Class T Fuse watts loss or series resistance

[Pappion]

I can't find these specs on any Class T fuse data sheet. Why should they be ignored or hidden? Is it no worse than a wire?
LET, LMT fuses have a Watt Loss spec. About 20w for 200A thru a 400A fuse. Equates to 0.0005 ohms. Loss seems excessive.
Someone with good equipment could do a 4 wire ohm test on a Class-T fuse.
But that's typically at 100ua. More useful would be 50% current rating, 120 sec soak time, current measured, voltage drop measured, ohms calculated.
I walked away from all that high dollar equipment when I retired.

 

[John Frum]

I can't find these specs on any Class T fuse data sheet. Why should they be ignored or hidden? Is it no worse than a wire?

Since the fuse protects the wire it seems unlikely that its worse than the wire.
Even the joinery to and from the class t fuse is ~.0005 ohms per joint.
wire to block, block to fuse, fuse to block and block to wire is ~0.002 ohms.
The actual fuse resistance would be on top of that.

For reference...

Strange BMS cell voltage readings

I have put together a 16S 272Ah Lishen cell pack for testing. I neglected these cells a little so not sure what to expect. Some of them were stored for quite a few months empty. Doing the capacity test, I've rigged up the computer to the UART interface of Jiabaida (JBD) 6S-21S 150A BMS...

diysolarforum.com

Does the diagram seem to be in the ballpark?
Maybe I've misinterpreted it?

 

[Pappion]

Not the answer I am looking for. I want way less than a 20W loss at the fuse.
I saw resistance specs for automotive blade fuses.
Don't have them for ANL fuses, but know they a way lower than circuit breakers.
Really want some numbers for Class-T.
I did engineering work and resent this "trust us, you don't need to know" BS.

 

[Pappion]

Victron came thru with some numbers.
Victron Midi, Mega, and ANL
Only 2 selections of ANL. That brings the 200A @ 12v (2400W), fuse loss down to 10W.
More for Mega
littelfuse_mega_datasheet

 

[John Frum]

Not the answer I am looking for.

I think you want published specs for the fuse resistance.

I want way less than a 20W loss at the fuse.

You could get rid of half the joinery resistance by not using a carrier.
I'm told BS88 fuses fuses don't require a carrier.
If a joint is worth ~.0005 ohms, can a fuse which is meant to protect the circuit by melting have lower resistance?

 

[John Frum]

In the diagram I linked a 40mm2 busbar which is 70mm stud to stud has ~.0007 ohms resistance.
The fuse has to be more than that, no?

 

[John Frum]

I think I see my mistake, you want to know the "hot resistance".
In other words how much energy is lost to heat after the fuse comes up to temperature at a specific current.

 

[Pappion]

I think I see my mistake, you want to know the "hot resistance".
In other words how much energy is lost to heat after the fuse comes up to temperature at a specific current.

Little Fuse document stated they use Positive Temperature coefficient material. So hot is a little higher resistance than cold.

 

[Pappion]

I think you want published specs for the fuse resistance.

You could get rid of half the joinery resistance by not using a carrier.
I'm told BS88 fuses fuses don't require a carrier.
If a joint is worth ~.0005 ohms, can a fuse which is meant to protect the circuit by melting have lower resistance?

I think you over estimate the Carrier/Joinery resistance. You could avoid it and bolt the cable to the same mounting posts for ANL etc.

 

[John Frum]

I think you over estimate the Carrier/Joinery resistance.

Do you mean me or the doco I linked?

 

[RCinFLA]

Rarely do high amperage fuses spec a resistance since the resistance changes depending on amount of current causing fuse metal to heat up and change resistance.

Sometimes they will spec a voltage drop at a given current level.

Resistance for high amperage T-class fuses are typically between 0.25 and 1 millohm, depending on current they are carrying.

High amperage fuses have significantly lower series resistance than a high amperage breaker which has a lot of guts causing 3 to 5 milliohms of breaker series resistance. They get very hot with high load current.

 

[John Frum]

The linked document indicates aluminum to copper joint.
Maybe that is higher resistance than copper to copper.

 

[Pappion]

Rarely do high amperage fuses spec a resistance since the resistance changes depending on amount of current causing fuse metal to heat up and change resistance.

Sometimes they will spec a voltage drop at a given current level.

Resistance for high amperage T-class fuses are typically between 0.25 and 1 millohm, depending on current they are carrying.

I am an adult, I can handle the spec. No need for them to hide it.

 

[Pappion]

I think they don't want to commit to a spec that is secondary. Prefer to be flexible on metals and construction, and meet primary specs.

 

[filippomasoni]

I'm interested to know more about this topic, t-class don't have much specs in general compared to the other HRC fuses from bussmann etc.
I'm planning to get a LET 180A, which has a 21W stated power loss. Is that considered at the max continuous 180A? What power loss can I expect at 50% draw and at 10% ? Is it linear?

 

[Pappion]

I'm interested to know more about this topic, t-class don't have much specs in general compared to the other HRC fuses from bussmann etc.
I'm planning to get a LET 180A, which has a 21W stated power loss. Is that considered at the max continuous 180A? What power loss can I expect at 50% draw and at 10% ? Is it linear?

That would be at max. Slightly non-linear, it should have a positive temperature coefficient, like most fuses..

 

[filippomasoni]

That would be at max. Slightly non-linear, it should have a positive temperature coefficient, like most fuses..

Thanks. That means the bigger the fuse, the less power wasted? The problem is that they get expensive, big and heavy as amps go up of course. Do you know which series is the most "efficient" in this aspect? LMT, LET, EET ?!

 

[brum]

I'm using the Bussmann 100FE fuse and its resistance is 1.5milliohms. This is measured directly on the two fuse side caps. Cold fuse, not under load.

The Tomzn 125A DC breaker is close to 1 milliohm and I also have one of these. It is used as a disconnect switch.

The total battery resistance (16S 48V LiFePo4) is close to 10milliohms (measured on the battery terminals) and 25% are in the two fuses. Other comes from wires, BMS, cells themself, and the busbars, lugs, and connections.

For losses - these are not linear, but exponential. The curve will get steeper when you get close to the fuse rating. If you go to 1/4 of the rating (50A for 200A fuse) you will have less than 1/4 losses. One approach to reducing the losses is to go with an oversized fuse. But don't even consider going with no fuse.

 

[filippomasoni]

Got it thanks. Every load will have it's breaker or fuse to protect wires, the main battery fuse is there to basically protect from a catastrophic short, that's why we need t-class or HRC fuses that can break tens of thousands of amps. So if it's oversized means it will only break in a short.
Blu sea t-class are rated at 20kA of interrupting capacity at 125VDC but don't have proper specs sheets for power loss or curves of activation.
LET or LMT are usually rated at 40kA and most times even higher 250VDC, so they must be much better and sometimes can be found for cheaper, but not that easy to find online.

Also since it can break, we need to have a spare, so double the expense.

 

[ouk]

Resistance for high amperage T-class fuses are typically between 0.25 and 1 millohm, depending on current they are carrying.

This scared me a little bit, and as this thread is the only place on the internet where the resistance / power loss of a class T fuse is mentioned and I wanted a definitive answer to that question, I asked Bussman directly. My main motivation was to compare the power loss of a 600A Class T (JJN-600) to a an NH3 630A (630NHG3B). The NH3s are bulkier, but have a higher DC voltage (250V), interrupt rating (120kA) and are significantly cheaper. Needless to say that if the class T fuse indeed had a resistance of 1 mΩ (dissipating a scary 360W at nominal current) my choice was going to be very easy.

Bussman's answer to my query was that a JJN-600 at rated current and 25°C ambient temperature had a resistance of 0.097 mΩ (min. 0.09mΩ, max. 0.104mΩ). Which leads to a nominal voltage drop of 58mV (max 62mV) and a power loss of 35W (max 37.5W).

From the datasheet, NH3 will dissipate 46W at rated current and 25°C ambient temperature. Which means a voltage drop of 73mV and a resistance of 0.115mΩ. To compare it with the JJN and assuming the resistance is roughly the same at 600A, the power loss at 600A would be about 41W.
I had no reply from the 2nd manufacturer of class T fuses I know of (Ferraz Shawmut).

These numbers make sense as given the NH has better IR and voltage ratings it is likely to expose a slightly higher resistance due to its internal construction. I think it is not absurd to extrapolate these number to other fuse ratings, with power losses of class T fuses slightly lower than the equivalent NH fuse as provided in the datasheet.

As I am on a boat, Wh and (mostly) space are precious, I think I will go with Class T even if they are 3 times more expensive.


EDIT: from another datasheet I can't find again and that was reporting power losses at 80% nominal current (which is generally the circuit's 'design' current), they were around 2/3 of the power loss at nominal current. If the extrapolation to a class T stands, a JJN-600 would be likely to dissipate something in the vicinity of 25W at 500A. With a few reasonable assumptions (fuse temperature at nominal current), it should be relatively easy to evaluate the order of magnitude of power losses for any current.