# Choosing Fuses or MCB in 24VDC vehicle circuit

I am currently designing a 24VDC circuit for a small vehicle. I personally prefer using MCB's since they are simple to reset and easy to detect faults in the system, although i do find it rather hard to calculate the correct size and type of MCB for the circuit due to its changing characteristics compared to normal AC circuits.

So it opens up a few questions on this matter:

1. If i use MCB, how do i handle a short-circuit if the battery is unable to supply enough current to break (eg. under-voltage guard?)?
2. Should i forget all about AC/DC MCB's and use normal fuses instead?
3. What is the best practice when calculating MCB for DC circuits, in means of size and type?

Since i have a very restricted budget, pure DC MCB's or electronic fuses are pretty much out of the picture (unfortunately).

I prefer fuses to breakers - they both "blow" when a fault occurs and so they should! The trick is to calculate wire thicknesses and current ratings of fuses so that there is no risk of fire and the circuit works reliably. Given that you are building a small electric vehicle, efficiency demands low volt-drop wires to maximize the energy available so, agian I'd say this reinforces the use of fuses.

What does my car use? It's got a small panel of fuses located somewhere and it's done 98,000 miles and I don't recall a fuse blowing. I bought it at 4,500 miles! Yeah I've had the older rogue car that seemed to pop fuses now and then but nothing major.

Diagnosing blown fuses - this should be amde as a simple as possible - if you have several circuits teed-off from the battery you should make your fuse panel accessible and easy to get a meter on the potential offending circuits.

Q) Low voltage battery may not supply enough current to blow the fuse/breaker on a short circuit fault. This is OK - it won't melt the cables because you've got the fuse rating right (well you need to have done this of course). Will it damage the battery - quite likely so you need an isolator to disingage the battery when not being used. What if it's needed to keep some circuits alive - use a low value fuse and appropriate cable.

Something that tells you the battery voltage is useful - a low power/energy moving coil meter will do the trick.

• I did absolute not put it that way. But i definitely agree that fuses has been used for decades and manages to work quite fine. Essentially they shouldn't blow if everything is running correctly. You mention that i need an isolator, what do you exactly define as such thing? Jul 10, 2013 at 18:31
• @JavaCake I wasn't trying to imply anything about your knowledge dude. No insult intended. Anyway, an isolator - a switch capable of breaking the supply from the battery so that nothing remains connected to it. Jul 10, 2013 at 18:38
• I did not take it that way, im sorry if my reply misleaded you to think that. I understand now, there will definitely be such switch implemented. Do you have any experience with MCB AC/DC in such curcuits? Jul 10, 2013 at 18:43
• @JavaCake I have no hands-on experience of those devices other than in household power circuits!! Jul 10, 2013 at 21:03