Can you use an MCB on DC?

Straight drop-in for a blade fuse, but tall, so the fusebox lid might not fit.

It's a '77 Kawasaki Z650 - it uses the older cartridge style fuse. I just happen to have a 20A MCB and was hoping that I could use that.

I might experiment by making a load out of some headlight bulbs and seeing what it drops out at.

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

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).

The datasheet (there's a link on the page) specifically allows DC up to 72V with a breaking capacity (max fault current) of 10kA.

I suspect any MCB would work well enough - and it's not a terribly critical application.

You can get smaller devices though - like panel mount with a reset button and similar.

Your biggest problem will be keeping it dry - these things will not like getting even slightly wet.

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.

Surely most of the load will be resistive? Lamp filaments.

Are you using the wrong sort of fuse? You should be using something that is slow blow. In any case I'd be inclined to stick a 25A in.

I have know auto fuses blow for no reason when they've had a life in which the current changes abruptly over a wide range ('off' to almost max, etc).

Bill

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

You can buy circuit breakers specifically designed for car etc use on DC. Vehicle Wiring Products, etc.