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My question is rather simple, when and why do one use a 1-pole or 2-pole circuit breaker in DC (+24VDC and GND)?

JavaCake
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  • Do you have (or know of) any example circuits that use either techniques? – Andy aka May 10 '13 at 14:44
  • A motorcontroller i used few months ago from roboteq demonstrates in the datasheet that the GND is directly connected to the battery, while the +24VDC supply is connected to a fuse/circuitbreaker. – JavaCake May 10 '13 at 14:48

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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.

supercat
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  • What about simple devices such as DC-DC converters? – JavaCake May 10 '13 at 16:56
  • @JavaCake: Decide whether you regard as plausible any fault condition where current could flow through either wire even if the other was interrupted. You can't guard against every conceivable fault condition (e.g. wires on both sides of the circuit interrupter somehow both getting shorted to some other wire); the question is whether fault conditions that could be resolved with a double breaker are significantly more likely than those which can't be practically handled at all. – supercat May 10 '13 at 17:09
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I think that in the case of dc batteries connected to a negative-to-ground system; there should be only one pole that can be interrupted as, if they were two, you could isolate the batteries from ground when the breaker trips

JoseChile
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  • Welcome to EE.SE, Jose. You could add a bit more explanation to your answer. Many vehicle battery isolators are in the negative lead between the battery and chassis so isolating the battery is not a problem. – Transistor Oct 09 '18 at 22:12