Maplin 100A Circuit Breaker

[paulrossall]

I have been concerned that my engine start battery does not have any fuse on it. I also have domestic battey with 100amp domestic (240 volt) switch and circuit breaker.
Is a 240 volt 100 amp CB ok or should I replace it with the Maplin CB which only cost £9.99? I reckon I can use the Maplin one as a switch just by pressing the "test" button which will cut the circuit. Space is always limited around batteries so I think this Maplin CB will act as both a CB and a switch.
Can I put one of these CBs on my engine start battery. Engine is a Yanmar 2GM20 and starter motor is rated at 1.0 kW or will it blow the CB?
Maplin also do a 60amp CB for £4.99 which is much cheaper than "marine" ones.
Paul

<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".

 

[Heckler]

i might have got this wrong but DC at 12volts pulls a lot more amps than domestic gear can stand.

thats why they use 11000 volts to transmit long distances.
the switch gear and cables can be a lot thinner at hi voltage than lo.

amps = watts divided by volts

for a given amount of watts, as the voltage drops so the amps rise

stu

<hr width=100% size=1>http://www.beneteau-owners-association.org.uk

 

[VicMallows]

You simply cannot use a CB (or switch of any kind) which is rated for AC (alternating current) on a DC (direct current) supply as you have in the boat. The contacts will just weld together. It's to do with the nature of the spark created when any contact opens or closes. Also, even if suitable for DC, your 100A breaker would certainly trip out on surge current when you tried to start the engine.

Vic

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[paulrossall]

Stu & Vic, What do you use?

I did wonder about the 240 volt ex consumer unit, but it did look very robust. Should I be useing a fuse rather than a Circuit Breaker to protect both engine and start batteries, and what happen if I try to start on domestic battery?
Perhaps I should not worry. Do I need any big fuse/CB protection on my batteries?? I do have appropriate fuses further down the domestice circuits.
Paul

<hr width=100% size=1>" there is nothing-absolutely nothing-half so much worth doing as simply messing about in boats".

 

[VicMallows]

Re: Stu & Vic, What do you use?

The question of 'Big Fuses' protecting everything downstream of the battery is a hot topic in the boating world, and as a professional will not get drawn into the debate. An observation is that to the best of my knowledge, no production car has ever had a fuse between the battery and the starter motor .. or the alternator. Of course, everything else should be protected in one way or another.

I'm sure this will turn into a long thread!

Vic

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[pvb]

And, of course, with a car...

The great thing about cars is that, if they catch fire, you can simply walk away from them. Unfortunately, with boats.......

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[pvb]

You should protect your wiring...

The purpose of fuses or circuit breakers is only to protect the wiring. If there's a short, the power in a battery can rapidly overheat the wiring and potentially cause a fire. That's why it's best to have a fuse or breaker as close to the battery as possible.

240v equipment isn't the most appropriate stuff to use in a boat, for lots of reasons - so you'd be better off fitting some proper 12v items designed for the marine environment.

"Mega Fuses" are easy to find and relatively inexpensive. I put one on my domestic circuit when I increased the battery capacity. At the same time, I put one on the starter battery too. I'd agree that starter circuits aren't usually fused, but was pleased I'd fitted a fuse when, whilst refitting an alternator, I managed to short out the circuit. The fuse worked perfectly!

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[andyball]

Re: Stu & Vic, What do you use?

Paul, most switches/relays/cb's will sometimes have a rating marked on them for current and voltage. Often one current figure for DC (like you 12V system) & one for AC. The maplin one you mention is presumably the car audio item? that should be fine, except possibly for starting, I don't know the size of that engine.




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[willothewisp]

Re: Stu & Vic, What do you use?

Am about to replace my Mastervolt charger with a Victron, and the diagram clearly shows two fuses, 40AT, one for each domestic battery, and a 25AT for the starter battery, close to the batteries. They all have separate connections to the charger.
I expect those people would know what they're doing, wouldn't they? Which is more than can be said for my person.
Coby

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[MainlySteam]

Re: Stu & Vic, What do you use?

But those are not fuses between the cranking battery, and the starting motor or alternator that Vic talked about though. They are fuses between a charger and the battery

I am in the industry and am with Vic on this one. If one thinks the likely fault scenarios through on a small boat (including arcing currents and the boat operation ones where reliability of cranking may be critical), a fuse or breaker in the cranking and alternator circuits is pointless unless the boat is so poorly wired that a hard dead short is possible between the cables, and I have to assume many forumites boats are, from what I read on the forum about their electrical practices, that poorly wired (but which is impossible in a correct installation), or the battery is a very long way from the engine (which it will not likely be due to voltage drop issues when cranking).

That will extend the thread by a good few posts I imagine, plus some hate mail!

John

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[andyball]

Re: Stu & Vic, What do you use?

John

Tend to agree about the starter cable, but having seenmany rectifiers on bikes short out (& usually blow fuses) on bikes, I'm inclined towards protection on the alternator cable.....or do car type rectifiers never short to earth?.

The few bikes I worked on without fuses on the rectifier cable, that have had internal shorts have caused quite a mess.

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[Alex_Blackwood]

Short reply. DO NOT use ANY device in ANY circuit unless it is rated for the VOLTAGE and CURRENT of that circuit. DO NOT use ac rated equipment on DC circuits. Domestic 240 volt equipment is only rated for ac use and should never, ever be used on DC circuits.
As a guide you can use DC equipment on ac circuits, you can use 24 volt equipment on 12 volt circuits but not 24 on 12. The current rating is important and should NEVER be exceeded.

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[Benbow]

If you fit a circuit breaker, eg to protect your main domestic supply, it is essential that you check its maximum interrupt capacity. Unlike a fuse, a circuit breaker faced with way too much current will not go open-circuit; in fact it will weld itself closed. Cheap breakers have max interrupts only a few times bigger than their trip value and so need to be backed-up by a fuse.

That’s why a circuit breaker is never used in a starter circuit, the wiring is so heavy that the current that could be delivered by a dead short is huge and the breaker is unlikely to trip (battery explodes, boat burns).


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[steve_cronin]

Re: You should protect your wiring...

Yes, if he must fit a fuse in the starter feed, a "Mega" is the most suitable but I recon that he'll never need to give it any attention. If he does of course, he'll find out that they are a bit fiddly to change requiring a spanner for the contacts and a screwdriver to flick the fuse insert out of the annoying plastic clips. There are better types of heavy duty fuse available of the stick variety which have a ceramic body and simply push into a sprung holder. They are better suited to windlasses though but have exposed contacts and are quite expensive. A Mega is far cheaper and safer for a starter circuit (again if he MUST fit one)

Steve Cronin



<hr width=100% size=1>The above is, like any other post here, only a personal opinion

 

[johna]

Look at <A target="_blank" HREF=http://www.indexmarine.co.uk>http://www.indexmarine.co.uk</A> for the right equipment.

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[chriscallender]

Re: And, of course, with a car...

I discovered some scary wiring on my car last weekend whilst trying to cure a misfire which had been introduced when it was serviced a couple of days before. The starter motor battery cable had corroded and half turned to white powder and lost its insulation over a length of about 6 inches. It would easily have been possible to move that cable to a position where it shorted to ground. Wasn't picked up on at the service - OK it also came back from the service misfiring until I "sorted" that out and it is an old banger (more money saved for the boat!).

So it could happen on a car. Which reminds me, I must replace that cable this weekend currently its um, er "fixed" with insulating tape!!! And no, I wouldn't have done that on the boat so I guess that proves that I don't worry so much about car engine fires!

Chris

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[MainlySteam]

Re: Stu & Vic, What do you use?

I know this is not the case with a number of small marine diesel engines, but the engine, alternator frame, instrument sender bodies, etc should be isolated from dc. If it is the case that mostly eliminates the chance of shorting onto the alternator frame internally (I would have thought) and from loose connections externally.

I have not heard of any fires attributed to an alternator rectifier shorting fault - I would have thought that if the diodes were implicated in the shorted current path then they would act as pretty effective fuses and fail open under those current conditions (especially seeing they have a habit of failing open or closed sometimes all by themselves). I would be interested in any experiences on that.

The most frequent source of ignition on alternators on boats that I know of is the output cables failing through fatigue at the terminals (or just coming loose) and shorting against the frame or engine if they are not isolated from DC ground.

Out of interest in bigger boats where the cabling is much longer and goes via remote bus bars etc from the engine alternator and cranking motor to the start batteries isolators are of course used because of the possibility of shorting. So I just had a quick look at what is installed on a smaller new build I am looking after at the moment and the isolators are rated at 250 A continuous and 2,500 A for 10 seconds in a 24 v system (the boat has over 3,000 hp installed between 4 engines, so those currents apply to an around 800 hp engine). I do not know whether a fuse or a breaker for a small yacht system could even be easily obtained with a similar failure characteristics at the much lower curents involved. Again I would be interested in any comment.

It's a long time since I looked, but the American Boat and Yacht Council standard allows starter motor cabling from the battery (in fact "batteries" because I think they require redudundancy for the cranking battery) to be unfused and so by implication the alternator cabling as well, and 6 foot (from memory) unprotected for other cables from batteries.

While one can get 600 -1000 amp fuses which I imagine is around what one might need for a mid sized yacht alternator/starter circuit at the battery, then I would suspect that any fault that opened one of those would be a big bang event with a puff of smoke and a spray of molten copper. While I would be interested in any experiences I suspect most fires are actually started by arcing type events from failed connections or chaff at unsuitable support (I have seen sharp edged things such as hose clips used, for instance) which probably draw much less current than that and so there is little chance of the protection opening.

John

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[tr7v8]

Re: Stu & Vic, What do you use?

no production car has ever had a fuse between the battery and the starter motor .. or the alternator

The vast majority of current cars have a fusible link in the main battery lead.
I believe it may even be regulation.



<hr width=100% size=1>Jim

Draco 2500

 

[VicMallows]

Re: Stu & Vic, What do you use?

Fusible Links in cars: In the feed to the starter motor? Certainly not on my 3yr old Mondeo. I agree they are frequently used in the main ignition switch feed and especially the glow-plug feed (on diesels), though note the Mondeo has gone back to conventional fuses for these.

Vic

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[Birdseye]

Re: Stu & Vic, What do you use?

2 years ago I was in port in N Spain when a boat limped in after a fire at sea. The boat was a well known British make, and the fire had been caused when the engine sump flange had chafed through the pos feed wire to the starter. The resulting dead short set fire to some of the woodwork, flashed almost all of the insulation off the battery feed wires and melted through the plastic on the tops of the three batteries where the feed wire lay. It was possible to touch the battery plates with a finger - I did so. The cabin had been filled with acrid fumes from the burning plastic together with battery acid fumes. The whole episode had occurred beating into rough weather, which is why the engine mounts had allowed the engie to move sufficient to cause the problem. Their great good luck was that the battery switch hadnt fused solid as I would have expected and they could turn the system off. Had the switch melted together, then the boat would undoubtedly have gone up in smoke.

I have no reason to suspect that the boat wiring wasnt standard as supplied when the boat was new.

Having seen the consequences of that short, I htink you are wrong. I cant imagine any way in which a properly installed fuse wouldnt be an additional safety feature, even if decently installed wiring in the first place would have made this particular accident less likely. You have to protect against the unforseen, and as far as possible against the things we all do but which in retrospect are stupid.



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