Fuse For Starter Circuit?

[<152587>]

I have recently fitted a battery fuse. Here's why:

I had just got back to my home mooring and was returning to the cockpit having secured the mooring strop when I became aware of an unusual smell, a bit like burning sugar. My first thought was that the engine was overheating and I looked over the transom to check the cooling water, but that seemed fine.
Then all the electrics went off.
I opened the lid of the engine compartment (possibly unwisely) and was greeted by acrid fumes, but no sign of flames. I quickly closed the lid again and went to turn off the battery, but the 1-2-Both switch was jammed. I got a spanner with a view to undoing a battery terminal, but when I opened the battery compartment I was greeted by more acrid fumes. By the time I had disconnected the battery the fumes had dissipated and I could see that the insulation had completely melted/vaporised on the negative battery lead and part of the negative battery post had melted.

What had happened was that the heavy cable between the 1-2-Both switch and the starter motor had been rubbing against the speed-control quadrant on the engine and had worn through the insulation, causing a short circuit between the engine and the starter cable (always energised when the battery is switched on as it is a pre-engaged starter with the solenoid mounted directly on the starter motor). The last straw had been when I closed the throttle after arriving at the mooring, nipping the starter cable in the speed-control quadrant and causing a major short-circuit, melting/vaporising the insulation on the starter cable between the point of the short-circuit and the 1-2-Both switch. The circuit was only broken when the spindle of the 1-2-Both switch got so hot it melted its plastic support and the rotating part of the switch came away from the fixed contacts (which is why I could not turn it when I tried to isolate the battery). I don’t know if this is a design feature or just an accident, but it does seem as if the switch acted as a high-current fuse, and without it I might well have had a fire on my hands.

I was initially puzzled as to why only the negative battery lead had melted its insulation while the positive leads were unaffected, but I then discovered that the positive leads were 50sq.mm while the lead from the battery negative to the engine block was only 35sq.mm, as was the starter lead (although the engine wiring diagram said 50sq.mm). I have now fitted a new 1-2-Both switch, replaced the burned-out cables with 50sq.mm tinned marine quality leads, and in the best tradition of stable-door shutting I have fitted a 200 Amp battery fuse.

Thinking back to fitting Paxman YHAM's to minesweepers, the start motors took a 2 man lift and consumed more wriggleys than any of you have on your boat. It was impressed into us that the wire must be secured to prevent chaff. So in the best tradition of the stable door, you have fitted a fuse so when the cable gets mixed up with the controls again the fuse will blow!

In my opinion by fitting a fuse in a start circuit you are engineering a potential failure in a essential circuit. Don't forget that fuses sometimes fail through tiredness rather than overcurrent.

 

[matthewriches]

[deleted] double post

 

[VicS]

Just for avoidance of ambiguity ( as the bishop said to the trans actress ), is that a fuse rated to BLOW at 200Amps - or for 200Amps continuous, with a higher 'blow' rating....?

The full specification for the fuses should include time to blow vs current data similar to the curves attached. There should also be a de-rating factor based on temperature.