bjarney,
With regards to the likelihood of a fault on a branch breaker tripping an upstream breaker. The important thing to consider is the amperage size of the devices and the instantaneous setting of the upstream device. Any fault that occurs that is equal to or greater than the instantaneous setting of the upstream device could trip both breakers. Since a small percentage of overcurrent conditions are short-circuits, the chance is relatively small, however, most people have seen this situation happen.
JBD,
You bring up a good point. Applying series rated combinations does not always cause a lack of coordination. However, the possibility of a lack of coordination always exists with a series rated combination, especially if a short-circuit occurs. That is how the upstream device can protect the underated downstream device, it opens.
Also, yes not all fully rated systems are coordinated. In fact, the majority of systems that use fully rated molded case circuit breakers are NOT selectively coordinated, because of the instantaneous trip feature with molded case circuit breakers.
However, fully rated systems can be selectively coordinated if you use either fuses and apply proper selective coordination ratios or if you use the more expensive (and bigger) low voltage power circuit breakers with short-time delay.
The important thing to remember, when applying series rated combinations, selective coordination is NOT the only thing to consider. You must consider other things such as marking, selecting proper combinations and application of these combinations, as well as other requirements and considerations. See the documents we have on the Bussmann Website for more information.
