I have an old motorbike which blows a 20A fuse about once a year - no obvious fault, just replace the fuse and it's good for another year. Instead of keeping a stock of spare fuses in my jacket pocket, it would be nice if I could have a 20A MCB under the seat that I just reset as needed.
The contacts won't be designed to break a DC circuit but the voltage is low and the regularity of use should be low too so you will possibly get away with it.
It'll work, but for how long is something else. A proper DC mcb contains magnets to spread the arc across the contact if it opens under load. Without the magnets there is a danger of an AC mcb damaging the contacts (through "pitting") and even eventually welding themselves closed. Also, IIRC, the DC rating of the mcb would be 0.707x the AC rating. That's
They aren't rated for it. There are two potential problems:
Contacts won't be rated to break a DC arc. 12V is very low, so you will probably get away with it in this case, particularly given that modern mains MCB's do try to extinguish the arc ASAP without waiting until the end of the mains half-cycle, so they are a bit more like DC contacts than older breakers were.
Whilst the slow thermal trip will work regardless of AC or DC, it's possible the fast magnetic trip might not work with only
12V available. If that's the case, you may get somewhat slower tripping than with a fuse. It's also possible you might have the opposite problem and the MCB trips on momentary inrush current from a headlamp filament, where the fuse wouldn't.
You are operating it outside its design ratings, so there will be some suck-it-and-see. However, there's also a possibility it might not provide all the circuit protection needed in some cases, and in the worst case, you might end up melting the wiring loom and doing bad damage to the bike.
What does the 20A fuse protect, and do you know what causes occasional tripping? Solving any problems here might be a better solution. Extra low voltage DC breakers do exist, but may not be easy to find (and if you do find them, you must dounle check that their breaking capacity is high enough).
Yes. I use Crabtree CUs with standard MCBs in them for all the 12 volt systems on our boat. I checked specifically that the Crabtree MCBs would work down to 12 volts. I'm sure most makes of MCB will be quite happy down to 12 volts but some are not actually specified to work at that voltage. The Crabtree ones are.
Many moons ago, a person I used to work with had a motor bike just like yours, and all was well for about three years, then one night the bike burned his garage down when an electrical short set fire to the sheet he had covered it with. I guessI'm saying might be safer in the long run to chase down the intermittent short. Brian
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