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Many electrical engineers thought that interrupting rating and interrupting capacity has a similar meaning. For this reason, we need to know its key differences and its effect when applied to electrical design or installations.
Interrupting rating is the maximum short-circuit current that an overcurrent protective device can safely interrupt under standard test conditions. One should notice the term “under standard test conditions” which means, it is important to determine how the overcurrent protective device is tested in order to assure it is properly applied.
According to the IEEE Standard Dictionary of Electrical and Electronic Terms, interrupting capacity is the current at rated voltage that the device can interrupt.
Standard Test Conditions - Fuses
The branch circuit fuses are tested without any additional conductor in the test circuit. For example, if a fuse has an interrupting rating of 300 kA, the test circuit is calibrated to have at least 300 kA at the rated fuse voltage. During the test circuit calibration, a bus bar is used in place of the fuse to check the proper short-circuit current. Then the bus bar is removed and the fuse is inserted; the test is then conducted. If the fuse passes the test, the fuse is marked with this interrupting rating (300 kA).
In the procedures just outlined for fuses, there are no extra conductors inserted into the test circuit after the short-circuit current is calibrated. A major point is that the fuse interrupts an available short-circuit current at least equal to or greater than its marked interrupting rating.
In other words, because of the way fuses are short-circuit tested (without additional conductor impedance), their interrupting capacity is equal to or greater than their marked interrupting rating.
Standard” Test Conditions - Circuit Breakers
Compare to fuses, it is not the case with circuit breakers. This is because of the way circuit breakers are short circuit tested (with additional conductor impedance), their interrupting capacity can be less than their interrupting rating.
When the test circuit is calibrated for the circuit breaker interrupting rating tests, the circuit breaker is not in the circuit. After the test circuit has been verified to the proper level of short-circuit current, the circuit breaker is placed into the circuit. Accordingly, in addition to the circuit breaker, important lengths of conductors are permitted to be added to the circuit after the calibration. This additional conductor impedance can result in a significantly lower short-circuit current.
So a circuit breaker marked with an interrupting rating of 22,000A may in fact have an interrupting capacity of only 9,900A.
- Cooper Bussman.