Easy to replace fuses in the 50 to 500 amps range - suggestions?

[tinnews]

I've been doing a major re-wire of the boat we recently bought in Belgium, it's going pretty well and much of the horror is now tidied up (it had suffered from the typical car wiring types of bodges, wires draped around the place, lots of ends poked into one terminal, etc., etc.)

I have installed a couple of consumer units to provide switching and protection for individual circuits but there are still two 'master' fuses between the batteries (separate leisure and starter batteries) and the consumer units feeding all the circuits.

The existing fuses are 'bottle' fuses and I'm wondering whether to retain them or to replace them. I certainly want easy to replace fuses (i.e. not needing a screwdriver or spanner) as one doesn't want to have to find tools in an emergency. I need something in the region of 60 to 100 amps though. Currently one fuse is 63 amps and the other is 35 amps. If I stay with the bottle fuses I want to make both 63 amps which means I need to change one of the fuse holders and getting a replacement fuseholder may be a problem although it looks as if Farnell have a possible DIN rail mounting holder, I'm not sure how easy this will be to fit.

So, are there any reasonably economical alternatives with 'replace by hand' fuses with ratings in the 60 to 100 amps region? They want to be reliable as well though don't need to be capable of use at high voltages, this is a 12 volt system.

Then, in addition, I want to protect the 'heavy' stuff. The starter battery also feeds the starter motor (surprise!) and the bow propellor. Currently (no pun intended) these are wired direct to the battery and I'm thinking some sort of protection for this wiring would be a good thing too. The connection to the bow propellor in particular is long and circuitous and a fault could easily start a nasty fire in an inaccessible place. The wires from the batteries to the existing bottle fuses are also unprotected (only a couple of feet or so, but still a fault is possible).

I'm thinking that the easiest approach for this is a fuse in the ground wire to the batteries, it's where the battery isolation switches are already wired so adding a fuse here would be reasonably accessible. It looks to me as if a couple of Littelfuse 'mega fuse' links would do here, they have screw connections but at around 500 amps I guess that's inevitable. Are there any practical alternatives to the 'mega fuse'?

[tony sayer]

Concur with that, we used some mains fuses on DC 24 volt supplies they blew at a lot more then what they were supposed to. Automotive sounds far more appropriate..

Suppose on of those car hi-fi places might have something suitable unless anyone else knows different?..

[Jules Richardson]

Not just a fuse, but fit a physical kill-switch too. I remember last year when the starter solenoid on the tractor jammed closed - cue starter motor turning the engine over continuously even with the ignition off; by the time I'd run to the workshop for a suitable spanner to disconnect the terminals, there was smoke billowing from the starter. Left a bit longer, I probably would have had a fire...

If there had been a fuse in the circuit it probably would have gone eventually, but a simple physical switch would have been nice.

I take it it's a wood / fibreglass hull then, with no chance of an accidental ground path back to the battery?

cheers

Jules

[Man at B&Q]

Not so much unsuitability for DC, just that the rating will be a lot lower when used for DC.

Arcs when breaking the circuit will self extinguish with AC when the current falls to zero each half cycle. When switching DC the arc is sustained until mechanically broken by the contact separation.

MBQ

[tinnews]

There are battery isolation switches which are fairly accessible so that's covered at least.

No, it's steel hull. What sort of accident/problem are you thinking of when you say "accidental ground path back to the battery". Surely I'm protecting against just that, the chance of one of those big, heavy wires shorting to the hull/superstructure - which can't happen on a wooden/plastic boat.

[John]

Would it make sense to have a separate battery for the bow thruster - located near the thruster. You must be getting quite a voltage drop at the moment. A local battery could be charged with thinner cable than the cable that is needed to transmit the motor power. It would also make the starter battery totally dedicated to engine starting.

[The Natural Philosopher]

I looked it up. 50A is the biggest blade type fuse.

I started looking at DC motor contactors/trips for low voltage, but got side tracked.

I am not sure that a fuse in a starter motor is something you want anyway. Those things can surge to several hundred amps.

There is a reason why they are connected directly to batteries via a heavy duty solenoid operated contactor.

I would be more inclined to wire them up directly, and have a simple car blade style fuse panel (any scrappie will have dozens) and make sure the (upstream) wiring to it is of the highest quality,

[Dave Osborne]

Not so. Fuses "blow" because the current flowing in them causes the fuse wire[1] to heat up - eventually to the point where the fuse wire ruptures.

The AC current rating of the fuse is defined as "symmetrical RMS current" which is mathematically equivalent to a direct current of the same magnitude. The fuse has no idea (and doesn't care) whether the current flowing is a.c. or d.c. until the point where it ruptures due to overheating. At the point it ruptures, if the current flowing is A.C., then the arc is quenched immediately. If the current flowing is D.C. then the arcing time will be longer. Therefore, the rated breaking capacity of a fuse is lower for D.C. than for A.C., but the current/time characteristic curves are the same.

The voltage rating and the rated breaking capacity of a fuse are defined in accordance with national and/or international standards which require the rated breaking current to be broken without the fuse cracking, falling to bits or indeed exploding.

The effect you may be experiencing (i.e. A.C. fuses blow more often on D.C.) is probably due to the fact that with the D.C. voltage being lower and therefore the load resistance being lower, the opportunity for a significant overload is somewhat enhanced cf mains-voltage a.c.

see here:

that some fuses have a D.C. rating and some don't. The ones that don't have a D.C. rating are just as suitable for D.C. as the ones that do have a rating, but the manufacturer does not test all product lines on D.C. and therefore does not publish a D.C. Voltage/breaking capacity for all lines.

Note further that for fuses that *are* rated for D.C. use, the manufacturer does not publish separate time/current characteristic curves for D.C. and neither do they publish any correction factors for D.C. use.

Ergo, direct current and "symmetrical RMS alternating current have the same effect on the fuse wire and have identical tripping characteristics.

[1] Fuse wire or equivalent "fusible link".

[js.b1]

PAL type automotive fuses go 100A (if not more).

Some french cars use a flat fuse for 100-150-200A as I recall, right at the battery terminals in a busbar arrangement. The objective I assume is to disconnect a starter cable short fault in event of an accident - high pressure (petrol) and very high pressure (diesel) fuel lines are usually extremely close to such cables on most cars.

Incidentally, check if there are any requirements of your boat insurance - by that I mean fusing or emergency disconnect. Nylon flexible conduit is good for protecting cables, which for marine should be of the tinned fine variety.

[Dave Osborne]

I presume you mean the cables should be tinned fine? I read this as "Nylon flexible conduit [...] should be of the tinned fine variety" ;-)

[David Hansen]

On Tue, 14 Sep 2010 16:23:00 +0100 someone who may be Dave Osborne wrote this:-

Within the limits of the discussion I agree, with the minor niggle that the arc will only be quenched "immediately" on AC if the current is less than the breaking capacity of the fuse. If it is more then the fuse cracking, falling to bits or exploding is possible. The damage if this happens has to be seen to be believed.

"Breaking Capacity

"The standardized values of Breaking Capacity for fuse-links to BS88 are 80kA for voltages of 415V a.c., and above, and 40kA for d.c., applications. The 240V a.c., designs have a breaking capacity of 16kA minimum."

Fuses are good devices, other types of AC equipment may not be suitable for use on DC at all.

[Dave Osborne]

But that (the physical destruction of the fuse) demonstrates either a flawed design or a catastrophic event, does it not?

The prospective short circuit current should in all cases be less than the breaking capacity of the protective device for all foreseeable, manageable risks.

For the vast majority of domestic installations, the prospective short circuit current is an order of magnitude less than the breaking capacity of the fuse. E.g. The breaking capacity of a BS1361 fuse in a house service cut-out is rated at 33kA and (I would say) the prospective short circuit current in the vast majority of dwellings is less than 3.3kA, so in fact there is a (worst-case) safety factor of 10 on the main supply fuse.

It's not that dramatic if it merely reduces your worst-case safety factor from 10 to 5.

Cheers, DaveyOz

[Tabby]

Fuse wire sounds ideal, but replace the 2 screws fixing it with thumb screws, wingnuts etc, so no tool needed. Its pretty much impossible to beat wire fuses for reliability & economy. Attach spare fuse wire to the holder.

NT

[Dave Plowman (News)]

Maxi-blade? The same as ordinary car ones but larger. Half way down the page.

[David Hansen]

On Tue, 14 Sep 2010 19:29:25 +0100 someone who may be Dave Osborne wrote this:-

A flawed design of the installation, yes. As I said it was a minor niggle.

[Jules Richardson]

Mostly misreading on my part, I think :-)

I just think of a metal hull as a nice chunky ground (similar to the chassis in a car), and of motors etc. as having their metal cases connected to ground too. Say the mounts for two of those motors wear or are installed incorrectly, you may end up with the two motor cases linked via the hull - something that could go unnoticed for a long time. A positive-side fuse would protect Motor 1, but a ground-side wouldn't because the motor would still have a ground path back to the battery via the hull and Motor 2.

It takes a couple of faults for it to happen, but I've seen that kind of thing done due to botched installations, manufacturing faults, or age- related wear. As for why it's relevant in your case? It's probably not ;-) I only realised later that you were talking about having a ground disconnect very close to the battery - and so long as there's no way that the -ve terminal of the battery could find a path to the hull, it's a non- issue...

cheers

Jules

[The Natural Philosopher]

This may be of interest.

you want a safety cutoff that will handle hundreds of amps.

[The Natural Philosopher]

Oh here is a 70A rated trip...

[tinnews]

It's still a whole lot better than setting fire to the wiring. I suppose the exploding fuse might start a fire but 10 metres (in my case) of smouldering insulation sounds decidedly worse than some bits of exploded fuse sprayed around the engine compartment.

In reality I doubt if I have any really serious worries as regards rupture capacity with fuses on a 12 volt system driven by just one (admittedly big, 1050 amp CCA) battery.

Actually, looking at the Littelfuse data, the 'interrupt capacity' of the Mega Fuse range is only 2000 Amps at 32 volts so it's possibly marginal for my application. Although what happens if you exceed the interrupt capacity is anybody's guess.

[tinnews]

Some boats are equipped that way but I think we'll stay as we are, it's not *that* big a boat and it doesn't make sense to add yet another battery.