W-C-375E
4.7.8 Short circuit (see 3.11). The power source, loading, instrumentation, calibration, wiring, and preparation for test shall be in accordance with the applicable portions of UL 489. Circuit breakers shall be tested in the enclosure specified (see 3.1). Circuit breakers shall be mounted as in actual service with the door or cover and any other openings, including unused knockout openings, closed. The knockout openings shall be closed on the outside of the enclosure by means of tape. Three thicknesses of cloth 1/ are to be taped to the inside sidewalls of the enclosure so as to cover the entire sidewall within .5 inch of front and rear vertical edges. A cotton pad 2/ at least .5 inch thick, and having a length and width four times the handle opening but not less than 3 inches in either dimension is to be centered and secured not more than .5 inch from the end of the circuit breaker handle. A small opening is permitted
in a cotton pad to pass the plunger to operate a push-type circuit breaker. The circuit breaker shall be tested with alternating current at rated frequency. The power factor of the test circuit with required current of the test circuit with required current flowing shall be as tabulated in table X, except that a lower power factor may be employed. The capacity of the circuit together with the total limiting impedance shall be such as to permit the applicable current specified in table XIII. The open-circuit voltage shall be not less than 100 percent nor more than 105 percent of the rated voltage of the circuit breaker. The minimum point on the voltage wave is not to be less than 90 percent of the rated voltage of the circuit breaker. A fuse shall be connected as specified in 4.7.3 for operations conducted on the individual poles. For common operations, the fuse shall be connected to the side of the supply least likely to arc to grounded metal.
Circuit breakers shall be subjected to the number and type of short circuit operations indicated in table XIV. Successive operations shall be conducted by alternating between the "O" and "CO" operations (see 6.7.2). The time interval between the short-circuit operations of a circuit breaker shall be two minutes, except that the time interval may be extended to whatever is necessary to allow the circuit breaker to be reset, but not more than 1 hour. Each circuit breaker is to be subjected to all the operations of the short-circuit test with one set of line and load leads, i.e., without replacement of any lead during the test.
Following the short circuit test, circuit breakers shall be closed, subjected to 100 percent of the rated voltage and checked for continuity, then the 200 percent rated current test at 25°C shall be conducted as specified in 4.7.2.
TABLE XII. Power factor of test circuits.
Test circuit in amperes |
Power factor |
10,000 or less 10,001-20,000 Over 20,000 |
0.45-0.50 0.25-0.30 0.15-0.20 |
1/ The cloth employed is to be bleached cheesecloth, 36 inches wide, running 14 to 15 yards per pound, and having what is known to the trade as a "count of 32 by 28". In placing the cloth in position for the short-circuit test, the material is to be folded in such a manner that cut or torn edges will not be exposed directly to the arc or flash resulting from the opening of the circuit breaker.
2/ The cotton is to be surgical cotton such as is commonly used for medical purposes.
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