Engine wiring

[alan54]

What are the current regulations regarding the best practice on wiring the engine alternator? Is it to fuse or not ! I am receiving different information and methods. Not to fuse the cable to the 3 way switch can lead to a fire to fuse can possibly prevent engine start when needed.

regards
alan

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[Althorne]

My understanding is that there is no hard and fast rule for the very reasons that you have given. While on an a boat electrics course recently the question was posed and it was finally determined that you can use either however if you decide to have a fuse it needs to be a slow blow type. The ones shown to use acollege were also bloody expensive...best part of £80. If I were in your shoes I would try to get the advise of the local boat electrician for the price of a decent few pints or fork out for him to do the work.

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[Ships_Cat]

For the reasons you and Althorne have given it really becomes a personal choice. One needs to also remember that for the reaons that the fuse being so big and also worse in that it is slow blow, it will likely only protect against a dead short between the positive and negative conductors. A very unlikely occurance on a properly wired small boat - although some of the wiring recommendations I see on here raise the odds considerably. Most faults causing fires are likely to be arcing which the fuse will likely not protect against.

In our own boat we had the alternators and starter wired with no fuses (two alternators) and also both alternators are not switched either both being straight onto the batteries (with interconnection to either set of batteries). That is fairly common with custom built boats here and totally precludes the possibility of live switching the alternators onto or off load and damaging them but does mean that before working on the alternators one has to remember to disconnect them from the batteries.

John

<hr width=100% size=1>I am the cat but I am only 6.

 

[boatmike]

Endorse entirely what ShipsCat says. Would add that if you have isolating switches on all batteries and turn them off when not in use it reduces any risk to minimal proportions even with his perfectly acceptable (but frowned on over here) method of wiring direct to battery. Even in the unlikely event of a short when underway the ability to isolate the battery manually will give peace of mind and puts the choice in your hands. I would personally chose to be able to access this isolating switch from outside the engine compartment to reduce the risk of fire but would not want a fuse to do it for me. But do ensure all "domestics" are properly fused downstream.

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[pvb]

On balance, the best practice seems to be to wire the alternator output direct to the batteries (via a charge-splitting device if required). I certainly wouldn't wire it via an isolator switch. However, I am a firm believer in fuses (ever seen a boat fire?). Both my engine start battery and my domestic battery bank are protected by high-amperage fuses right next to the batteries. The output from the alternator is connected to the load side of the fuse, not the battery side, so that the alternator output wire is also protected. The fuses are Littelfuse 200A (E6050/6051 - about £15 from <A target="_blank" HREF=http://www.indexmarine.co.uk>Index Marine</A>). These fuses have a slow-blow characteristic. On the one occasion that I inadvertently and foolishly grounded the alternator output lead when replacing the alternator, the fuse blew straight away.

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[SlowlyButSurely]

I may have misunderstood your description, but wasn't the reason that you had a short circuit when changing the alternator due to the fact that you didn't have an isolator switch?

I wouldn't connect anything directly to the battery. I have an isolator switch for each battery, then when they're switched you know they're off and nothing is live.

I can't undertsand this fear of switching off the battery with the engine running and damaging the alternator. How often does one switch off the ignition when driving a car?


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[pvb]

As I said, I foolishly grounded the wire; what I should have done was to disconnect the battery at the terminal first.

Glad to see you also have separate isolator switches instead of a 1/2/Both switch - this is the best way to wire. I still think that wiring alternator output direct to the battery is the right solution. Your analogy about driving isn't strictly applicable because you don't have loads of other people with access to the ignition switch in a car!

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[Robin]

Switching off the ignition on a car doesn't disconnect the battery and the alternator is still connected to the battery. Switching off the key on a diesel boat engine normally only switches off the instruments and again doesn't disconnect the battery. If you switch a 1/2/both to off or switch off a battery isolator switch or disconnect the battery whilst the engine is running you will blow the alternator most times. If you have 2 batteries charging via a diode splitter and one is accidentally disconnected you will be OK.

<hr width=100% size=1><font size=1>Sermons from my pulpit are with tongue firmly in cheek and come with no warranty!</font size=1>

 

[dickh]

I also have a fuse in the alternator circuit which I accidently blew and didn't notice till the alternator failed to charge - took ages to find it as it was wrapped in insulation in the wiring loom!(Perkins Perama M20). Went to a local electrical distributor to get a replacement but ended up getting two from RS as they had them in stock. Mucj cheaper the Perkins spares prices.

<hr width=100% size=1>dickh
I'd rather be sailing... /forums/images/icons/smile.gif

 

[roger]

I have the battery outputs isolatable by switches ( one switch each) and no fuses on the charging side. I also have the batteries connected permanently to the alternator via the diode splitter and of course the alternator diode stack. The remaining risk is that lead from diode splitter to battery gets earthed. A large fuse in each battery line after the splitter might be a good idea. I dont think they would all blow together and if only one survived it would protect the alternator.
Yes the battery outputs are fused

<hr width=100% size=1>Roger

 

[Jean]

Re "it will likely only protect against a dead short between the positive and negative conductors" - That is what they are primarily used for, ie, to protect the cable under short circuit conditions (hopefully rare but certainly possible). Even though they are "slow blow", they still have a time/current characteristic and stand a much better chance of blowing quickly under short circuit, very low resistance conditions. Liken them to the incoming fuses to a domestic supply, really mainly there to protect the local (large core) cabling. Also re arcing, generally more likely from "short circuit", particularly in an unfused supply from a battery as far as boat electrics are concerned. As an electrical specialist, I'd certainly prefer to see appropriate fuses connected in the battery supply at the battery terminals (AND appropriate spares carried on board of course!)

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