Is fuse wire DC rated?

[mascip]

One of my client has on his boat an old Wylex fuse box with fuse wire, like the one in that video.
It's used on his 12V DC system. I think that it's not safe, but wanted to check with yous first.

As explained on this page, AC fuses which are not DC rated should not be used on DC circuits. DC fuses must create a gap quick enough and big enough in the circuit, that the arc created won't cause damage or keep on letting current pass. I am pretty sure that fuse wire does not comply with this, as it is not explicitly DC rated. But an electrician whose opinion I respect tells me that it is good enough, that he has seen it used on DC systems many times over the years, etc. But because he hasn't seen it cause accidents, doesn't mean that it cannot cause accidents.

Any opinions on this please? Does anyone know for sure whether fuse wire is DC rated or not?

 

[Wilko]

I am always worried about what can go wrong, and a couple of things come to mind.
Good of you to show the board ! As JW says the 230V fuse wire is intended to carry periods of overload without melting, but to melt quickly or vapourise with a fault current. There's a performance graph it has to meet and all. I'd be surprised if the circuit current vs melt time was the same for 230V AC fuse wire and 12V DC fuses for your car, say. Then, if there was a short circuit and the fuse wire vaporised as it is intended and if there was an explosive atmosphere in the hold of the vessel, well the fuse may become a different kind of fuse... Just saying.

 

[mascip]

Unless some company somewhere makes DC rated fuse wire... But I doubt that.

For explosive atmospheres (say, near a petrol engine) you would use ANL fuses, which do not produce a spark when they blow. They are rated "ignition protected" for this. But it is not an issue on that particular boat (diesel engine, and the fuse box is far from the engine, in a different room)

 

[UKMeterman]

More detail needed, what is the load, what is the supply and where is the fuse box?

 

[mascip]

I am not sure why more detail is needed, to answer my specific question about fuse wire and DC ratings, but here you go:

Supply: 2 x12V batteries in parallel
Loads: various low power appliance (about 10 LED light bulbs, 3A water pump, and a few more things, all small)
Where is the fuse box: in a ventilated cupboard inside the boat, about 4 meters of cables away from the batteries which are outside of the boat under the back deck, very ventilated.

 

[Lucien Nunes]

Of course wire fuses work on DC - how do you think they protected DC circuits before cartridge fuses became popular? All the old cast-iron switchfuses etc with a rotary switch lever on the side and porcelain fuseholders were designed for use on DC mains as well as AC, and that could be up to 250V DC. The arc when a fuse ruptures can last longer and dissipate more energy on DC, but in this case because it's ELV, there's much less arcing than on 230V AC. The Wylex fuse box will state 'AC Only' because of the micro-break main switch; this would not reliably interrupt 230V DC but will be OK on 12V.

What is more interesting on a 12V circuit is the time/current characteristic of a rewireable, as the short-circuit current of a long thin 12V circuit is usually very low and the usual requirement to derate the circuit may apply due to the higher fusing factor of the rewireable. But this is because of the very low voltage, not a matter of whether it's AC or DC.

The article linked above contains some inaccuracies and I would not use it as a guide.

 

[Clive P]

I have used MCB's before on canal boat consumer units. These were schneider, and I used them following the guidance in their catalogue. They have a complete page detailing their usage on DC circuits which I am guessing is mirrored to other makes as well.
Not used rewireables,but can only assume they would be acceptable using the correct information to determine their breaking capacity etc.
Regards.

 

[westward10]

As Lucien states the breaking arc will last longer because of the constant current flow. AC constantly changes cycle and when this hits the zero point it immediately stops.

 

[UKMeterman]

It might be an idea to consider an automotive blade fuse folder?

 

[HandySparks]

At 12V I wouldn't worry about it.

It might be an idea to consider an automotive blade fuse folder?

Good point.
However, when you consider the size of push-in automotive fuses, the existing BS3036 rewirables should be fine.

 

[mascip]

Very informative answer Lucien, thanks a lot

The big question then, is how to apply fusing factors to wire fuses, when being used on a 12V DC system.

If I cannot answer this question, then I cannot fuse the circuits appropriately, or guarantee to my client that he is appropriately protected.

Changing to an automotive fuse box is a good idea, but it seems like an unecessary expense if the current fuse box is good enough for his needs.

But I would need to figure out how to apply fusing factors to these wire fuses. It sounds like something simple to learn, but by searching on Google for 20 minutes I wasn't able to find the relevant information. Anyone please?

 

[GMES]

Just thought I would pop this in the thread purely based on the topic of fuses, I have not read it so please read in detail before using it as a guide in future.