It sounds like you are asking about a fault condition, such as a breakdown in insulation, with a value of resistance to ground that is low enough to cause significant leakage current, but still high enough to cause the total current (i.e., load current plus fault current) to be below the threshhold of tripping the breaker. Is that it? Do you have reason to suspect that this is happening?
I would start by measuring the current at the source and at the load. Put the clamp on ammeter as close to the source (i.e., within the panel, right at the breaker) and as close to the load as you can get. If you are seeing a significant difference, then shut down the load and perform an insulation resistance test. Actually, now that I've said that, it's probably best (safer process) to start with the insulation resistance test.