The short-circuit rating of a circuit breaker is established by the symmetrical component of short-circuit current in rms amperes, designated rated short-circuit current, to which all required short-circuit capabilities are related. All values apply to both grounded and ungrounded short circuits on predominantly inductive or resistive 3-phase circuits with a normal frequency phase-to-phase recovery voltage equal to the operating voltage.
Figure 1— Relation of Symmetrical Interrupting Capability, Closing Capability, Latching Capability,
and Carrying Capability to Rated Short-Circuit Current
Rated Short-Circuit Current.
The rated short-circuit current of a circuit breaker is the highest value of the symmetrical component of the polyphase or phase-to-phase short-circuit current in rms amperes measured from the envelope of the current wave at the instant of primary arcing contact separation which the circuit breaker shall be required to interrupt at rated maximum voltage and on the standard operating duty.
It also establishes, by fixed ratios, the highest currents which the breaker shall be required to close and latch against, to carry, and to interrupt. For numerical values of rated shortcircuit current, refer to the tables of preferred ratings in ANSI C37.06-1979.
The relationship of rated short-circuit current to the other required capabilities is illustrated graphically in Figs 1 and 2.
Figure 2— Ratio of Circuit Breaker Asymmetrical to Symmetrical Interrupting Capabilities
Related Required Capabilities.
The circuit breaker shall have the following related required capabilities which are based on a relay time of one-half cycle, but may be used with any permissible tripping delay.
Required Symmetrical Interrupting Capability for Polyphase and Phase-to-Phase Faults.
For polyphase and phase-to-phase faults, the required symmetrical interrupting capability of a circuit breaker is the highest value of the symmetrical component of the short-circuit current in rms amperes at the instant of primary arcing contact separation which the circuit breaker shall be required to interrupt at a specified operating voltage on the standard operating duty and irrespective of the direct current component of the total short-circuit current.
The numerical value at an operating voltage between 1/K times rated maximum voltage and rated maximum voltage shall be determined by the following formula: In no case shall the required symmetrical interrupting capability exceed K times short-circuit current.
The numerical value shall be equal to the product of a ratio S, specified below and illustrated in Fig 2, times the required symmetrical interrupting capability of the breaker determined for the operating voltage. The values of S shall be 1.4, 1.3, 1.2, 1.1, or 1.0 for breakers having primary arcing contact parting times of 1, 1.5, 2, 3, 4, or more cycles, respectively.
The values of S for primary arcing contact parting times between those given above shall be determined by linear interpolation. The primary arcing contact parting time shall be considered equal to the sum of one-half cycle (present practical minimum tripping delay) plus the lesser of:
1) The actual opening time of the particular breaker, or
2) 1.0, 1.5, 2.5, or 3.5 cycles for breakers having a rated interrupting time of 2, 3, 5, or 8 cycles, respectively.
NOTE — Any combination of symmetrical and direct current components is permissible, provided that the following conditions are met at the instant of primary arcing contact separation:
1) The symmetrical component does not exceed the required symmetrical interrupting capability
2) The degree of asymmetry does not exceed 100 percent
3) The total short-circuit current does not exceed the required asymmetrical interrupting capability
Required Interrupting Capability for Single Line-to-Ground Faults.
For single line-to-ground faults, the required symmetrical interrupting capability and the required asymmetrical interrupting capability of a circuit breaker shall be 1.15 times the corresponding values for polyphase and phase-to-phase faults. In no case are the capabilities for single line-to-ground faults required to exceed K times the symmetrical interrupting capability (that is, K times rated short-circuit current) and K times asymmetrical interrupting capability, respectively, determined at rated maximum voltage.
Required Closing-Latching-Carrying-Interrupting Capabilities.
The circuit breaker shall be capable of performing the following duties in immediate succession:
1) Closing and, immediately thereafter, latching any normal-frequency making current which does not exceed 1.6K times the rated short-circuit or whose maximum crest (peak making current) does not exceed 2.7K times the rated short-circuit current.
2) Carrying a short-circuit current I for any time up to the permissible tripping delay determined in accordance with 5.8.
3) Interrupting any short-circuit current which, at the instant of primary arcing contact separation, has a symmetrical value not exceeding the required asymmetrical interrupting capability.
