AIC ratings of CB's are tested by mounting them inside a panel with a bolted short accross the CB output. The input current is regulated (10K, 14K, 22K, etc) and they either interupt the fault current without being damaged, or they don't (and typically blow up). If they pass, fine. If not, the CB line looses it's rating and the mfgr needs to make some manufacturing mods.
Available fault current from the poco is simply the max current their equipment will put out with a bolted fault at the lugs on the poco secondary. Some utilities have fused outputs (like So Cal Edison) and their fault currents are relatively lower (48KA max on 480V systems, 10KA on resi). Some don't (like LA-DWP), and their fault currents can be guite high. We were on a project at Ports-O-Call in San Pedro and LA-DWP gave us a fault current of something like 140,000 amps at their transformer. Seriously. Our service was about 300' away, so the wire resistance knocked that down quite a bit - meaning if a fault occured at our service, the actual fault current would be much lower than if the fault occured at the transformer.
Fualt current is different at different points in your electrical system, primarily because there is differing amounts of impedance between the source transformer and different points within the system. The specific fault current at any point is a combination of what's actually available from the poco equipment and the cumulative impedance between the poco equipment and the point you're trying to calculate for. Frankly, I just used wire resistance to come up with rough fault current numbers, the actuals using impedance would be lower.
Radar
