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current is maximum during the first cycle or loop, because of the presence of the d-c component and because the motors contribute the most short-circuit current a t that time. Thus, the short-circuit stresses on the circuit breakers and other parts of the circuit are maximum during the first loop of short-circuit current. During the time from the inception of the short circuit until the circuit- breaker contacts part, the current decreases in magnitude because of the SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES 25 decay of the d-c component and the change in motor reactance, as explained previously. Consequently, the current that the circuit breaker must interrupt, four or five cycles after the inception of t.he short circuit, is generally of less magnitude than the maximum value of the first loop. The fact that the current changes in magnitude with time has led to the establishment of two bases of short-circuit-current ratings on power cir- cuit breakers: (1) the momentary rating or its ability to withstand mechanical stresses due to high short-circuit current and (2) the inter- rupting rating or its ability t,o interrupt the flow of short-circuit current within its interrupting element. What Comprises the Circuit-breaker-rating Structure. Circuit- breaker-rating structures are revised and changed from time to time. It is suggested that where specific problems require the latest information on circuit-breaker ratings the applicahlc American Standards Association (ASA), National Electrical Manufacturers Association (XEMA), or American Instituteof Elect,rical Engineers (AIEE) standards he referred to. To illustrate the various factors that comprise the circuit-breaker- rating structure, an oilless power circuit breaker for metal-clad switchgear rated 4.16 kv 250 mva* has been chosen. The complete rating is shown on line 5, Table 1.1. The following will explain the meaning of the several columns of Table 1.1, starting at the left. The rircuit-breaker-type designation, column 1, varies among manufacturers. For the sake of com- pleteness the General Electric Company nomenclature is used in this col- umn. The remainder of the items are uniform throughout the industry. 1. Type of Circuit Breaker (AM-4.16-250) AM = magne-blast circuit breaker 4.16 = for 4.16-kv class of circuits (not applicable to 4800- and 4800- volt circuits) 250 = interrupting rating in mva a t 4.16 kv 2-4. Voltage Rating 2. Rated kv (4.16): the nominal voltage class or classes in which the circuit breaker is rated. 3. Maximum design kv (4.76): the maximum voltage a t which the cir- cuit breaker is designed to operate. The 4.16-kv circuit breakers, for example, are suitable for a 1330-volt system plus 10 per cent for voltage regulation or 4.76 kv. (Note: 4330 is 4% X 2500.) Some utility syst.ems operate a t 1330 volts near the substation. 4. Minimum operating kv a t rated mva (3.85) : the minimum voltage a t which the circuit breaker will interrupt its rated mva or in this case it is 3.85 kv. At any voltages below this value, the circuit breaker * blegavalt-amperes (see Appendix). t i. 16 SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES I ! I ( t a / I SHORT-CIRCUIT-CURRENT CALCULATING PROCEDURES 27 is not designed to interrupt the rated mva but will interrupt some value less than rated mva. This is very significant in the rating of power circuit breakers for, as poiuted out later, the circuit hreaker will interrupt a maximum of only so many amperes regardless of voltage. At any voltage less than the minimum operating voltage the product of the maximum kiloampere interrupting rating times the kv times the square root of 3 is less than the mva interrupting rating of the circuit breaker. 5-6. Insulation Level (Withstand Test) 5 . Low-frequency rrns kv (19): the 60-cycle high-potential test. 6. Impulse crest kv (60) : a measure of its ability to withstand lightning This is applied with an impulse generator as a and other surges. design test. 7-9. Current Ratings in Amperes 7. Continuous 60 cycles (1200 or 2000): the amount of load current which the circuit breaker will carry continuously without exceeding the allowable temperature rise. 8-9. Short-time Rating 8. Momentary amperes (60,000) : the maximum rms asymmetrical cur- rent that a circuit breaker will withstand including short-circuit cnr- rents from all sources and motors (induction and synchronous) and the d-c component. This rating is independent of operating voltage for a given circuit breaker. This is just as significant a limitation as mva interrupting rating. It defines the ability of the circuit breaker to withstand the mechani- cal stresses produced by the very large offset first cycle of the short- circuit current. This rating is nnusually significant because the mechanical stresses in the circuit hreaker vary as the square of the current. It is the only rating that is affected by the square law, and therefore is one of the most critical in the application of the circuit breakers. The rating schedules of power circuit breakers are so pro- portioned that the momentary rating is about 1.6 times the maximum interrupting rating amperes. 9. Four-second (37,500): the maximum current that the circuit breaker will withstand in the closed position for a period of 4 sec to allow for relaying operating time. This value is the same as the maximum interrupting rating amperes. 10-13. Interrupting Ratings 10. Three-phase rated mva (250): the three-phase mva which the circuit breaker will interrupt over a range of voltages from the maximum design kv down to the minimum operating kv. In this case the 28 SHORT-CIRCUIT-CURREM CALCULATING PROCEDURES interrupting rating is 250 rnva between 4.76 and 3.85 kv. The mva to be interrupted is obtained by multiplying the kv a t which the cir- cuit breaker operates times the symmetrical current in kiloamperes to be interrupted times the square root of 3. The product of these must not exceed the rnva interrupting rating a t any operating voltage. 11. Amperes a t rated voltage (35,000): the maximum total rms amperes which the circuit breaker will interrupt a t rated voltage, i.e., in the case of the example used above 35,000 at 4.16 kv (4.16 X 35.000 x fi = 250 mva). These figures are rounded. This figure is given for information only and does not have a limiting significance of particular interest to the application engineer. 12. Maximum amperes interrupting rating (37,500) : the maximum total rms amperes that the circuit breaker will interrupt regardless of how low the voltage is. At minimum operating voltage, 3.85 kv, this corresponds to 250 mva, and, for example, a t a voltage of 2.3 kv this corresponds to 150mva. The circuit breaker will not interrupt this much current a t all volt- ages, i.e., i t will not interrupt this much current if the product of current, voltage, and the square root of 3 is greater than the mva interrupting rating. This current limit determines the minimum kv a t which the circuit breaker will interrupt rated mva (column 4). At any voltage lower than that given in column 4, this maximum rms total interrupting current determines how much the circuit breaker will interrupt in mva. Therefore, when the voltage goes below the limit of column 4, the mva which the circuit breaker will interrupt is lower than the rnva rating given in column 10 by an amount propor- tional to the reduction in operating voltage below the value of column 4. 13. Rated interrupting time (8 cycles on 60-cycle basis): the maximum total time of operation from the instant the trip coil is energized until the circuit breaker has cleared the short circuit. What limits the Application of Power Circuit