I"ve done some designs using PTC's for fuses. That 100A PTC will have a voltage drop of 0.2 to 0.4V at full current, rather high.
These are best used with systems where the overcurrent protection is difficult to reach, and it is easy to "unplug" or rewire the problem. Also, to reset, the load must be removed and a cool down period is involved.
I don't recommend a wholesale change out to PTC's in 12V boats. They are best INSIDE electronic equipment to protect agains reverse polarity wiring, as an example.
What is the current which you call "full" - tripping(40A), holding(21A), maximum(100A)? Actually, high currents are present only during short periods. My alternator is 50A, but we all know how short that current will be present, if ever (if batteries are flat). My AC charger/inverter is also Victron multiplus compact and although it is capable of delivering 70A for charging I have configured it to give charge with maximum of 20A since my house battery is AGM 130Ah and start battery is flooded 148Ah (actually 74Ah x2 in parallel to save my back), so when paralleled around 10A to each bank (if we say everything is ideal, which is not, but that is the life). I do expect that most currents at steady states are much, much below holding current of 21A.
A few things I would want to check on if it were me:
- Cycles - I see some of these have cycle times as low as 350. If it trips every time you start your engine, that seems pretty low. Not listed in the data sheet.
- Most of the resistance and trip specs are listed at 25°C. I would want to know how these change at engine room temps much higher than that.
- Time to trip shows 13 seconds at 120A and 25°C - is this OK? What happens at 40°C?
- Hold current listed as 21A - is this OK?
- Power dissipation once tripped is only 6.2W (I^2*R) - is this OK?
Just some thoughts - it may be fine.
I designed that all of my "electrics" (for which is possible) and batteries are out of engine room and in very good ventilated space, so temperatures are not critical, although I did pay that with thicker cables.
Tripping times of PTC are similar to thermal fuse, just the current values are a bit different. And this is the strongest PTC I could fine.
According to my steady states, 21A should be OK, and transients I want to dampen with PTC anyway.
Dissipation is not critical. Even the ANL fuse, although being ignition protected is in IP enclosed cabinet so it cannot start a fire.
Just saying if you do replace the current unit, switch to Blue Sea, don't get a bigger Cyrix.
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Watch out for fuses with excessive delays, in a dead short scenario sparks flying, quicker the better.
I myself would focus on fixing the underlying issue instead.
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"Too big"? Makes no sense to me. Costly maybe, but there's no downside to high ampacity
except where you specifically want to limit charging current, e.g. to keep wire sizes reasonable.
Plus, never have to replace that ACR, even if you put in HO alts, high-CAR chemistry banks etc.
And self-jumpstarting built in, for even big engines.
It is spark protected as I have already explained, fuse part enclosed and fuse enclosed in IP protected cabinet.
ACRs are relays, so eventually, they have to be replaced, contacts die sooner or later, therefore price is important. Ampacity is good, but it also does cost. My engine is already having very big starter, it is an old V8 from Cummins. My cables for starter have 85mm2 cross area. If jump starting needed, I like the idea that I can also disconnect start group of batteries with selector in case it is completely dead (well, I hope I will never experince that, but there is some Murphy guy always listening). If jump needed, than again I prefer manual selector.
So, if the house is sometimes paralled with the start battery during start, a 100A fuse should not be in that part of the circuit. You want the fuse to be in series with the house loads, near the battery. Another cable should go to the combiner.
You might consider the house to combiner over current protection of a manual reset 100A circuit breaker, if you feel the need to protect the combiner.
I copy/pasted that part of schematic with the fuse from Cyrix datasheet, here is the link:
https://www.victronenergy.com/upload/documents/Manual-Cyrix-i-400-EN-NL-FR-DE-ES.pdf
Other fuses are out of the scope of the problem, so I have not been describing them (from batteries to loads, etc), this is the only one in line with the combiner.
Using manual reset DC circuit breaker is also excellent idea and easier to find. But, when solar charger comes in the circuit, I question myself how often will I have to do that, if often, I prefer PTC, otherwise manual. Both adapters would be wise to build. Tanks for the advice.
Thanks everybody for the brainstorming!!
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