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Hello All, Wondering why the 48v input model requires an external input fuse but the 12v and 24v models have an internal fuse, i.e. what is the input fuse protecting with the 12v and 24v models? Is it short-circuit and overload protection? or internal circuitry? Knowing what's protected might help me size the 48v fuse. Thanks, Peter
@petermcgowan, all circuits, including both the input and output of a DC-DC converter, need to be independently fused. The purpose of a fuse is more to protect the wiring of that circuit than necessarily the device that the wiring is connected to; the fuse selected must be selected first appropriate to the rating of the cable used and second appropriate to the maximum current draw of the device x1.25 (generally speaking).
Victron very rarely specifies the size of the fuses to be used in device installation because the fuse is cable-specific before it's device-specific - For instance, a device that pulls 60A (just as an example) but is connected using light-gauge cable that's only good for 40A continuous current should be fused at ~40A rather than ~70A.
Hi Justin, I'm following, I think! Can you help me see where my logic is off... In the case of the 12v and 24v units those internal fuses are not protecting the input wire, the cannot as they're "downstream" of the supply, correct? They're only protecting the internal circuitry? If I wanted to truly protect the wire to those units I'd need a fuse suited to the input wire gauge located close to the battery, correct? Now, If I have similarly located fuse protecting the 48v wire to the unit, what sort of fuse would I need at the device to protect the internal circuitry? With 6AWG wire over a couple of feet I'm reasonably sure a fuse that protects the wire (70amp) won't protect the device. I can find a 30amp fuse that's suited to 48v but I'm not sure that'll protect it either. I can't find any 48v fuse that goes lower than 30amps. It may simply be impossible to save that devices internal circuitry and that's why there's no internal fuse. In which case maybe I've answered my own question. I just want to be sure that I've come to the correct answer.
yes, typically for each power converter there should be:
1) a fuse upstream, located at the far end away from the converter (at the battery in your case). This as you say is protecting the cable run from the fuse to the converter. Ideally you want as short a run as is possible between the battery itself and the fuse, because that run relies on physical protection only (insulation/barrier/etc).
2) a fuse downstream, located as close to possible as the converter. this could be a single fuse or a DC distribution box with fuses for subcircuits.
If the only suitable fuse you can find is a 30A one for the battery fuse, then your cabling from the battery to the converter should have an ampacity of greater than 30A. This nominally ensures that even with a 30A short in the converter (so the fuse doesn't blow), the cable can handle 30A without the insulation overheating and causing fire/smoke.
edit: reread the question. I would suggest that you don't need to worry about applying another layer of protection at the input side of the converter. You do need to have a level of trust that the Victron designers have done their job in engineering the product. If you really want to protect the device, my suggestion would be conservatively fuse for the downstream loads (so the downstream fuse blows before the overcurrent protection kicks in). I don't necessarily agree with that position, but I suspect its more effective than fusing upstream at the converter.
