Do I have this right regarding fusing?

Being a non-electrical engineer (but an engineer in another unrelated field) I have a question regarding fusing.

If I have a 72V-12V DC-DC converter that is rated @ "50A", I assume the rating is for the 12V side -- correct? As such, I would attach a 50A fuse to the 12V output before the load and I should be good to go.

But for completeness, I'd like to add a circuit breaker the 72V input to the DC-DC converter. Because of the higher voltage, I assume I would need a much smaller amperage circuit breaker. Something on the order of 50*12/72 = 8.3A. Or approximately 10A. Do I have this right?

Thanks
M

Sounds right, but the converter is not 100% efficient, so the current will be higher by the actual efficiency (could be anything from as low as 50% to as high as something in the 80s, maybe more). If there's no specs on any of that you might want to assume the worst case.

Breakers usually have various ratings about time vs overcurrent, so check the one you're going to use to be sure that it will pop in a short enough time at a low enough overcurrent to protect what you want it to, but will not pop at any of the continuous usage you need the system to run at.

Also make sure it is a DC breaker--AC breakers may not break a DC circuit as the arc doesn't get interrupted like AC, and they have no way to cause it to break if it doesn't do it on it's own, so itll keep arcing and intensely heating inside, and could catch fire. (but still won't break the circuit).

Keep in mind the fuse and breaker are not just to protect the devices; they are also to protect your wiring. So size them also so that a wiring fault downstream would blow them before letting the wiring heat to a point the insulation fails, and place them as close to the upstream sources as possible.

If you want to output the full 50 amps, you’ll need to account for the efficiency losses of the unit (may need more than 8.3). If the breaker is being used to protect the run of wire between the battery, I think you want to size the breaker based on the capacity of the wire, since that’s what you’d be protecting from melting. The output fuse protects the unit.


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