Measuring resistance across the line and load of a breaker does not measure resistance of the "bimetallic element. The resistance that is measured is that of the contacts themselves. Now, I know where this will lead.
Now I would expect the reply now to be "contact resistance is not good in that the contacts would heat." Contact resistance can vary all over the map. Each time the breaker is opened and closed it changes as the moving and stationary contacts rub together. And when measured with an ohmmeter the voltage used to measure the resistance is minimal when compared to that which the breaker is actually applied to. Under actual operating conditions with voltage applied one would have to actually measure the line to load voltage which represents the voltage drop due to resistance within the breaker. That voltage along with the current value could be used to calculate the resistance using ohm’s law.
Remember that voltage drop equals heating which affects the temperature rise of the breaker. Breakers do give off heat.
Should one elect to measure resistance across the line load of a breaker the results should never be used to determine the condition of the beaker unless the readings a very high of show and open circuit. The breaker should be exercised. Also in many cases what may look like a questionable condition often changes to an acceptable condition when placed into service over time.
The industry recognized testing of molded case circuit breakers is NEMA AB4.
The best thing that a person can do is simple take a breaker apart. It takes a lot if guessing as to what is happening inside away.
