Why all those current ratings?

The basic rating is continuous current, the value of amperes intended to flow through the device. (This is sometimes described as "operational current" or "thermal current.") When we speak of a "100 ampere fuse," for example, we mean that a load drawing 100 amperes continuous current is the maximum for which that fuse is intended to provide fault protection. How long is "continuous"? "More than three hours," according to one definition.
Although we tend to think of "inductive" and "resistive" circuits as two separate entities, the distinction is only one of degree. Whether a-c or d-c, all real circuits contain both resistance and inductance. Opening a current-carrying circuit will therefore always result in an arc across the opening contacts. How intense-and therefore how damaging-that arc may be depends upon the relationship between resistance and inductance, expressed by the circuit power factor.
As long as the current (and therefore the arc intensity) does not exceed a design level, a fuse or circuit breaker will snuff out the arc before too much damage is done. Current at that point is the interrupting capability of the device. Sometimes called the breaking current, it will far exceed the basic "continuous current rating."
It's often assumed that this is the amount of current that will actually cause the fuse or breaker to open the circuit. That's not the case. As the National Electrical Code explains, in Article 100, this rating is "The highest current at rated voltage that a device is intended to interrupt under standard test conditions." Those test conditions are outlined in UL standards. Suppose a voltage source can theoretically supply 50,000 amperes to a short-circuit. The complete circuit including that fault must have an impedance that includes the connecting wire from the power source to the breaker being tested, and that impedance will reduce the actual current the breaker sees during the test to something less than 50,000 amperes. Thus, there may be no contradiction in applying a circuit breaker tested to "only" 40,000 amperes, for example, in a power system capable of supplying 50,000 short-circuit amperes (see Figure 1 for typical relationship).
Two different interrupting ratings may be assigned. In a medium-voltage power circuit breaker, for example, the full magnitude of electromagnetic stress exists immediately upon initiation of a fault. That imposes a short-time or momentary current limit, such as 60,000 amperes for a 1200 ampere unit. The thermal stress accumulates with time, leading to a lower interrupting current limit of 35,000 amperes. (Low-voltage breakers operate so quickly that this duality does not exist.)
Whenever abnormally high current flows through a circuit component, that component is subject to two kinds of damage. The first is the combination of thermal stress and mechanical force created by the flow of current. Depending upon the device and the applicable standard (usually UL or NEMA), a withstand rating exists to define the maximum amperes that can flow without causing such damage to the device itself. Any switch or contactor not intended to clear a fault must have a withstand rating with which upstream protective device behavior must be coordinated.
Interestingly, if you look up withstand rating in the index to the National Electrical Code, in the IEEE Dictionary, in power system design literature, or in industry standards for various devices, you won't find the term listed. One definition of this rating is: "the ability ... to withstand the let through energy of a short circuit protective device under fault conditions. . . ." Some documents do mention surge voltage withstand, which has nothing to do with current.
Adding confusion to all this is the wording of one UL control equipment standard in which a table of "interrupting current" values is referred to in the text as "withstand values."
A second source of damage to any switching device (and its surroundings) is contact arcing. This applies only to devices intended to open the circuit either when a fault exists or when a control function dictates either opening a closed current-carrying contact, or closing an open one in a "live" circuit.
Source: findarticle.com