Fuse or no fuse.

Clyde_Wanderer

Clyde_Wanderer

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I am in the process of wiring up a Jabsco water puppy bilge pump which is rated at 15 amp, and wiring it through a relay, using main power cable rated at 27 amp direct from the battery.
Should I fit a fuse rated at say 20amp or no fuse at all?
Would I not be taking a big risk without a fuse?
 
I guess you already know the pros and cons of the "no fuse" argument.
I'm shortly going to wire up an electric bilge pump. I'm not going to use a fuse because I don't want to be sitting up to my knees in water with an in-line fuse holder and a blown fuse at any point (even if I do have spare fuses, that's time that could be spent pumping water by both electric pump and yours truly on the manual one).
Anyway there has got to be a bit of electrical resistance in the connections at that fuseholder which must - even infinitessimally - degrade the pump performance.
I'd rather be pumping water than heating metal!
Besides that - weigh up the possible consequences - in the event of the pump being required to run continuously you already have a serious leak which needs addressed - are you going to compromise the reliability of your pump in the off-chance you have a wire meltdown or boil up your battery?
Personally I'd go for the immediate problem (sinking) and do what I could to avoid the fire scenario by careful cable selection and routing.
Only my opinion though, I'm sure there are other takes on it!
Mike
 
The fuse or breaker is intended to protect the cable not the pump. Imagine this scenario - you have 3 metres of cable to the pump, with no protection fuse/breaker. The cable becomes damaged near the pump, causing both conductors to short out (or a fault in the pump gives a similar situation). Now the cable will allow as much current as the battery will allow, and proceeds to behave like an element in an electric fire. The rubber insulation melts, maybe alerting your sense of smell if you're nearby, or ignites surrounding materials. Shocking example maybe. A fuse/breaker situated near the power source (battery) would have likely tripped or blown before the cable got hot, if rated properly. Now do you think it's a case that you'd rather chance wet feet instead of a fuse? If you don't want to be searching for a replacement fuse in the event of a leak, fit a circuit breaker.
 
Bythor is correct .You need to fuse close up to the battery to protect the cable say 30amp.If this blows you have a problem anyway and the way to cater for that is to have an entirely different second circuit and pump! the likelyhood of either a fuse or cb malfunction is much less than a fault on the circuit or the pump.
 
If you have ever seen the result of a short circuit at 12 volts you would not contemplate not having a fuse or breaker in the line to protect the cable. Nothing like a coupleof metres of electric string glowing almost white hot to concentrate the mind. You have more chance of recovering a sunken boat then one destroyed by fire.
 
I would never consider having an unfused line in the boat (except direct from the start battery isolation switch to the starter solenoid)

As has been said earlier the fuse is to protect the wire.

The potential to set the boat on fire is huge.

I use car blade type fuses extensively. You can get in line holders in a rubber housing and the range of ratings of the fuses is very wide.

An equally important thing is that they are readily available and at a low cost.

Iain
 
I ain't going anywhere with Mike! I've seen the result of a bilge pump fuse being shorted out to keep the pump going and it's not pretty. The "protect the wiring "guys are right, and protecting the wiring helps stop boats going on fire. Have you seen the pictures of boats burned down to the waterline?
fit a slow-blow of the load rating, about 20% higher if instant trip. If the wiring is marginally overloaded, it may melt soldered joints first if you're lucky.
 
Not sure what is meant by "damaged", rats chewing through it perhaps ?
Get an armoured cable for the job and make sure it can withstand locked rotor current.
If you have a cable that can maintain a white hot glow, you've got the wrong cable, and if you've got a cable with insulation that can burst into flames when heated, you've got the wrong cable.
If you want to pile flammables against a cable that might get hot in an unlikely event, you've got a....
Can't think of one for that!

Mike
 
All the commments are valid to varying degrees.
I have a friend who destroyed a good set of sails and nearly lost the whole boat when he had a short in mast head light wiring. He had only got out onto the jetty to see if the light was glowing. It was the wiring that was glowing. No fuse.
The wire needs to be thick enough to handle the fuse current. (the current needed to blow the fuse)

I would suggest for reliability that you get one of those automotive bolt in fuses.(though they may not be available in low enough amp rating) The cable has a crimp which is bolted to the fuse lugs. It is a job to change the fuse but very reliable compared to a fuse holder.
If you use an ordinary fuse get a fuse holder that has a really tight clamp onto the fuse ends. Spring presssure onto the ends will fail quite quickly. Springs that clamp around the outside of the caps are better. Screw down pressure onto the ends are OK.
The fuse needs to be near the battery terminal. I would suggest perhaps a 30 amp fuse and approprite cable. 20 amp would be ok.
If you go for 30 amp there is the remote possibility of the pump itself getting very hot before the fuse, battery or cable fail. So consider if the pump location would be fire risk if it was very hot.

If you decide not to fuse the pump wiring then you should look at the wiring run +ve and -ve to see the consequences of red hot wire in a fault situation. You can get wire designed for aircraft with insulation (like teflon) which will not burn. (Captan is one brand name) which is far safer than plastic insulation. But it will still get hot

We do not fuse a starter wiring but then the wire size is such that the battery may not have the capacity to bring the wire to incandescent heat and wiring runs are kept short anyway.

What is vital with bilge pumps is to ensure that there is no wire with +ve 12 volts (at all times) that can get under water. This especially so for wire joints. The electrolysis action of 12 volts in salt water will corrode the wire within days. ie do not put 12 volts to the pump and switch the -ve wire. Rather put -ve to the pump and switch the +ve having the 12 volt feed to the switch or relay as high as possible out of the water. Hopefully using water proof joins and wire.

olewill
 
[ QUOTE ]
Why not fit one of these in the cable. I used them in the battery meter wires on Ruffles. They seem to seal quite well though not for immersion.

[/ QUOTE ]Why not fit one of these in the cable. I used them on "RB". They seem to seal quite well even if immersed. /forums/images/graemlins/grin.gif
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hammer.thumb.gif
Support your local and check out the bargains available.
 
Thanks all for your replies.
William_H do you think a 30 amp fuse would be wise with 20 amp rated cable?
I will only be using this pump while the boat is on the mooring, (unattended) as I have a smaller pump which will be on auto while I am using the boat.
As said I would rather see the boat sink, than see it burn.
The main power supply to the relay is 27amp rated cable 3mm direct from the batt +ve and is only about a metre long The switch wires to the relay are, one via a float switch and one from manual, and are fused.
Cheers, Eamonn.
 
A 30 amp fuse on 27 amp rated cable? Suggest you check your copy of the 16th Edition a bit more clearly. I'd also suggest the OP checks his cable de-rating table a bit better too.
 
I had a reply all prepped last night with links to a surveyor who sided with me - just to show I wasn't out on a limb! But the site crashed as I was previewing it - and I can't be bothered re-writing.
Would ask the question though:
Why did he short out his fuse if he knew it was dodgy?
What was the less than pretty result?
What was the root cause of the failure?
Only from answers to these will we learn anything here.

Anyways - how about this for a compromise:
wire a fuse into the supply that feeds the relay and wire a manual bypass for the fuse/relay using a high current switch for emergencies.
That way when you are up to your a$$ in alligators you can flog the pump but feel safe when using it on a float-switch.

Mike
 
I should have made it clear - I meant the POWER side of the relay to be fused as well as the solenoid side!

Mike
 
He was in the french canals and his pump was forever tripping due to debris being sucked in. He got fed up with the fuse blowing under stalled conditions, so bypassed it. the result was a melted motor and wiring, but in this case because he was onsite and in charge, no fire......but of course no pump either, so he had to stop and rethink anyway. It is a 1930's wooden TSDY of Dunkirk vintage so would have gone up quite nicely, and been a sad loss.
 
Its a pity to let the bilges get that dirty that the pump gets clogged up enough to set it on fire, especially in an old dunkirk classic.
Simon T, I wasent suggesting that I would be using a 30 amp fuse with 27amp rated wire, but actually questioning someones ( William's ) suggestion about using 30amp fuse on 20 amp rated wire, but I think he meant it the other way round.
Thanks again all for your replies.
 
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