 72.5 kV. Determine the maximum symmetrical interrupting capabilityof the breaker and explain its significance at lower operating voltages.SOLUTION:The maximum symmetrical interrupting capability is given by:This value of symmetrical interrupting current must not be exceeded. From thedefinition of K we have:
Professor Arturo Bretas | EEL 4251 - PSAExample 3:64Hence, in the operating voltage range 72.5 -60 kV, the symmetrical interrupting currentmay exceed the rated short-circuit current of 19.000 A, but it is limited to 22,990 A .For example, at 66 kV the interrupting current can be:Breakers of the 115-kV class and higher have a K of 1.0.