Interrupting rating; aka Interrupting Capacity (IC) is the amount of current that an Over Current PROTECTIVE DEVICE (OCPD) can SAFELY stop from flowing. So it only applies to things like circuit breakers and fuses. When a high amount of current is flowing, as during a fault of some sort, that current will WANT to continue flowing even as the breaker or fuse acts to stop it. An arc is formed across whatever gap begins to open, and as that arc widens, it acts kind of like a capacitor and the votlage across the gap increases, which serves to keep the arc intact. But the OCPD is specifically designed to keep increasing the gap faster than the arc can keep up with it, until eventually the dielectric strength (the ability of the air gap to stop the flow of electricity) gets high enough. During that "clearing time", the mechanical stress on the device itself becomes tremendous as the magnetic fields increase in strength exponentially to the increase in current flow. So by the time the device extinguishes the arc and stops the flow of current, the mechanical stress is hundreds of times more than it was under normal operation. So the IC rating of the device speaks to how well that device can handle those mechanical stresses at the rated current without turning into a bomb and killing everyone in the area. When an OCPD says for example it has an IC rating of 35kA (35kAIC), that means at 35,000A of current rushing through it during a fault, it will not turn into a bomb. But if the AVAILABLE fault current in the system is 60,000A, it might! So you would get a device rated for 65kAIC if that's the case.
Let-Through current; is related to HOW the OCPD acts to interrupt current flow, and is the PEAK amount of current that gets through it during a high current event in a given amount of time. There are numerous ways of achieving different amounts of Let-Through current values, especially between Circuit Breakers and Fuses. But it's also a lot more complicated than it appears on the surface, so you can't simply look at a chart and pick something out. The concept is very dynamic and there are a lot of what-ifs to consider. Let-Through values are often USED in what are called "Series Ratings" or devices, where the let-through of one device affects the devices down stream. But when that is done, it is generally done on paper, then TESTED to confirm that. A registered PE can sometimes be persuaded to do a calculation-only approval of a series string, but not the average lay person or electrician.
Withstand rating; similar to IC, the withstand rating, now called the Short Circuit Current Rating (SCCR), is related to those high mechanical forces created during a fault, but applied to EVERYTHING in the circuit, not just the protective device. So bus bars, terminals, contactors, switches etc. All of the devices in the current path have to be able to stay together until the protective device can stop the current flow. In a series string of devices, the overall SCCR is going to be the lowest rating component in that string, basically the "weakest link". But since these other devices are not actually DOING the interrupting, it's a little different. You can attain an SCCR rating by coordinating WITH an OCPD, based upon the "Let-Through" current, but again, only via testing in series strings. Most mfrs of equipment now will provide you with a chart showing the SCCR of their devices when protected by specific OCPDs. In the case of Circuit Breakers though, you will find that Mfr. A will only test their devices with their OWN breakers, but not with those of Mfr. B or C. When an SCCR is done with Fuses, the tests are often cdone with a CLASS of fuse, meaning any listed fuse of the same class can be used, regardless of Mfr. You should always double check that though.
Ground Fault Protection (GFP) or more accurately, GFPE (the E is for Equipment); has nothing to do with any of the above, other than indirectly as the way an OCPD is triggered to stop a fault. But having GFP is mandated by code in some circumstances, for example Services that are 1000A or higher. GFPE is NOT the same by the way as GFCI. GFCI is specific to "Class A" personnel protection in North America and is required to act at 4-6mA of current. That becomes VERY difficult to attain as the current in the circuit goes up and above 60A, is almost impossible to attain. But GFPE, for EQUIPMENT, can be anywhere from 30mA to 100% of the total circuit current. So on a 2,000A breaker, the GFP can be set to 100mA, 100A, 1,000A or even 2,000A, depending on what the user wants to protect. The intent however is for GFPE to act BEFORE the long time thermal trips act, so is generally much lower.
