Starter protection for Perkins 4.236?

[Kettlewell]

How would I figure out the minimum ANL fuse size for the starter on a Perkins 4.236? It's a big ol' marine diesel with a starter motor the size of a loaf of bread, so I suspect it draws a lot of current. I know that theoretically I should size the fuse for the capacity of the wire, but in the case of a starter I'm not sure that is feasible or desirable. You basically want a fuse in there that will blow in the case of a catastrophic short of some sort, but won't ordinarily blow no matter how hard you are cranking the diesel. I've had a 350-amp ANL fuse in there for years and it has not blown, so I suspect it is a bit of overkill. Is there any way to determine what the size should be? (I realize that ABYC doesn't require a fuse on a starter circuit.)

 

[Gudenuph]

Kettlewell

My first thought is to use the "cut and try" method. Just try successively smaller fuses until one blows. Then use the one that didn't. Or, just wait for an answer. That's just me and my simple way. LOL

Geohan

 

[Kettlewell]

Gudenuph: I often do things that way, but in the case of the starter motor I just know that when I find the fuse that is too low an amp rating it will be 2 AM and I will be dragging anchor--not a great time to be changing fuses. Usually in boating we go the other way--when in doubt, make it stronger. That doesn't work too well with fuses.

 

[Ron20324]

The first question should have been "what size wire is it"?

 

[Bill Roosa]

Yea, what Ron said
The fuse is to protect the wire not the starter. If the starter shorts it will pull enough current in VERY short order to blow the fuse IF (and you need to check this) the fuse is sized to protect the WIRE.

Fuses do not prevent shorts from happening. If the starter shorts out then the fuse will blow but the starter is fried at that point. No amount of "proper fusing" will stop a starter from shorting out. So wither it is a "small short" or a big one the starter is still dead and the fuse (pick any size bigger than the starter needs but smaller than the ampacity of the wire) will blow.
It is OK to put a smaller fuse on a circuit than the wires can handle and greater than the appliance needs but what is the use? It does not protect the appliance one bit.

Another way to say this is any short in a starter is going to be of such a magnitude that fusing for the max wire ampacity will be exceded and the fuse will blow pritecting the wire from setting the boat on fire. the starter was dead before you put the key in the ignition you just did not know it yet.

 

[mitiempo]

You are fusing for short circuit protection, not overload. ABYC actually allows fusing for short circuit to be 150% of the wires ampacity. In addition ANL fuses will handle a larger amount of current for their rating. See table below.

Knowing wire size would help. If you are using 2/0, which should be the smallest on a large starter I would think, it can handle 280 amps in engine spaces - 330 outside. A 275 amp ANL will handle 420 amps for 500 seconds (over 8 minutes) and a starter is only run for 10 or 15 seconds at a time. It will handle more than 420 amps for very short periods of time.

 

[Maine Sail]

The first question should have been "what size wire is it"?

Bingo!! We need to know your wire size and jacket temp rating.. You are allowed to go as much as 150% of the table IV ampacity rating for short circuit protection..

If your wire is 1/0 or larger and rated at 105C then the 350A fuse is within the 150% rule..

 

[Kettlewell]

Thanks folks. I did size the 350 amp fuse for the max. load on the 1/0 cable when I first put it in, and I do set the maximum fuse size by the wire size, but no more than is needed by whatever is connected to the wire. In other words, I'd prefer to use a 250-amp fuse in the starter wire if I could get away with it--margin for error. So I was wondering if there is any way of determining what the typical inrush is for various starter motors.

 

[Maine Sail]

Thanks folks. I did size the 350 amp fuse for the max. load on the 1/0 cable when I first put it in, and I do set the maximum fuse size by the wire size, but no more than is needed by whatever is connected to the wire. In other words, I'd prefer to use a 250-amp fuse in the starter wire if I could get away with it--margin for error. So I was wondering if there is any way of determining what the typical inrush is for various starter motors.

In-rush does not really matter for fusing as it is less than 0.20 seconds in duration. You want to size for the average draw which is quite a bit lower than in-rush. Some of the Perkins motors have in-rush loads as high at 500+ but average just 250 or so. Other engines barely break 100 amps average... In order to test for in-rush you need at a minimum a Fluke 376 (captures in-rush at 100mS) or similar or an o-scope with DC amp clamp..

I would leave the 350 as it well protects against dead shorts. Not much sense in going any smaller, especially on a starting circuit. For starting circuits you generally want the biggest fuse the wire will support that will not melt the wire before blowing..

 

[Kettlewell]

For starting circuits you generally want the biggest fuse the wire will support that will not melt the wire before blowing..

Thanks. That was my original reasoning when I put it in. Since ABYC doesn't require circuit protection on starter lines, and I have never had a car starter short out in a way that burnt the cable either in 40+ years of driving all sorts of decrepit vehicles, I wonder how much danger there is in not fusing the starter? Seems like failure on a properly sized and protected cable would be pretty far down the list of potential problems. On the other hand, the 350-amp fuse in my starter line has worked fine for seven years so I'll probably just leave it in there.

 

[mitiempo]

You are protecting against chafe which is more likely on a boat than a car because of longer battery runs and more chance of wire movement.

 

[Maine Sail]

Thanks. That was my original reasoning when I put it in. Since ABYC doesn't require circuit protection on starter lines, and I have never had a car starter short out in a way that burnt the cable either in 40+ years of driving all sorts of decrepit vehicles, I wonder how much danger there is in not fusing the starter? Seems like failure on a properly sized and protected cable would be pretty far down the list of potential problems. On the other hand, the 350-amp fuse in my starter line has worked fine for seven years so I'll probably just leave it in there.

Do cars do this? Do they get wired by every Tom, Dick & Harry as opposed to a state of the art factory that employs strict, repeatable assembly, that has been thoroughly tested before final production, and goes together the same every time?

Starter wire that WAS fused. This one saved a Sabre from burning to the waterline......:

Starter wire that was not fused (just two minutes before this fire erupted there were a number of 8 & 9 year old junior sailors on this boat):

No fusing on the battery cables, deemed electrical fire:

The ABYC does not say to NOT fuse, they give an "exemption" IF the "starting circuit" wires are protected.

 

[Kettlewell]

MaineSail: Good punctuation for this thread! Like I said, I do fuse the starter circuit. Somewhere I read that BoatUS lists electrical fires as near the top of the list on the causes of boat insurance claims. I've had some near misses over many years. A boat I purchased many years ago when I was learning came with a clause in the contract that the boatyard was responsible for making sure the engine was running properly once launched. Thinking that was a good thing I was happy when the motor fired right up and I was able to motor away on my delivery trip only to soon have black smoke pouring out of the cabin. Apparently during the boatyard's commissioning someone had managed to pull down some electrical wires that ran over the top of the engine room, leaving them lying on the engine manifold which eventually burnt through the insulation and it turned out it was a completely unfused circuit that went right back to the battery. The burning insulation damaged a bunch of stuff, but luckily I was able to catch it before the boat burnt on my maiden voyage! It was amazing how quickly the boat was filled with choking black smoke that made visibility near zero. Ever since than I have been cautious about my electrical systems. Some friends of mine lost their boat in the Caribbean when it burnt to the waterline due to an electrical fire of unknown origins--luckily someone noticed their dog swimming around the harbor and he was saved.