How interchangeable are AC and DC circuit breakers?

A friend recently asked my engineering opinion on the matter of using DC (Direct Current) circuit breakers in the place of AC (Alternating Current) circuit breakers in a circuit. Now I’d be there first person to tell you that my engineering skills are a bit rusty, but I guess like learning to ride a bicycle it never quite goes away. Forward, onto the breach!

Disclaimer

I’m not an expert even though I sometimes pretend to be one online. This advice is for information only and it is up to YOU to ensure that your electrical installation meets local laws and guidelines.

AC in the place of DC

Now this seems to be the most common situation. People want to use the more commonly available AC MCB (Miniature circuit breaker) for their DC based system. This is a bad idea. For exhibit one I call the following video:

This video shows the fundamental difference between AC and DC currents. AC current has a handy zero crossing every half cycle that makes it easy to extinguish an arc. DC on the other hand is full on, all the time. It takes a lot more effort to extinguish the arc in a DC circuit. DC circuit breakers are specially designed to extinguish DC arcs and have special magnets to pull the arc away to get it extinguished.  Thus, don’t use AC MCB’s for DC circuits. For a more in depth explanation have a look at the links below.

References:

For further reading have a look at

DC in the place of AC

This arrangement is a bit more tricky. From the outset it looks like this arrangement should be fine as long as the breaker meets the required voltage isolation levels. However therein lies the rub, if the device does not meet the required isolation levels or have the right approvals, it does not meet legal requirements. There are certain MCB’s that can be used for both AC and DC circuits but these would be clearly marked. Information on this arrangement was a bit harder to find. Have a look a the thread below.

References:

For further reading have a look at

Other considerations

This diagram from an ABB MCB datasheet shows how the trip curves differ for AC and DC. So in other words the performance varies depending on the current type. This is another reason not to use these breakers interchangeably without proper design consideration.

Screen Shot 2016-05-09 at 8.44.31 pmThis is for a circuit breaker that is designed for both AC and DC. You can find the datasheet here.

For a much more detailed discussion on the intricacies of DC circuits and safety I can heartily recommend the ABB circuit-breakers for direct current applications technical application papers.

Conclusion

My recommendation would be to use DC MCB’s for DC and AC MCB’s for AC. There are special design considerations that go into the design of these devices and as an ex-colleague told me when I asked him to check this article: “I got burnt with 110V DC in a panel so don’t play with that thing. Make sure its the right breaker that trips fast.
Can’t argue with real world experience.