standards / manuals / guidelines for small hydro development - AHEC

component of fault current in addition to AC component to determine the critical currentvalue existing at the time of primary arcing contact parting can not be calculated. Furthernetwork analyzers is a fixed place study. Now-a-day computer studies are carried out forsuch application.8.1 Staged Short Circuit TestsStaged fault short circuit tests adequately controlled on actual systems have been carriedout mostly on new equipments and systems to determine circuit breaker capability. Theseare accurate, costly and not always possible as selection of circuit breaker precedes powersystem installation.8.2 Circuit Breaker Rating for Short Circuit DutySteps involved in fixing circuit breaker short circuit rating are as follows:1. Determine normal current duty of the circuit breaker and select higher available ratedcurrent from standard values as per IEC/IS 13118 clause 4.4.2. Short Circuit studies be carried for following types of faults which are considered worstaccording to IEEE std. C37010-1999.a. Three phase ungrounded faultsb. Phase to ground faultMore severe of the short circuit faults be taken for selecting the short circuit rating.3. Determine short circuit currents for the required accuracy by a suitable method. For lineto ground faults, the required symmetrical interrupting capability is 15% higher.4. Circuit breaker having the rms value of the ac component of the short circuit higher thenshort circuit duty as calculated from table X A of IS 13118 (table 2.4 for commonly usedvoltages) be selected.5. A circuit breaker having adequate symmetrical interrupting capability will normally haveadequate capability to meet normal asymmetrical requirement. Maximum symmetricalinterrupting capacity of new circuit breaker is as follows:Rated short current x Rated maximum voltage operating voltage For higher X/R ratio or other special conditions refer the detail methods given in IEEEstd. C37010-1999.4-18