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Old 28-01-2025, 04:08   #1
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Fuses for Lithium Power Systems

I bought some Heschel 400A NH2 fuses for my lithium power system, and now that I've received them, I see that data sheet does not state any DC voltage spec. Only AC (and 400v).

Most of these fuses are rated 400V AC and 250V DC so way more than needed for an approx 28v boat system.

The NH2 fuses are bulkier than Type T but have higher interrupt rating (120kA vs. 20kA) and are cheaper. Type T is not much used in Europe; NH seems to be favored for lithium systems.

Should I be concerned about the lack of a DC rating? As far as my extremely limited electrical knowledge goes, fuses work the same way whether AC or DC, right? Or not?
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Old 28-01-2025, 05:31   #2
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Re: Fuses for Lithium Power Systems

In general, AC-Only rated fuses SHOULD NOT be used in DC voltage circuits, unless the fuse manufacturer provides DC application ratings.
I’d ask the manufacturer, prior to using on DC.*


When you blow a fuse in DC system, a plasma can form, and continue conducting current. AC will too, but when the cycle crosses zero volts, the plasma dies, and the circuit is open.
Because of this zero-crossing, AC arcs are easy to mitigate, as they are self-extinguishing. With DC power, this zero-crossing never occurs, and electrical arcs may persist to the point of causing significant damage/harm.
Fuses that can be used for AC and DC will have different ACV and DCV interruption capacity ratings. AC will always have higher IC ratings.
Alternatively, a fuse intended for DC, could have a larger gap area, to help extinguish the plasma arc, and thus maintain a similar [to AC] IC rating.

* Hensel USA - German made? [I've never heard of Heschel]
800-318-8013
info@hensel-usa.com
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Old 28-01-2025, 05:55   #3
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Re: Fuses for Lithium Power Systems

Thanks. That's not encouraging. I'll query the manufacturer.
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Old 28-01-2025, 05:56   #4
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Re: Fuses for Lithium Power Systems

Another question about fuses --


The datasheets for NH type fuses also have a number for "power dissipation" -- as much as 30W at nominal capacity.


30W? That's like a 7% loss. Why would this be the case? Seems crazy.
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Old 28-01-2025, 06:51   #5
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Re: Fuses for Lithium Power Systems

My understanding is that a fuse is simply a resistor designed so that at a target current its effect is sufficient to melt itself. Effect is current squared times resistance. So there is always going to be some resistance in a fuse. At a nominal 100A (alternator charging for example) 30W effect corresponds to a resistance of just 0.003 ohm. That seems pretty reasonable to me.
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Old 28-01-2025, 07:15   #6
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Re: Fuses for Lithium Power Systems

It was stated confidently by a number of members that the spec sheets for these fuses actually state that they are rated for DC. I never questioned that because I don’t use them. Kudos for checking this instead of ignoring it: I am very curious on your findings!

About power dissipation at the point where it needs to blow, i.e. it’s rated amperage, it needs to generate enough heat to melt the internal wire/metal to interrupt the circuit. There will be graphs showing how long this takes for a certain current. The power dissipation is what generates this heat.

This is why I like to see cabling bigger than required: this allows bigger fuses and less losses at normal use.
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Old 28-01-2025, 07:21   #7
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Re: Fuses for Lithium Power Systems

Thanks to everyone for the education! Learned a few things today. Not just about plasma (!), but about fuse resistance. It's all entirely logical but I had just never thought about it! Electrickery is such a complicated subject.



For this particular case, to be on the safe side, I'm sending these NH fuses back (thank you Amazon returns policy) and I've ordered Blue Seas Type T, 400A ones, with holders. I had not thought they were available in Europe, but it turns out they are, and not massively more expensive than the NH ones. With the additional benefit that they are much more compact.
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Old 28-01-2025, 08:12   #8
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by s/v Jedi View Post
...
About power dissipation at the point where it needs to blow, i.e. it’s rated amperage, it needs to generate enough heat to melt the internal wire/metal to interrupt the circuit. There will be graphs showing how long this takes for a certain current. The power dissipation is what generates this heat.

This is why I like to see cabling bigger than required: this allows bigger fuses and less losses at normal use.
A fuse's resistance can also change with temperature. Hence, why values are often stated as cold resistance @ 25°C test temperature.
Manufacturers may provide a chart showing the effects of ambient temperatures on specific fuses.
A fuse element doesn’t react directly to the current level [amperage)], so much as it does to all combined sources of heat, that may cause it to reach its melting point. Resistance to current is a function of the fuse, but outside sources also contribute to the temperature rise of the fuse element [localized area] material.
Fuseholders, cabling, enclosures, soldering, and many other outside features can add to localized fuse heating [beyond ambient].


For UL rated fuses, the continuous operating current should be no more than 75% of the fuse rating.
IEC fuses can operate continuously at 100% of rated current.
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Old 28-01-2025, 10:56   #9
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Re: Fuses for Lithium Power Systems

What brand of NH fuse did you buy? I'm sure there are plenty out there, but i've never seen one not rated for DC - often 125v or 250v DC.
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Old 28-01-2025, 12:24   #10
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Re: Fuses for Lithium Power Systems

Quote:
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What brand of NH fuse did you buy? I'm sure there are plenty out there, but i've never seen one not rated for DC - often 125v or 250v DC.
I believe he wrote Heschel in the OP…
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Old 28-01-2025, 16:52   #11
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by s/v Jedi View Post
I believe he wrote Heschel in the OP…
Being from the USA, I'm not familiar with the brand. I took a look at my Siemens catalog and they do show a few AC only NH fuses, but it looks like less than 10%.... seems Dockhead had bad luck on that Amazon order.

Good to know for the future that there are AC only NH fuses out there.
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Old 28-01-2025, 23:53   #12
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by Matt Johnson View Post
What brand of NH fuse did you buy? I'm sure there are plenty out there, but i've never seen one not rated for DC - often 125v or 250v DC.

As others have written, Heschel.


I decided against using these even if the maker informed me that they would be OK for DC use (and they haven't responded). I don't want my installation to have even the appearance of having some big design flaw like wrong type of fuses.


I found Blue Seas Type T fuses in stock in Europe for a reasonable price, and they also sell a very nice, compact, covered fuse block from Blue Seas. This seemed like a better solution.



Also, my boat was built with Type T fuses in the heavy DC circuits (thruster, windlass, winches), and I'd prefer not to have two different types of fuses on board.
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Old 29-01-2025, 00:39   #13
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by Dockhead View Post
As others have written, Heschel.


I decided against using these even if the maker informed me that they would be OK for DC use (and they haven't responded). I don't want my installation to have even the appearance of having some big design flaw like wrong type of fuses.


I found Blue Seas Type T fuses in stock in Europe for a reasonable price, and they also sell a very nice, compact, covered fuse block from Blue Seas. This seemed like a better solution.



Also, my boat was built with Type T fuses in the heavy DC circuits (thruster, windlass, winches), and I'd prefer not to have two different types of fuses on board.
My boat was built 44 years ago with NH fuses for engine, domestic and windlass. Dutch built boat. Lithium system also has NH fuses for the same reason you are staying with class T
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Old 01-02-2025, 23:52   #14
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Re: Fuses for Lithium Power Systems

Quote:
Originally Posted by GordMay View Post
In general, AC-Only rated fuses SHOULD NOT be used in DC voltage circuits, unless the fuse manufacturer provides DC application ratings.
I’d ask the manufacturer, prior to using on DC.*


When you blow a fuse in DC system, a plasma can form, and continue conducting current. AC will too, but when the cycle crosses zero volts, the plasma dies, and the circuit is open.
Because of this zero-crossing, AC arcs are easy to mitigate, as they are self-extinguishing. With DC power, this zero-crossing never occurs, and electrical arcs may persist to the point of causing significant damage/harm.
Fuses that can be used for AC and DC will have different ACV and DCV interruption capacity ratings. AC will always have higher IC ratings.
Alternatively, a fuse intended for DC, could have a larger gap area, to help extinguish the plasma arc, and thus maintain a similar [to AC] IC rating.

* Hensel USA - German made? [I've never heard of Heschel]
800-318-8013
info@hensel-usa.com
The NH fuses are originally made for grid stations and have all AC and DC rating as thats part of the certification for grid stations use.
In DC voltage they always one level down so a AC 690V AC can do 400V DC. A 400V aC NH2 can do 250V DC. The AIC rating goes down for DC by 25% so 120KA AC is 90kA.
All eg 690V AC 250A NH2 fuses are the same as they are identically tested to same certification. On older NH you have always both ratings but the newer mostly only the AC rating is printed on and stated on the data sheet. You can only find their DC rating in the certification document. That's on purpose as some manufacturer offer a seperate NH 00 nearly identical DC only rating line for marine (the whole beneteau group switched to NH main fuse when you order the lithium option) and solar for much higher prices.
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Old 02-02-2025, 02:00   #15
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Re: Fuses for Lithium Power Systems

Dockhead, sorry don't have time to investigate, but do you have the right NH model for DC?

I used NH00 https://www.bimblesolar.com/DC-fuse-disconnect-1pole


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