To protect, or not to protect?

[SvenglishTommy]

I have a dilemma regarding the protection of my electrical system. I recently completely re-wired my boat and changed the layout a little. The main problem is that the batteries are located at the front under the v-berth and there is a long run back to the starter and everything else.

To make things easier I have tried to bring the battery terminals aft by using oversized wire to heavy duty connection blocks that are much to closer where I would ideally like the batteries to be. I then base all my voltage drop calculations from this position as the drop over the 10M (there and back) run is negligibale to anything but the starter motor.

My concern is the length of unprotected wire that this leaves. I have two 100A circuit breakers that I was thinking of installing close to the batteries as a protective measure, but I'm not sure if this is a good idea as they would then be part of the starting/charging circuits. Sorry if this doen't make much sense, maybe I should post a diagram, but any advice would be much appreciated.

 

[irawnsley]

Starting current could be as much as 250amps therefore your 100 amp circuit breakers may well be overloaded

 

[ccscott49]

100 amp circuit breakers will not be enough. But why would you want batteries in the bow of your boat? Anyway, you should use a circuit breaker, but it will need to be bigger.

 

[pvb]

Use Mega Fuses...

To protect your cables, install Mega Fuses next to the batteries. Mega Fuses bolt on to a substantial base, and are easily available in various ratings. The advantage of the Mega Fuse is that it has a good slow-blow characteristic.

I can't imagine that your boat would have anything like a 250A current draw on starting. Compis 28s usually had a smallish Yanmar or Farymann engine, and I doubt that the starter takes more than 100A. A 100A or 125A Mega Fuse would therefore be OK for the starter circuit (and these will actually withstand double their rated current for about 10 seconds before blowing). It's a good idea to keep a spare fuse on board, although in an emergency you could simply swap a cable on the fuse mounting block to take the fuse out of circuit.

 

[PBooth]

You could use what are known as 'Mega Fuses' they are available up to 500 amps and bolt down into a neat holder, they are non-spark, so they are safe to have in the battery box. The fuse must be rated lower than the current-carrying capacity of the cables. In all wiring, the fuse or circuit breaker is there to protect the wiring, not the equipment, if the equipment needs protection, it will be built in by the manufacturer.

Peter

 

[marcusgillies]

Need i say any more!! Well covered.

 

[SvenglishTommy]

You're right pvb, the Compis 28s do have small engines. Mine has had the Yanmar replaced with a Volvo MD2010, and should only draw about 60A. My breakers will take 135A for short a short time which is double the expected current, so do I really need to fork out more cash for one of these mega fuses, they dont look cheap.

Why cant I use the breaker I have already? It would be easy to reset if it blew, do they resist that much more than a fuse?

 

[pvb]

If you have the breakers...

If you have the breakers, and if they'll take the current you claim, you can certainly try them.

 

[William_H]

Re: If you have the breakers...

Let us look at the worst case scenario........ that the heavy cables have been shorted together at your junction box. The cables I imagine would easily carry 500 amps or maybe 1000 amps before they get so hot they will ignite surrounding structure. This fusing current may be had from the wire data and is the max fuse rating for the wire. Then you ask will the battery provide this kind of current for long enough to start a fire? possibly not unless you have a very high capacity battery. All this assuming that the shorting material or mechanism can itself carry the current without melting.

If you consider that cars do not fit a fuse to the starter wiring and morris minis and Rolls Royces had battery in the back and the fact that a steel car has much more likely hood of a short to body earth then I would suggest that in your F/G boat you have no need for fuses.

So to be doubly safe without fuses you need to carefully support the cables in the run and if possible keep them away from flamable material ie fibreglass and even heat insulate with metal at points where they go through bulkheads etc. And of course protect terminals from foreign objects shorting the terminals.
Regards olewill
Yes I can understand why you would want the batteries up forward to get the stern up a bit.

 

[Ships_Cat]

Re: If you have the breakers...

I go along with what you say William.

Many also forget that many problems are started with relatively low current arcing, ie the cables are not clamped hard together into good low resistance contact so the fuse won't protect anyway.

Some years ago I had a draft accident report to review where the vessel had a fire on board due to a short. The accident investigator had a land type electrical consulting engineer do the technical investigation of the elecrical aspects and the draft came out full of the need to fuse the batteries and how bad it was not to do so and that it was a serious concern for all vessels (he had worked out that they were not commonly fused).

It seems that the marine reviewers were all of the same mind on this and all that alarmist comment was removed and a little piece inserted that the need for fusing of batteries should be reviewed as a separate exercise insofar as it affected all boats. The review came to nothing.

In the case of the actual accident it was quite plain that the reason was the rats nest of wiring and improper clipping (among other things was clipped in places with sharp edged hoseclips /forums/images/graemlins/crazy.gif). In my view instead of worrying about fuses people should ensure that the cabling is correctly done. It will, in fact, only take a good marine electrician a relatively short time to run through a boat tidying it all up.

In the end it is a personal choice but for myself I would avoid fusing unless there was some unusual reason to do so.

(Note: In case of misinterpretation I am only talking about the main high current positive and negative cables from the batteries here - any other cabling taken to radios, bilge pumps, etc direct off the battery terminals should be fused).

John

 

[pvb]

The old car analogy...

[ QUOTE ]
If you consider that cars do not fit a fuse to the starter wiring and morris minis and Rolls Royces had battery in the back and the fact that a steel car has much more likely hood of a short to body earth then I would suggest that in your F/G boat you have no need for fuses.


[/ QUOTE ]
Ah yes, the old car analogy crops up again. Surprising how people forget that with a short on a car, you can simply get out, stand safely on the edge of the road and watch it burn merrily. But with a boat....

 

[cindersailor]

I went through this dilema last winter when I rewired my boat. In the end I decided that I would rather have a burnt-out alternator than a burnt-out boat and a swim! When you consider that the alternator will only be burnt-out if the engine is running when the fuse blows and only then if no other battery is in line at the time (quite unlikely if you have a 2 battry setup - who ever only charges the engine battery?) the risk is actually quite low. The fact that I only paid £7 for my alternator from a scrapyard may have influenced the decision however! If I had paid through the nose for a "marine" alternator I can understand the hesitation. My opinion is that ALL cables should be fuse protected. I fitted a 150A megafuse in the engine start battery cable and a 60A strip fuse to the domestic one. Even with this the domestic alone will start my Yanmar 1GM without blowing the fuse. Genuine Megafuses are available at £4.50 including the holder (far far less than the "marine" price!) from http://www.premierwiring.co.uk/

 

[PBooth]

Re: If you have the breakers...

In a short, even ordinary batteries can deliver amazing amounts of current, easily into the thousands of amps. The only limiting factor will be the internal resistance of the battery itself, which is very little, unless the battery is really shot. Good installation practice is the only way to be sure to prevent shorts/fires etc. Low voltage DC seems very safe when you can touch it without harm, but the high amperages that batteries can develop need to be treated with respect.

Peter