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September 21, 2025 3:31 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 11:26 am
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
