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Where to get DC Fuses


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1) When does one need to put a fuse in on battery cables? Is it even when using 1 battery, or only 2 batteries and higher etc? Doesn't an inverter have sufficient protection to stop as soon as there is an overload?

2) I know it has been asked before, but haven't seen any recommendation of what/where to buy fuses from?


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10 hours ago, alkit said:

When does one need to put a fuse in on battery cables?

The short answer is: Always.

The longer answer:

All conductors, even the thickest welding cable, have a certain internal resistance and if you put enough current through there it will get hot. If it gets hot enough, it melts its insulation, and if it gets really really hot it starts a fire. The cable will therefore have a rating, an "ampacity", the maximum current it can carry, which as a rule of thumb is 5 times its cross-section in square millimeters (eg 25 mm^2 can handle about 125A). Of course it takes times for the cable to heat up, so it won't be setting a fire the minute you go over that limit.

A fuse heats up faster than the cable. It ensures that if you go over the limit for an extended period of time, the small encased-in-fire-resistant-plastic fusible link blows first.

On a dead short (5 times the fuse rating or more) it blows instantaneously.

Aaaah, you might say, but what if I make the cable thick enough to handle the short circuit current of the battery? Well sure, you won't be setting the cable on fire then, but the battery isn't going to be very happy with you either. They heat up internally, often destroy themselves, and may even explode in the process. So you still want the fuse.

10 hours ago, alkit said:

Doesn't an inverter have sufficient protection to stop as soon as there is an overload?

Current is "drawn". The current won't flow unless there is a low-enough impedance path to allow it. Most inverters do indeed have sufficient protection to deal with an overload, and will only draw as much current as its been designed for. A Victron Multiplus, for example, will allow 200% of the nominal power rating for one second an no more.

The fuse isn't meant to protect against overloads, and it should be rated high enough to not blow during an overload. You are correct that the inverter has the ability to deal with this and doesn't need the help of a nanny-fuse.

But that's not why you put in the fuse.

The mantra is this: The fuse is there to stop the cable from going up in flames if something goes wrong.

This includes unforeseen things, eg shoot through in the MOSFETs that make up the H-bridge inside an inverter would cause a dead short across the DC bus and something is going to go up in fire... in this case it will be the inverter.

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2 minutes ago, alkit said:

Now just waiting to see if anyone can recommend where to buy them from and what is available?

VisN provided a link above that you should also read. For a battery fuse most people will go for either the Jean Muller or the Mersen units. For smaller setups you can use a MegaFuse. When using a Megafuse, note that there are 32V fuses and 60V fuses. Get one with a sufficient rating.

Most solar places should be able to source these for you. Eg sonop has it and will ship, but they may not be the cheapest option. Do take a look on pricecheck as well.

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