Mega Fuse on DC supply

[superheat6k]

What is the general opinion of Mega Fuse supply cable protection fuses ?

I am updating my boat's domestic electrical distribution, replacing all the old cap ended fuses with miniature MCBs.

I have bought 3 Blue Sea 12 way fuse blocks and these shall be fitted with the ETA 1610-21 series breakers. Thinking about a 125 amp Mega Fuse on the supply line from the domestic battery bank.

Necessary or overkill ?

 

[prv]

The purpose of the megafuse would be to protect against a short of the supply from the battery to the breaker panel. How likely do you think this is? To a large extent this depends how long the cable is, and what sort of places it's travelling through.

Personally, my preference is to fit one.

Pete

 

[KevO]

The purpose of the megafuse would be to protect against a short of the supply from the battery to the breaker panel. How likely do you think this is? To a large extent this depends how long the cable is, and what sort of places it's travelling through.

Personally, my preference is to fit one.

Pete

+1

 

[macd]

Thinking about a 125 amp Mega Fuse on the supply line from the domestic battery bank.

Not commonplace, but increasingly used these days: threads on the subject pop up every so often.

Only downside I can think of with your suggestion is that, should the starter battery develop a fault, 125A may be insufficient to utilise the domestic bank, instead.

 

[David2452]

Fit a fuse? Yes. Fit a 125a fuse? Who knows as the fuse rating will depend on the weight of cable from the domestic bank to the distribution panel.

 

[prv]

Only downside I can think of with your suggestion is that, should the starter battery develop a fault, 125A may be insufficient to utilise the domestic bank, instead.

The size of cable from the domestic battery to the distribution panel is unlikely to be enough to run a starter motor either. So if the fuse did blow, it would be doing its job and protecting the cable. The answer is that any paralleling link should come directly off the battery terminal before the fuse. Being (presumably) very short and wholly inside the battery box, I would say that the link doesn't need a fuse of its own.

Pete

 

[superheat6k]

Not commonplace, but increasingly used these days: threads on the subject pop up every so often.

Only downside I can think of with your suggestion is that, should the starter battery develop a fault, 125A may be insufficient to utilise the domestic bank, instead.

The starters are fed from the two separate banks, port shares the domestic, stbd shares with the windlass, each section having it's own manual battery isolation switch, plus there is a cross connect solenoid powered by the stbd bank, so starting isn't an issue, with the stbd bank backing up the port if the domestics get a bit too low.

I estimated 125 amps based upon the cable size feeding the new main terminal - 35mm2, and the volume of separate MCB protected circuits. I wouldn't expect the domestic load would ever go above 60 amps, so 125 amps would provide reasonable protection for any heavier cable developing a fault. Blue Sea also suggest a 125 amp supply fuse on the 12 way fuse box supply, and I will have three of these in parallel, although the majority of individual circuits are rated at 6-8 amps on devices pulling only 1-2 amps.

This upgrade was prompted by one cable / fuse in one of the original cap ended fuse boxes starting to smoulder, to the point where the fuse cover has partially melted. Rather than mess around with the 27 year old fuses I prefer to use modern MCBs, and generally sort out the snakes wedding that is the electrics cupboard. I just wondered whether have the extra discrepancy is necessary. Anyway I found a holder with fuse for £11 on ebay so I ordered it this morning.