1-pole and 2-pole DC circuit breaker
In many applications, it is reasonable to assume that under any plausible fault conditions breaking one particular power supply connection will safely interrupt any fault currents.  In such applications, a single-pole breaker is appropriate.  In some other applications, however, there may exist plausible fault conditions in which either supply connection might by itself be able to supply fault currents.  As an example, suppose that many reversible motors are driven by the same relay, and the wires feeding each motor would of a gauge sufficient to handle one motor's current, but insufficient to safely handle the current of the main breaker.  Assume further that one of the main supply wires is connected to the frame.  The wires for each motor should be protected by an interrupting device whose trip current is low enough that they can't overheat, melt, ignite, etc. but interrupting one wire from each motor would be insufficient to ensure safety if the other wire happens to short to the frame.  The solution would be to have a breaker that will disconnect both wires if either wire has an over-current condition.