32V rated fuse in 24V system?

[Jesswrk88]

Hello all,

I’ve been told I should look for a fuse rated higher than the typical 32V for my 24V system. Is this true? If so, what is the minimum voltage rating I should use?

 

[FilterGuy]

The general rule of thumb I use is 125% of max expected Voltage. 24V LiFePO4 goes up to 29V so Something around 36V min.

 

[Dzl]

I am not speaking from a place of certainty here but, unlike current ratings, I have always trusted maximum voltage ratings from reputable manufacturers to be accurate and to have already accounted for proper safety margins. 32V is a common maximum voltage for marine components. I personally would feel comfortable using a 32V fuse in a 24V nominal system. The blue sea fuse block, and (some of) the red battery switches, and their ANL fuses are a few examples of 32V rated devices that are commonly used on 12 and 24 volt systems.

That said, I have two rules of thumb that dictate that you shouldn't listen to what I just said:
1. When presented with two conflicting opinions on an issue that matters with no clear right answer, go with the more conservative one until you know more.
2. Filterguy tends to be right

 

[FilterGuy]

If you google around on fuse ratings, you see discussions about building in margin for current... but don't often see anything about voltage other than the manufacturers rating so @Dzl is probably more on target than I am. I have no real reason for a 25% margin other than the fact that Murphy's law is optimistic and Karma hates me. Consequently I tend to over-design and never like to run something right at it's max rating.
(My Alphred E. Newman Avatar with "What, me worry?" is a sarcastic jab at myself)

A 32 volt rated fuse has a ~10% margin over the max voltage you are likely to ever see on a 24V LiFePO4 circuit.... If you can find something DC rated for more than 32v DC without paying an arm and a leg i would go for it. If you have a problem finding something affordable, you will probably be just fine with 32V.

If you think about it, voltage rating on a fuse only comes into play for 2 reasons:

1) When it blows, you don't want an arc forming causing issues.​

2) Insulation properties in normal operation (You don't want arcing between conductors in normal situations).​

My guess is the rating is more about the first reason. It would probably have no problem at 100 volts as long as the current stays low.
Even at 32 volts, the fuse sustaining an arc when it blows seems very unlikely unless the fusible link is extremely short.

I just found this at DigiKey:

As mentioned earlier, the voltage rating for a fuse is also very important. The voltage rating is the maximum voltage that the fuse can safely operate at when an overcurrent situation happens. The voltage rating on the fuse can be higher than the voltage present on the circuit, but not the other way around. If a 250 VAC fuse is used on a device that uses 120 VAC, there should be no problem. If a 125 VAC fuse is used on a 250 VAC supply, the fuse will be damaged and the likelihood of arching is great when the fuse blows.

This goes along with what @Dzl said...... but I still like to over-spec when I can.

 

[Supervstech]

If you google around on fuse ratings, you see discussions about building in margin for current... but don't often see anything about voltage other than the manufacturers rating so @Dzl is probably more on target than I am. I have no real reason for a 25% margin other than the fact that Murphy's law is optimistic and Karma hates me. Consequently I tend to over-design and never like to run something right at it's max rating.
(My Alphred E. Newman Avatar with "What, me worry?" is a sarcastic jab at myself)

A 32 volt rated fuse has a ~10% margin over the max voltage you are likely to ever see on a 24V LiFePO4 circuit.... If you can find something DC rated for more than 32v DC without paying an arm and a leg i would go for it. If you have a problem finding something affordable, you will probably be just fine with 32V.

If you think about it, voltage rating on a fuse only comes into play for 2 reasons:

1) When it blows, you don't want an arc forming causing issues.​

2) Insulation properties in normal operation (You don't want arcing between conductors in normal situations).​

My guess is the rating is more about the first reason. It would probably have no problem at 100 volts as long as the current stays low.
Even at 32 volts, the fuse sustaining an arc when it blows seems very unlikely unless the fusible link is extremely short.

I just found this at DigiKey:

This goes along with what @Dzl said...... but I still like to over-spec when I can.

Heh... the quoted article thinks an overvolted fuse will have a high likelihood of arching... I guess fuses do kinda lean over when they blow...

 

[Dzl]

but don't often see anything about voltage other than the manufacturers rating so @Dzl is probably more on target than I am.

Well shit...
I have a rules of thumb for when two people disagree with each other... I haven't yet developed a rule of thumb for when two people disagree with eachother but then both tell you the other guy is probably right

I think this is spot on:

A 32 volt rated fuse has a ~10% margin over the max voltage you are likely to ever see on a 24V LiFePO4 circuit.... If you can find something DC rated for more than 32v DC without paying an arm and a leg i would go for it. If you have a problem finding something affordable, you will probably be just fine with 32V.

If you think about it, voltage rating on a fuse only comes into play for 2 reasons:

1) When it blows, you don't want an arc forming causing issues.​

2) Insulation properties in normal operation (You don't want arcing between conductors in normal situations).​

My guess is the rating is more about the first reason. It would probably have no problem at 100 volts as long as the current stays low.
Even at 32 volts, the fuse sustaining an arc when it blows seems very unlikely unless the fusible link is extremely short.

From looking at AIC ratings, I think you are probably correct about the reasons and the implications. In regards to point #1, regardless of max voltage rating, from what I have observed Ampere Interrupt Capacity (AIC) is higher at lower voltage and vice versa.

As an example an MRBF fuse is rated for 10,000 AIC @ 14V may be rated for 5,000 AIC at 32V and 2,000 AIC at 58V. Other fuses like an ANL fuse do not specify different AIC at different voltages, they only state a maximum voltage 32VDC and AIC 6000 @ 32V

I would guess your point #1 would be the primary factor in a fuses voltage rating with #2 being a factor as well (either for the fuse or the holder). And I would guess #1 would not be a catastrophic hard limit (e.g. 32V safe 33V unsafe) but rather a tested reference point (e.g. tested to break 5000A at 32V). <----But this is speculation that should not be taken seriously without evidence.

 

[HaldorEE]

The good thing about a fuse blowing is it almost certainly going to happen due to an overload on the battery, not because you were charging the battery too hard.

During an overload, the battery voltage is going to be lower than nominal, not higher. So a 24V battery (nominal 25.6V) is almost always going to be below 24V if the fuse blows. Therefore I have no reservations about using 32V rated fuses or circuit breakers with 24V systems.

Course I did buy a 48V, 150A circuit breaker for my 24V, 280 AH battery pack. Does this make me a hypocrite? Not really, the 48V wasn't significantly more expensive than the 32V part so I bought it instead because, why not.

Blue Sea Systems 7189 Surface Mount 285-Series Circuit Breaker 150 Amp

I am using 1/0 wiring and powering a 2000W inverter so 150A is plenty (nominal current draw for a 2000W inverter at 90% efficiency is 86A). Circuit breakers and fuses don't trip or blow until you exceed 130% of rating for several minutes (or over 200% for 10 seconds or so). Even when delivering the 4000W max surge power my inverter is rated for, I don't expect the circuit breaker to trip. If it did at that point then I am probably going to be very glad it did.

Here is the time vs % rated current trip chart for the linked circuit breaker.

 

[Jesswrk88]

Thanks guys, I really appreciate your advice! I think I’ll try to get my hands on a higher rated fuse but if I need one in a pinch I’ll feel ok with 32V. I have another question too if y’all don’t mind me picking your brains.

I came across these two fuse holder/busbar combos and think I’d like to give one of them a shot rather than using a simpler bus with separate fuse holders in-line. The Victron uses mega fuses and the Littelfuse model uses MRBFs. Any experience with these busbars or words of wisdom in picking between the two? Preferences of one fuse type over the other? I have a 3000w inverter and I don’t really see myself hitting the 6000w momentary surge it’s capable of but plan to use a 300a fuse there in case I get close every once in a while.

Littelfuse 0FHZ0005Z FHZ Series ZCASE Fuse Holder, 5 Stud Assembly | Waytek

Littelfuse 0FHZ0005Z FHZ Series ZCASE Fuse Holder, 5 Stud Full Assembly with M8 Input, 400A is in stock and ready to ship. Order today!

www.waytekwire.com

Victron 6-Way Mega Fuse Holder w/Busbar - 250A

 

[HaldorEE]

Thanks guys, I really appreciate your advice! I think I’ll try to get my hands on a higher rated fuse but if I need one in a pinch I’ll feel ok with 32V. I have another question too if y’all don’t mind me picking your brains.

I came across these two fuse holder/busbar combos and think I’d like to give one of them a shot rather than using a simpler bus with separate fuse holders in-line. The Victron uses mega fuses and the Littelfuse model uses MRBFs. Any experience with these busbars or words of wisdom in picking between the two? Preferences of one fuse type over the other? I have a 3000w inverter and I don’t really see myself hitting the 6000w momentary surge it’s capable of but plan to use a 300a fuse there in case I get close every once in a while.

Littelfuse 0FHZ0005Z FHZ Series ZCASE Fuse Holder, 5 Stud Assembly | Waytek

Littelfuse 0FHZ0005Z FHZ Series ZCASE Fuse Holder, 5 Stud Full Assembly with M8 Input, 400A is in stock and ready to ship. Order today!

www.waytekwire.com

Victron 6-Way Mega Fuse Holder w/Busbar - 250A

They both look good. The first one is rated for 400A max, the Victron is 500A. Either one should work fine.

Thanks for the link to wayteckwire. I hadn't seen their website before.

 

[Dzl]

Waytekwire seems like a solid resource, haven't bought from them yet, but they are one of the first places I look for fuses. They have good prices on quality brands, and a long track record.

As to the busbars both Littelfuse and Victron are reputable brands. I don't know much about mega fuses, but I have a high opinion of MRBF fuses. They are also pretty cheap if you buy them from Waytek.

 

[Dzl]

As to voltage:
Here is what I found in the ABYC code (not sure what NEC would say):

Fuses shall have a voltage rating of not less than the nominal system voltage.

 

[Dzl]

Fuses shall have a voltage rating of not less than the nominal system voltage.

The interesting implication of this is that the ABYC felt confident enough that fuse voltage ratings have enough built in safety margin to allow a fuse to have a voltage rating equal to the systems NOMINAL voltage. Meaning a fuse rated at 24V would be allowable in a nominally 24V system that could see up to ~30V.

Personally I would not cut it this close, but its nice to know that informed and conservative people believe that you could do so safely.

 

[HaldorEE]

If there is anything that was not going to push the margins of safety I would hope it is a fuse or a circuit breaker.

Then again, FPE was a real thing.

FPE Stab-Lok® Electric Panels - Exxon Buys A Scandal-Federal Pacific Electric Scandal Along With A Company, Business Week Article

inspectapedia.com

I worked for Reliance Electric/Toledo Scale when Exxon bought them. The whole FPE thing came to light during Exxon's in depth audit of all the companies they bought when they purchased Reliance Electric.

I took part in the FPE recall. Toledo Scale had a large national service organization that Exxon drafted into replacing literally 10s of thousands of Industrial Breakers. How Exxon got away with not having to recall the residential FPE breakers I will never understand. Those Stab-Lok breakers were far worse than any of the industrial breakers we replaced. Course there where millions of those out there.

 

[HaldorEE]

FPE wasn't Reliance Electrics fault. They got scammed too. FPE was just a corrupt organization. They were paying off the UL inspectors stationed at their factory to look the other way.