50 or 30 amp

The idea that all faults are low impedance (meaning direct line to line or line to ground short circuits that produce very high currents), is just not correct. There are numerous examples of high impedance faults on circuits that can produce limited current flows which either by themselves or in combination with actual loads result in currents which (for example) can be in excess of 30A but less than 50A.  Otherwise you could just use one circuit breaker size regardless of wire ampacity and call it good. But you can't. 

Example: You have a standard outlet, we know most are circuit breakered at 15 or 20 A. Somebody wired this outlet to a 50A CB (STUPID), you plug a lamp into it. The lamp has a shorted wire. The wire will get HOT, it could cause combustible materiels near it to light. If the current draw exceeds the CB it will kick, probability is wire will burn in two before CB kicks at somewhere less than 50As.

Example 2: When I first moved into my building, I was tracing outlets back to CBs in panel. I came across a standard outlet under a work bench that had 220 volts on it. Standard outlet. Traced back to panel and it had a double 100A CB on the line. I removed the double 100s, put in a single 20, went back to the outlet feed and removed the now disconnected 120. Now I had a standard 20A outlet.

Lostagain,

Understand better. Hair dryer is an example, heaters, microwaves, anything that has a high current draw. The key in my mind is the CB and/or the GCFI, I'm just talking about a regular outlet device. In a shop for an example, heaters, welders any high current device. The key is the protective devices. Taking the example if I was to take a 50A stove and put it on a 30A circuit with 30A wire, is it going to cause a fire, in my opinion Nope. It's going to kick the 30A CB, due to over current, the chances of fire are less than the fact the apparatus won't work. Adding 50A wire won't effect the load of a 30A CB. In a standard RV, regardless of source, internally the RV is controlled by CBs on the outlets, for example. Example I can run one cube heater in my Pod if I run 2, I trip the GCFI. If you could hook a hair dryer up to a 50A circuit and the cord shorted the wire would burn in two before the CB would kick.