farmelectrics, look at the front of a MCB. It should have two sets of specification information (beyond BS numbers and manufacturer's ID info). A typical example might be B6A (indicating a 6A breaker as typically used for overcurrent protection on a domestic lighting circuit) and 6000 (or 6kA) (indicating the maximum withstand current under fault conditions). The breaking capacity of 6kA may be written on the side of the MCB, or it may, for example, be presented as 'M6' (on MEM devices)
So we are looking at two different things here - overcurrent protection for the circuit and the fault current capacity of the device that provides the overcurrent protection.
Typical examples:
A lighting circuit is overloaded with light fittings. All lights are switched on. They draw 8A of current (yes, a lot of lights, but not impossible to achieve!). The 6A MCB protecting the circuit will (eventually) trip.
Alternatively, a lamp blows. There is a momentary short circuit of the lamp element, and a large current is drawn. Perhaps in the order of 100A. This will trip the MCB.
In both of the above cases, the MCB, which is rated to be able to withstand a fault current of 6kA, will operate and can be safely reset and used again.
It also helps if you understand that fault current rises, it is not instantaneously at its maximum. The longer a fault is allowed to exist, the higher the current rises. In the second example above, the fault existed for perhaps a few milliseconds before the element disintegrated.
Another scenario - a line conductor comes loose and touches an exposed conductive part of the installation. This fault continues for seconds, minutes even. Potentially huge earth fault current ensues (4.3kA in the example you quoted). If this fault current is less than 6kA then the MCB will safely trip and can be reset and reused.
If the fault current was 8kA then it is possible that either the MCB will just melt and the contacts will be welded together so that the MCB is unable to trip, or the MCB may actually explode and could harm persons nearby or could start a fire.
What limits the fault current is the Earth Fault Loop Impedance. If the impedance is too low (yes, low impedance can be a bad thing!) then the fault current would be too high. We need to design our circuits and select our equipment to maintain an acceptable balance. On the one hand, we want to encourage fast-rising fault currents (by keeping Ze and Zs low enough) but we do not want Ze and Zs so low that the fault current rises above the withstand capability of the protective devices used.
I hope that this, along with Tel's description above, helps you now understand the difference between the two specifications.
