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Old 10-10-2010, 22:10   #16
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Voltage drop across circuit protection devices. From the datasheets:

AIRPAX IPA 15A breaker: 0.108V
BUSSMAN AGC-15 fuse: 0.140V

About the same as 5 feet of 12ga wire.
About the same as the loss thru a couple of new crimp and screw terminals.
These losses are what limits the beneficial gains from using excessively large conductors.

Proper selection of a breaker can take some considerable engineering. It's easier to just go with whatever is common in the marine industry (E.g. what Blue Sea uses).
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Old 10-10-2010, 22:54   #17
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daddle and others,

I would question the data. The circuit breaker must have a calibrated voltage drop across the breaker in order to determine the current. Since the typical circuit breaker is reused the contacts can get burned from the circuit interruption process and since they are often used as switches, this also causes the contacts to further erode in time. The higher the current the higher the voltage drop.

The fuse must also generate a voltage drop to create the heat needed to melt the link. The higher the operational current the more heat until such time as the link melts. The voltage drop is then directly proportionate to the current. The fuse holder is not effected by the fuse burnout and when the fuse is replaced the device is effectively new again.
 
The fuse can be sized for very exacting use and the breaker can not. There are very limited breaker sizes in marine equipment and breaker trip currents can vary as much as 25% on a new breaker. What chance do you think that ten-year-old breaker is accurately protecting your expensive electronics gear?

As to the comment on changing out a fuse as per re-setting a breaker, I would wonder if either would fix anything? If something went over current it needs to be resolved and this usually involves more than just a reset, hoping the issue went away while you were not looking.

Your figures for voltage drop through various connectors is also questionable as there are not enough perimeters. Would each and every crimp connector have a similar voltage drop? No. Connector losses depend on too many factors to give out general answers to complex questions.

While I agree that bad connections can certainly negate the benefits of oversize wire, the important considerations are:
1: To minimize unnecessary connections and
2: Make good appropriate connections.

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Old 10-10-2010, 23:59   #18
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Again, the data is from the datasheets. Datasheets are the lifeblood of engineering. They are usually quite accurate if read and understood. Both breakers and fuse holders age. I've a fuse serving my refrigerator compressor that runs quite hot. The holder is probably failing.

If the breaker's contacts are burning prematurely then someone has failed to select the proper breaker. Marine breakers of the AIRPAX type are rated as switches. The current sensing, as I understand it, is more magnetic than resistive.

There is a maximum interrupt current for breakers. Especially important in DC circuits. If the breaker trips under in a short circuit type situation, and is installed electrically close to a big battery, then it's datasheet rating has probably been exceeded and it must be replaced. For example, if one shorts some terminals accidently with a wrench and trips the breaker, the breaker is probably toast. Especially if huge wire has been used to serve the breaker panel. It's all in the datasheets.

That is not the voltage drop of a bad connection. It's the voltage drop of a laboratory-made connection. Again, from the AMP datasheet. I'm sure any marine connection in service for a year or two is significantly worse. Those are not my figures.

So if the breaker has two crimp & screw connections, the return one, and the solar panel has two more, at a minimum we have 8 interfaces, each losing 3mV/A at 15A gives a connector loss of 0.360V. Plus the 0.108V of the breaker. Almost half a volt where none was expected. Plus the wire drop. Plus the internal resistance of the solar panel, plus the internal resistance of the battery, plus the voltage drops across the battery cable terminals and switches....
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Old 11-10-2010, 03:41   #19
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I have to agree with goboating now. Fuses are old tech. At night in a dark boat, pitching and rocking at sea, it sucks to look for a fuse and replace ..
How do you know a solar pannel fuse has blown when its night time?
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Old 11-10-2010, 04:42   #20
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How do you know a solar pannel fuse has blown when its night time?

Sounds like a riddle.....?

let me see.....

Its easy.. you hear the click..... zap as the lightning hits the boat?

I was generalizing about fuses vs breakers... no mention of my solar panel breaker in that statement.
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Old 11-10-2010, 06:36   #21
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Cool Just checkin up on ya.


It was a gag
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Old 11-10-2010, 07:45   #22
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um saudale, listen to your betters.

No fuse "protects " electronic devices, thats a fact, elctronic devices fail faster then a fuse can protect, What a manufacturer does with typical inline fuses is to ensure that a potential short circuit ( from the electronics failing faster then the fuse) does not result in an electrical overheat.

In practice breakers are a better solution, the fact that there are not supplied with the equipment is purely down to cost. In the case of the solar panels, teh fuse provides almost no protection anyway. Just put in a breaker sized above the maximum normal current epected.

Fuses are a 18 century throwback. The only way to protect electronics is to use electronic protection circuits ( crowbars, foldback systems, reverse polarity etc), which manufacturers employ to varying degress, depending on how cheapskate they are being.

PS arguing about voltage drop is utterly irrelevant, voltage drop is purely a concern if the maximum drop in a circuit results in the circuit not functioning, power loss ( usually inheat) is the concern and power loss is tiny, theres more lost in th efficiencies of the comsumer device then anywhere else. Your argument is angles on a pin type of thing.

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Old 11-10-2010, 08:04   #23
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Im lost now. I have a refer that is protected by a breaker and fuse's. The breaker as I understand protects the wiring in event of a short. One fuse is there because the fan may fail and the fuse will blow before it goes up in smoke. The other fuse is for the compressor same thinking. Seems like this protects the electronics and the boat.So sometimes it seems like a fuse is a good idea. I agree you dont want these things all over the place. But sometimes its better to have them then a unprotected piece of charged wire. My vote is with marks set up even though I paid out for the fancy thing.
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Old 11-10-2010, 08:16   #24
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sabray, what you have got is a "custom and practice " thing. Firslty since say the fridge manufactuer is too scabby to include breakers he puts it fuses, Also since he doesnt control what circuit breaker the boat builder put in he puts in fuses to try an prevent short circuit overheating.

In practice what you should have is a breaker that protects the device its connected to from short circuits , that also protects the wire. Its a mis-truths that breakers protec wire, thats merely how they are installed by the boat builder, but not the only way things should be.

I tend to run new DC feeds for any expensive or high load device and I protect them with a suitable circuit breaker. I remove inline fuses from electronics especially,like my VHF chartplotter radar etc.

FUses are never a good idea, there are much better one available nowadays.

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Old 11-10-2010, 08:24   #25
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I hear that but the refer has several current draws that are way less then the over all load. So the circuit breaker turns the refer on and it's rated at max load. The fuse is protecting lesser wiring and devices. Trying to think of things that are similar like The auto pilot which has the computer ram and control. In line fuses seem to protect these circuits and the lesser wiring. Right
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Old 11-10-2010, 10:41   #26
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So if the breaker has two crimp & screw connections, the return one, and the solar panel has two more, at a minimum we have 8 interfaces, each losing 3mV/A at 15A gives a connector loss of 0.360V. Plus the 0.108V of the breaker. Almost half a volt where none was expected. Plus the wire drop. Plus the internal resistance of the solar panel, plus the internal resistance of the battery, plus the voltage drops across the battery cable terminals and switches....
To get around all that voltage drop, I guess the only course of action is to hold off on all cruising until physics and progress gives us superconductors for the entire boat's electrical system.
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Old 11-10-2010, 11:09   #27
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I should be heading out now anchored up the severn a bit. But Im afraid my volts will drop off. So Im waiting. Seems all a little to difficult. Maybe there is an extra good way to do things and then reality is Im doing things. If your up this way dont look Im the guy with his volts around his ankles
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Old 11-10-2010, 11:26   #28
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18th century or not fuses are a good protection. I use circuit breakers on the mains for convenience, and fuses on each device. For the voltages and currents involved volt drop is irrelevant. For the panels just use the recommended inline fuse, (and locate it where it can be replaced conveniently).
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Old 11-10-2010, 11:27   #29
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"Its a mis-truths that breakers protec wire, thats merely how they are installed by the boat builder, but not the only way things should be."



Wow! I am not sure if you fail to understand the physics or the concept of hardware!

Any OCPD protects the circuit from TWO possible occurrences, a ground fault, in which the purpose of the OCPD is to trip out under the resultant maximum current draw and thus prevent the fire potential of a burn-through. The second purpose is to trip (or blow) when the current in the circuit exceeds the wire ampacity. Both are important, not just one or the other. One is theoretically infinite amps and the other is simply the wire getting too warm as a result of the circuit drawing too much current. This second condition does not have to imply a circuit is faulty but that there is too much total load on the wire.

The best example of the second case is in the typical 120VAC receptacle circuit. One can go on forever stringing duplex receptacles on a circuit, as long the total amp draw does not exceed the wire rating then the circuit is safe. Plug in a light here and there and the circuit perimeters are not exceeded. Plug in two blow dryers and the 20 amp circuit would trip out. A good example of this in 12 volts is the lighting circuit. It does not protect the individual light or wires within the fixture, only the wire runs between the fixtures are protected by the OCPD.

If an OCPD is used in a dedicated device it can then be sized to protect the device, such as a motor control circuit. BUT, if there is more than one application such as a transformer to supply the lower control voltage used in the controls, then that transformer circuit must be individually protected.
 
 
If the fuse is such an antique device why does the NEC prohibit the use of circuit breakers for OCPD in most motor control circuits? Must be a bunch of qualified electrical engineers bought off by the fuse people!

I have personally seen expensive breakers operating in clean conditions that would not trip at a current even close to the rated capacity. Plus or Minus 25% is a typical operating range of a used breaker. I am not saying that I would not have breakers on my boat because I do have and use them. I am saying that they are not yet the simple cure-all that you make them out to be.

I will tend to agree that any overcurrent protection device will not prevent circuit failure in delicate electronics. Crowbar circuits work but only if the OCPD works in turn. In the case of motor control the fuse will always be better than the breaker unless the breaker is custom made for the application. The big safety of the fuse in an electronic device is protection from reversed polarity. I am man enough to admit that I have gotten pos and neg crossed in an installlation and was darn glad the little fuse popped!

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Old 11-10-2010, 11:50   #30
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thats what i thought. gone sailing
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