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Topic starter
September 21, 2025 4:31 pm
I was just referred to your review of the Epoch 460Ah battery, and noted the following phrase in that review: "I’d never seen a class T rated above 20ka AIC".
Please have a look at the Mersen (Ferraz Shawmut) data sheet:
https://us.mersen.com/sites/default/files/medias/PIM/files/DS-A3T-A6T-Class-T-Fast-Acting-Mersen.pdf
Their A3T Class T fuses are rated for 50kA DC interrupt at 160VDC. They are readily available (at least in the US). I've used many of them.
I don't know why the marine electrical market, including ABYC is stuck on the idea that Class T fuses are limited to 20kA DC interrupt rating. Perhaps its because Blue Sea sell fuses only certified to 20kA.
My understanding is that Class T fuses are made primarily for AC applications. Not all certification labs can test for DC interrupt ratings above 20kA, and the tests are expensive. Perhaps those fuses rated for 20kA DC are actually much more capable, but simply weren't certified for higher currents (IIRC, Mersen have in-house test capability for at least 50kA).
December 20, 2025 12:26 pm
While a higher Interruption rating is a good thing, what are the odds that a conceivable short circuit on a recreational vessel could reach even 20kA? At 24V, the total circuit resistance (including the internal resistance of the battery) would have to be less than .0012 ohms. Such a resistance would have a voltage drop of .12 volts at 200 amps - something that seems rather unlikely, at least in the installations I've examined. The worst case, of course, would be to drop a big, fat conductor across the battery terminals - THAT might achieve a low enough resistance, but unless your fuse is inside the battery (like the Epoch), you're still in trouble.
FWIW, .0012 ohms is about 20 feet of 2/0 cable, not counting terminations. At 12 volts, the maximum resistance for 20kA is half: .0006 ohms.
I took a look online, and be very best of LiFePO4 batteries seem to have perhaps .0005 ohms (0.5 milliohms) of internal resistance per cell. Every out-of-optimum parameter (age, temperature, cycles, etc.) makes the resistance higher. So achieving 20kA would seem to be rather difficult even across the battery terminals.
Hartley
S/V Atsa
Topic starter
July 31, 2026 11:30 pm
Obviously, I'm way late replying, but to answer your question, with large and especially higher voltage (e.g., 48V) LFP installations it is easy to exceed 20kA short circuit current. We have such an installation on our boat - 16 x 12.8V 330Ah Victron LFP batteries in a 4S, 4P configuration. The interconnect wiring and busbars are short and heavy, and result in a calculated short current between the positive and negative buses of nearly 50kA. This is taking into account the battery data, and the resistance of wiring, switches and fuses in the battery system.
August 4, 2026 6:40 pm
Interesting, Graeme. I would be interested in the values you used to reach your "calculated" 50 kA short-circuit current. Do you have a value for the internal resistance of those Victron batteries?
Hartley
Topic starter
August 4, 2026 9:21 pm
I used 0.8mOhm per battery, which is Victron's specified value, and seems consistent with other data I've seen/measured. The short circuit current you would calculate from that (around 16kA at nominal voltage) is of the same order as ABYC recommends assuming. Apart from that, I included all the calculated/measured resistance of fuses, switches, cable and bus leading to the bank output terminals. However you cut this, the available short circuit current for the bank is likely to be well above 20kA. The previous poster's comments about "unlikely" etc. simply don't hold up when you use published data and do the calculations.
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