Which fuse type

[noelex]

The Mega fuses do not have high interupt capacity (IC) rating. It is only 2,000A. I suspect this may not be enough for your installation, or at least would be marginal as a main battery fuse especially with your planned battery upgrade.

The cube fuses have a better IC rating and these should be fine, but personally I am not a great fan of a fuse directly on the battery terminal.

If you want a fuse wih a conventional fuse holder the ANL fuses would be a safer choice. The interupt rating is 6000A which is adequate for the main battery feed on most boats, although larger boats may need a T class fuse which have a IC rating of 20,000A.

(The above numbers are all for a 300A fuse).

The required IC can be estimated given the battery size and type, wiring gauge etc, but frankly it simpler to go for a better fuse design in marginal cases such as yours.

In simple terms the IC rating is basically the maximium current that will blow the fuse without melting the whole lot together so that the circuit is not actually broken. Using a 300A mega fuse means in the event of a short circuit of the main battery terminals if the batteries can deliver more than 2000A, even briefly, the fuse may not be capable of disconnecting the short circuit. It is important.

 

[zoidberg]

Noelex's post frankly confused the hell out of me.

I asked Paul Ballard of 12VoltPlanet to guide me further....

Thank you for your enquiry.

The IC of mega fuses is not an issue for starting as the cranking current would never exceed 2000A but if you had a dead-short your batteries may theoretically be able to produce more than this. I would recommend discussing with a marine electrician who can take a look at your system and recommend the best approach based on what current your batteries could produce in a dead short situation. Without being able to look at all aspects of your system it's only possible for us to make a general recommendation I'm afraid.

Best regards
Paul Ballard

I think I'll just follow the guidance' of the ISO of reference and put no fuse on the start circuit, because it's short. As for the other cabling - which is 25mm/sq 'rated' at 170A, I'll just put 100A fuses where I think I can and should, and be done with it.

Some days it's better just to hang up your overalls and go do something different....

 

[Baggywrinkle]

https://www.amazon.co.uk/RKURCK-12V...face-Mount-Trolling/dp/B07K7MS3Z1/ref=sr_1_15

I used a few of these or similar ... one season done and working well. It's also very quick to isolate the batteries or loads when needed.

 

[noelex]

Noelex's post frankly confused the hell out of me.
.

Sorry about that.

My aim is not to confuse.

Basically (in simple terms) with a 300A mega fuse. If the current exceeds 300A for a moderate time the fuse will blow protecting everything from overheating and setting on fire. However if the current is above 2,000A, even for a very short time, a mega fuse may not disconnect the circuit (because it melts and the contacts fuse together).

So the question is:
Can your battery and battery cable in a short circuit situation deliver 2000A?. If it can (and I think yours probably qualifies) once the fuse has melted the short circuit remains and overheating fire etc may result so the fuse has not provided protection.

The bigger the battery bank the more likely that the system can provide the 2000A for a short time.

So with a larger battery bank, and thicker battery cabling, fuse protection is required to be able to interrupt higher short term currents, otherwise it is worthless for short circuit protection.

The answer you have recieved from Paul is spot on “your batteries may theoretically be able to produce more than this”

Mega fuses have a quite low interruption current. They are designed more for branch circuits, or primary battery protection on boats with a small battery. A small battery cannot deliver a very high current even if the terminals are shorted. Branch circuits have small wires which limit the maximum short circuit current and these will be fitted with lower capacty fuses.

ANL and T class fuses are designed to interrupt the higher short term currents that can be generated by larger batteries or battery banks.

I would not use a mega fuse for your installation, in a primary battery circuit. An ANL fuse holder would be fine and will only be slightly more expensive. As a bonus ANL fuses are more readily available if a replacment is needed in a remote location.