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#1
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Fuse the Wire or Fuse the Device?
I am about to open a debate on fusing in Gliders. Which of the
following fusing methods do you believe should be used and why? 1) Place one fuse at the main battery sized to protect the wire. That is if the wire "fuses" (vaporizes) at 20A then the fuse should be 20A. Therefore it takes a 20A device fault to blow the fuse. 2) Place one fuse at the main battery sized to the devices being sourced. That is if all the devices being powered have a maximum draw of 5A, then the fuse should be 5A or slightly larger. Therefore it takes a 5A fault to blow the fuse. 3) Place one fuse at the main battery sized to protect the wire (see #1). Place one fuse at each device sized to the devices being sourced. 4) Place one fuse at the main battery sized to the devices being sourced (see #2). Place one fuse at each device sized to the devices being sourced. 5) fill in the blank Thanks, John |
#2
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I use 5A fuze at the battery and a 5A C/B for everything in the panel.
Don't ever remember blowing a fuse. I have had wires break, fuse holders not keep good contact, plugs fail to do their thing, etc. My latest problem was a "clunking" noise, turned out to be the plug at the end of a 6 inch battery lead, clunking against the deck. After about 10 flights of clunking, the wire broke at the end of the solder connection to the battery. Now have the plug epoxied to the battery. BTW, we need 2 batteries and 2 varios and 2 GPS's and 2 data loggers, etc to make everything 100 % reliable. When my wire broke I was 40 miles from home, switched the Borgelt B-40 to "internal" power and had a vario and audio to get old JJ back home. |
#3
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ContestID67 wrote:
I am about to open a debate on fusing in Gliders. Which of the following fusing methods do you believe should be used and why? 4) Place one fuse at the main battery sized to the devices being sourced (see #2). Place one fuse at each device sized to the devices being sourced. I use a slow fuse somewhat bigger than the total my devices will draw on the battery. This prevents damage to the battery and the main wires to the instrument pannel. Note that it's hard to say at wich current your cable will melt. This is not only determined by the wire as it is when you just installed it, but also by the (soldering) connections, againg plastics, damaged isolation, the time a short circuit continues, etc. For each instrument on it's own, I have a faster fuse. 1 Amp for most of them. Now, if a device causes a problem, only that device will be out of order. The rest will continue to function just fine. You can use automatic fuses for these if you want. André |
#4
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#5
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The fuse is there to protect the wiring not your devices. The best bet is to do the following: 1 Large fuse at the battery just bigger than all of your devices, take this to a fuse box and fuse individual devices at their correct current rating. ie if the device is rated at 1 amp fuse it at 1 amp. Do NOT fuse two devices from one larger fuse. Use the correct size wire for the current to be drawn, many nasty things can happen if you get to larger voltage drop along the wire and you start driving a 12 volt device with a lower voltage. Be aware that the fumes released by some wire is very toxic and you may find you need to leave an other wise perfectly serviceable aircraft for no other reason that you did not fuse your wires correctly. One other point, if the device blows a fuse the chances are that there is something wrong with it. Do not start putting larger fuses in the circuit get the item looked at. With speacial thanks to Steve K, who recently lectured on this subject on a Form 2 course I attended, I hope I did you justice Sean -- jorgie ------------------------------------------------------------------------ Posted via OziPilots Online [ http://www.OziPilotsOnline.com.au ] - A website for Australian Pilots regardless of when, why, or what they fly - |
#6
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I really like scenario # 3. Most of the time you'd be OK with just the
devices fused, but would you want to have an electrical fire in the glider at altitude, with oxygen floating around in the cockpit? I have mine fused as close to the battery as possible. I'm a telecom manager for a class 1 railroad, and we do all our installations with this fusing method. We have a lot of specialized electronics in locomotives and vehicles. We don't want electrical fires in any of those, either. Be safe up there... Jack Womack |
#7
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I use scenario #3. In the UK it is mandatory to have
a fuse 'as close to the battery as possible' to protect the whole system and I use a 3 amp fuse. There is also a seperate fuse, rated to the individual instruments mainly to try and prevent one single instrument taking the whole system down. That does not always work as the only problem I have had was with a fault in the distribution box/power supply for the PDA/Logger/GPS. All the output leads from the box are fused but a fault in the box blew the main battery fuse. The power requirements of all my instruments are well below 3 amps. A standard battery is 7ah a current draw of 3 amps would flatten the battery in 2.5 hours (max) and this is one of the reasons why we are so set against adding further avionics such as transponders over this side of the pond. At 03:00 20 April 2005, Jack wrote: I really like scenario # 3. Most of the time you'd be OK with just the devices fused, but would you want to have an electrical fire in the glider at altitude, with oxygen floating around in the cockpit? I have mine fused as close to the battery as possible. I'm a telecom manager for a class 1 railroad, and we do all our installations with this fusing method. We have a lot of specialized electronics in locomotives and vehicles. We don't want electrical fires in any of those, either. Be safe up there... Jack Womack |
#8
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Don Johnstone wrote:
The power requirements of all my instruments are well below 3 amps. Have you checked the amperage while the radio is transmitting? A glider with a GPS (.2 -.3 amps), PDA (.4 to .8 amps), vario, and a good radio transmitting (Becker 4201 2+ amps) can draw 3 amps, so you might be closer to the fuse rating than you think. And if you turn on the T&B (.7-.9 amps), you will almost surely exceed 3 amps during transmission. I suggest you go to at least 5 amps, since you use smaller fuses to protect the individual devices. This will also reduce the voltage drop across the fuse, a good thing. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
#9
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"ContestID67" writes:
I am about to open a debate on fusing in Gliders. Which of the following fusing methods do you believe should be used and why? 1) Place one fuse at the main battery sized to protect the wire. That is if the wire "fuses" (vaporizes) at 20A then the fuse should be 20A. Therefore it takes a 20A device fault to blow the fuse. The idea is right, but you should use a fuse well under the rupture current of the wire. You want the fuse to blow, not both! Also check the inrush current of your panel, you may get a supprise at how high it is when the master switch is flicked on with all of your stuff already switched on. Also check the battery rating, the fise should be below that. Dumb to protect the wire and boil the battery. 5) fill in the blank Ask the insrument maker what fuse they sepecify. It could be none, unit has internal protection, or a type of xA. Unless you use a fast blow, most electronics get no protection from a fuse. The electronics are dead before the fuse heats up enough to blow. Carry spares of ALL sizes you use. Don't even think about doing the trick of wrapping foil around a blown fuse to fix it; the blow current of cigarette packet fiol over an AG3 fuse is 3000-10000A! Yes, THOUSANDS of amps. The type could be a standard(ish) fuse, a slo-blo or a fast blo, or an specific part. The correct rating for fuses is not Amps, but I^2T, that is, the higher the current, the less time it lasts, up to a limit where it lasts `forever'. -- Paul Repacholi 1 Crescent Rd., +61 (08) 9257-1001 Kalamunda. West Australia 6076 comp.os.vms,- The Older, Grumpier Slashdot Raw, Cooked or Well-done, it's all half baked. EPIC, The Architecture of the future, always has been, always will be. |
#10
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